# Clearing the Neighbourhood: Definition of 'Orbital Region'?

I am trying to determine how wide a 'neighbourhood' a planet is expected to clear.

However, the Wikipedia article on the matter provides no clarity, simply stating that a planet is expected to clear it's 'orbital region' with no definition provided.

I suspect that the correct answer is that a planet is expected to clear a region around it's orbit equal to the radius of its Hill Sphere, but I am not certain of it.

Furthermore, for a planet of Earth mass, the Hill Sphere radius is a measly 0.01AU, which doesn't seem like much of a neighbourhood!

Is there a formal way to define the 'neighbourhood'?

The IAU gives no definition, it leaves the term somewhat vague.

Various calculations of the "planetariness" of solar system bodies use slightly different notions of the "neighbourhood"

Soter's $$mu$$ considers bodies to be in the neighbourhood if they share a common radial distance (ie the orbits cross each other) and the orbital periods differ by less than an order of magnitude

Margot's $$Pi$$ uses $$2sqrt 3$$ times the Hill radius.

Stern and Levinson's $$Lambda$$ in the original paper consider the neighbourhood to be bodies that cross the orbit of the planet, while not in an orbital resonance, to be in its neighbourhood, in a somewhat complex way that requires knowledge of the relative inclinations and and orbital velocities of bodies that cross orbits.

Changing the details of these values doesn't change the conclusion of the calculations: There is a clear and measurable difference between bodies like Mars and Neptune that are (to use Stern and Levinson's term) uberplanets and Ceres and Pluto that are unterplanets (or dwarf planets as termed by the IAU).

No, it is a vague term that isn't defined anyhow by the IAU. If you consider the object's hill sphere as what is meant, then Pluto and Eris "cleared their neighbourhood" too. If you consider it the way that Pluto's orbit overlaps with Neptune for instance, then no planet cleared their orbits. There are Mercury-crossing asteroids, Near-Earth asteroids, Trojans crossing Jupiter's orbit et cetera. If we consider it as being meant that the bodies must be the most massive ones in their area (which again is vague) then Eris and Ceres should be planets and perhaps Pluto too (if you look three-dimensionally on the ecliptic). If you set an arbitrary border, either Ceres would be a planet or there wouldn't be planets at all.

So the 2006 planet definition is pure nonsense and shouldn't be considered valid and it isn't by many.

## Planets in deep space

Well, we just became aware that there are likely thousands of planet sized objects in wide orbits around the sun. There could be thousands more planet sized objects ejected from orbit.

We can measure the mass of the galaxy by the orbital speed of the objects in it. Not easy because they are so big, but doable for many.

There are many ways planets can end up in deep space. Interactions with other planets (especially early in the solar system's life-cycle when there are dozens of large objects forming, colliding, and affecting each other), gravitational interactions from passing stars and black holes in which a planet gets caught in the crossfire and expelled from its solar system (more common in star clusters and closer in towards the center of the galaxy), being "released" from a solar system due reduced gravity of a parent sun when it goes through the red giant phase and expels most of its mass in fierce solar winds (a future possibility for Earth), or even due to supernovae. All of these are possible and even likely throughout the galaxy. Raw guesses on numbers are not really possible, but billions of interstellar wanderers may be plausible from what I have read. And over the vast eons of time, almost any interaction that can occur, will occur, throughout the galaxy and universe. Not only that, many of these wandering "dark" planets may have internal sources of heat, sub-glacial oceans, and primitive life. So the answer to your question is most definitely "yes", but actual data is hard to come by because we can't see these objects at all.

As for your question relating to overall mass of the galaxy, I would guess the contribution from a few billion interstellar planets would be negligible in a galaxy with hundreds of billions of stars, considering that stars can be millions of times the mass of a single planet, and super-massive black holes billions of times more massive than the stars.

## Types of resonance

In general, an orbital resonance may

• involve one or any combination of the orbit parameters (e.g. eccentricity versus semimajor axis, or eccentricity versus orbital inclination).
• act on any time scale from short term, commensurable with the orbit periods, to secular, measured in 10 4 to 10 6 years.
• lead to either long term stabilization of the orbits or be the cause of their destabilization.

A mean motion orbital resonance occurs when two bodies have periods of revolution that are a simple integer ratio of each other. Depending on the details, this can either stabilize or destabilize the orbit. Stabilization occurs when the two bodies move in such a synchronised fashion that they never closely approach. For instance:

• The orbits of Pluto and the plutinos are stable, despite crossing that of the much larger Neptune, because they are in a 2:3 resonance with it. The resonance ensures that, when they approach perihelion and Neptune's orbit, Neptune is consistently distant (averaging a quarter of its orbit away). Other (much more numerous) Neptune-crossing bodies that were not in resonance were ejected from that region by strong perturbations due to Neptune. There are also smaller but significant groups of resonant trans-Neptunian objects occupying the 1:1 (Neptune trojans), 3:5, 4:7, 1:2 (twotinos) and 2:5 resonances, among others, with respect to Neptune.
• In the asteroid belt beyond 3.5 AU from the Sun, the 3:2, 4:3 and 1:1 resonances with Jupiter are populated by clumps of asteroids (the Hilda family, 279 Thule, and the Trojan asteroids, respectively).

Orbital resonances can also destabilize one of the orbits. For small bodies, destabilization is actually far more likely. For instance:

• In the asteroid belt within 3.5 AU from the Sun, the major mean-motion resonances with Jupiter are locations of gaps in the asteroid distribution, the Kirkwood gaps (most notably at the 3:1, 5:2, 7:3 and 2:1 resonances). Asteroids have been ejected from these almost empty lanes by repeated perturbations. However, there are still populations of asteroids temporarily present in or near these resonances. For example, asteroids of the Alinda family are in or close to the 3:1 resonance, with their orbital eccentricity steadily increased by interactions with Jupiter until they eventually have a close encounter with an inner planet that ejects them from the resonance.
• In the rings of Saturn, the Cassini Division is a gap between the inner B Ring and the outer A Ring that has been cleared by a 2:1 resonance with the moon Mimas. (More specifically, the site of the resonance is the Huygens Gap, which bounds the outer edge of the B Ring.)
• In the rings of Saturn, the Encke and Keeler gaps within the A Ring are cleared by 1:1 resonances with the embedded moonlets Pan and Daphnis, respectively. The A Ring's outer edge is maintained by a destabilizing 7:6 resonance with the moon Janus.

A Laplace resonance occurs when three or more orbiting bodies have a simple integer ratio between their orbital periods. For example, Jupiter's moons Ganymede, Europa and Io are in a 1:2:4 orbital resonance. The extrasolar planets Gliese 876e, Gliese 876b and Gliese 876c are also in a 1:2:4 orbital resonance (with periods of 124.3, 61.1 and 30.0 days). [ 1 ] [ 2 ]

A Lindblad resonance drives spiral density waves both in galaxies (where stars are subject to forcing by the spiral arms themselves) and in Saturn's rings (where ring particles are subject to forcing by Saturn's moons).

A secular resonance occurs when the precession of two orbits is synchronised (usually a precession of the perihelion or ascending node). A small body in secular resonance with a much larger one (e.g. a planet) will precess at the same rate as the large body. Over long times (a million years, or so) a secular resonance will change the eccentricity and inclination of the small body.

Several prominent examples of secular resonance involve Saturn. A resonance between the precession of Saturn's rotational axis and that of Neptune's orbital axis (both of which have periods of about 1.87 million years) has been identified as the likely source of Saturn's large axial tilt (26.7°). [ 3 ] [ 4 ] [ 5 ] Initially, Saturn probably had a tilt closer to that of Jupiter (3.1°). The gradual depletion of the Kuiper belt would have decreased the precession rate of Neptune's orbit eventually, the frequencies matched, and Saturn's axial precession was captured into the spin-orbit resonance, leading to an increase in Saturn's obliquity. (The angular momentum of Neptune's orbit is 10 4 times that of that of Saturn's spin, and thus dominates the interaction.)

The perihelion secular resonance between asteroids and Saturn (ν6 = g -g6) helps shape the asteroid belt. Asteroids which approach it have their eccentricity slowly increased until they become Mars-crossers, at which point they are usually ejected from the asteroid belt by a close pass to Mars. This resonance forms the inner and "side" boundaries of the asteroid belt around 2 AU, and at inclinations of about 20°.

Numerical simulations have suggested that the eventual formation of a perihelion secular resonance between Mercury and Jupiter (g1=g5) has the potential to greatly increase Mercury's eccentricity and possibly destabilize the inner Solar System several billion years from now. [ 6 ] [ 7 ]

The Titan Ringlet within Saturn's C Ring represents another type of resonance in which the rate of apsidal precession of one orbit exactly matches the speed of revolution of another. The outer end of this eccentric ringlet always points towards Saturn's major moon Titan.

A Kozai resonance occurs when the inclination and eccentricity of a perturbed orbit oscillate synchronously (increasing eccentricity while decreasing inclination and vice versa). This resonance applies only to bodies on highly inclined orbits as a consequence, such orbits tend to be unstable, since the growing eccentricity would result in small pericenters, typically leading to a collision or (for large moons) destruction by tidal forces.

In an example of another type of resonance involving orbital eccentricity, the eccentricities of Ganymede and Callisto vary with a common period of 181 years, although with opposite phases. [ 8 ]

## Orbital

Astronomical observations of distant stars tell us that many have planets orbiting them, and that some are of a size and density and orbital distance such that temperatures suitable for life are theoretically possible.

Scientists can search for exoplanets only within a limited range of orbital distances.

Eventually, the goal for Musk is to have Starship making regular and frequent flights — for point-to-point flight on Earth, for orbital missions closer to home and for long-distance runs to the moon and Mars.

There’s protection from micrometeorites based on a similar design for the current orbital EMUs.

Such conjunctions recur every two decades owing to their orbital periods.

These companies include SpaceX, Orbital Sciences, Sierra Nevada Space Systems, and more.

Though it would mow your lawn at International Space Station orbital speed—17,000 mph.

What is basically being attempted is a ballistic, sub- orbital flight, like a ride atop a short-range missile.

Now, Orbital Comics Gallery is seeking to address the issue through an exhibit titled 'Image Duplicator.'

When it was over, Keith had been stabbed in the shoulder, and Brandon had a fractured eye socket and orbital wall.

But, because of it's great mass, our orbital velocity is something terrific!

Our orbital velocity is seven hundred thousand kilometers per second!

At the orbital velocity of the ship, focussing on the star was indeed a difficult thing to do.

The rotations of the planets and the sun are in the same direction as the orbital motions and nearly in the same plane.

Is there any connection between their orbital distances, or between their orbits and the times of describing them?

## Part 2

In order to understand where the fuel for the solar nebula came from, it is important to examine the stages of the life cycle of a star, which results in the formation of the heavier elements that we find in our solar system. This “stardust” is thought to have been a part of our solar system since the beginning, when it was a diffuse nebula.

-Nebula: a cloud of gas and dust in space.

– Interstellar medium: gas and dust found in the spaces between stars.

• Interstellar dust: microscopic grains of dust, composed of elements like carbon, silicon, oxygen and iron.
• Molecular cloud: when a gas cloud cools sufficiently, the atoms can form into molecules creating a molecular cloud (not all elements can form molecules).

– Thermal pressure: pressure in a gas arising from the motions of its
particles in relation to the object’s temperature.

-Protostar: A forming star which has not yet reached temperatures and pressures required for sustained fusion to occur in its core.

-Protostellar disk: A disk of material surrounding a protostar. Sometimes
these disks coalesce into planetary solar systems.

-Convection: the energy transport process in which warm material expands.

rises while cooler material contracts and falls to the centre.

-Self -Sustaining Fusion: Fusion which occurs where the energy being created in the fusion process is radiated away at the same rate, effectively halting gravitational contraction, and bringing the star in equilibrium.

### Stardust as “Fuel” for Solar Nebula

With the exception of hydrogen and helium, many of the elements found in our solar system were not formed during the Big Bang, but were instead formed during the various stages of the life cycle of a star. By analyzing the processes by which these elements are formed, it possible to achieve a greater understanding of how solar nebula are formed, and how our solar system was formed as a result.

In order to understand how stardust becomes fuel for solar nebula, it is important to examine the stages of the life cycle of a star that result the formation of the elements that we find in our solar system.

### Formation of Solar Nebula

To understand the formation of the solar nebula, we need to understand the life cycle of a star. Stars are born in an area of high density Nebula, by condensing into a large globule of gas and dust that contracts under its own gravity. Specifically, they begin forming in cold and dense regions of the interstellar medium known as molecular clouds. The temperature of these clouds is between 10K and 30K (Kelvin) and have an average density of 300 molecules per cubic centimetre, which is almost a million trillion times less dense than the Earth’s atmosphere. Although this doesn’t seem dense enough to form into anything, it is far denser than empty space. These clouds consist mostly of molecular hydrogen (i.e. H2) since helium atoms do not combine with other atoms to form molecules, but there are also elements like carbon, silicon, oxygen and iron present in the form of interstellar dust, which will be discussed below.

By overcoming the thermal pressure of the gas cloud, gravity causes the densest regions (often referred to as “clumps”) of these clouds to contract, coalescing into increasingly dense regions of interstellar gas and dust. The contraction also eventually results in the fragmentation of the clump from the gas cloud, caused by the dense regions of the cloud contracting (overcoming the thermal pressure) to the point that they break away. The contraction converts some of the clouds gravitational potential energy into thermal energy. Due to the opaqueness of the aforementioned interstellar dust within the cloud, this heat cannot be radiated away as fast as the heat is created by gravitational contraction. The object that results is an increasingly hot and dense molecular mass called a protostar.

### Protostar to Main Sequence Star

The protostar stage is the first stage in a star’s life cycle, occurring after the initial gravitational collapse of the nebula, but before the star begins fusing hydrogen to create heavier elements. The stages leading up to this “hydrogen fusion” stage are summarised in Table 1.1. [v]

 Convective contraction The protostar surface temperature remains near 3000K while convection is the dominant form of energy transport. Gravitational contraction continues increasing the core temperature. The reason the core temperature rises while the surface temperature stays constant is that the energy at the core rises until it reaches the layer at which the photons can escape to space. This layer is 3000K because at higher temperatures, the collisions that occur can strip electrons from hydrogen atoms creating positively charged hydrogen ions, which absorb light photons better than neutral charged hydrogen atoms (which occurs at 3000K). Radiative contraction The primary energy transport moves from convective to radiative diffusion. This is done as the contracting protostar’s core becomes hot enough to strip almost all of its atom’s electrons, making it easier for radiation to flow freely through the star’s interior. Surface temperature begins to increase, up to about 5000K. During this stage, the protostar releases about half of the thermal energy it gains from contraction. If this did not occur the thermal pressure would balance out the gravitational contraction before nuclear ignition could occur. Hydrogen nuclei begin to fuse into helium nuclei, but the amount of energy released by the process is small compared to the amount radiated away. Thus, fusion is non self-sustaining at this point. Self-sustaining fusion Gravitational contraction continues, increasing the core temperature to between 10 and 15 million K. At this point there is sufficient temperature and pressure to achieve a fusion rate that balances with the rate which energy is radiated away. Equilibrium is achieved as the thermal pressure matches gravity. The result is a self-sustaining fusion process. At this point, the protostar becomes a star. Once a star begins the self-sustaining fusion process, the energy it generates balances the energy it radiates into space, and the internal pressure of the star becomes stable. Once equilibrium is achieved it is called a Main Sequence Star.

### Heavy Elements in Solar Nebula

Mass plays an important role in the life cycle of each main sequence star. A star with a higher mass comes into equilibrium at a much higher temperature than a lower mass star thus, fusion energy is released at a much higher rate. Because of this a higher mass main sequence star will consume hydrogen at a much higher rate, resulting in a shorter life despite having a greater amount of hydrogen to fuse. A star with 8 solar masses or more, is considered a high-mass star. Stars of this mass behave in a similar fashion to a low mass star in the early stages of life, with the exception of a much faster rate of fusion. Once the core hydrogen is spent, the paths of a high-mass and low-mass star diverge, as a high mass star continues to fuse heavier elements in its core due to higher core pressures and temperatures which result from higher mass. The life cycle of a high-mass star continues as summarized in table 1.2.

 Red Super Giant After the core hydrogen is exhausted, the core shrinks and temperature rises. Hydrogen begins to fuse in a shell around the helium core (helium is the result of hydrogen core fusion), pushing the outer layers of the star out due to increased thermal energy. Helium- core fusing supergiant Once the core temperature becomes hot enough to fuse helium into carbon, helium core fusion begins. The core expands, hydrogen shell fusion slows and the star’s outer layers shrink. Multiple shell fusion super-giant As the process continues the star begins to fuse many different elements in shells around an iron core. Supernova Iron cannot provide fusion energy, so as the heaviest element in the process, it accumulates in the core until fusion ceases. Once this happens, outward thermal pressure ceases. Gravity becomes the dominant force, allowing the outer layers of the star to collapse inward, starting a chain reaction creating the rest of the elements. The collapse halts once the outer layers reach the central core of neutrons and violently bounce back, sending the elements created in the fusion process out into space to become the ingredients for the birth of next generation stars.

The aftermath: The birth, life and death of a high-mass first generation star (a star formed from hydrogen and helium only) injects the interstellar medium with elements formed in the fusion process, which are heavier than hydrogen and helium. Together, all of these elements combine, cool and coalesce into molecular gas clouds. Through gravitational contraction, these clouds form clumps, which continue to contract and eventually break away from the larger cloud and form a protostar. As the cloud surrounding a protostar contracts, the gas begins to rotate. This rotational speed increases (due to the physical law of conservation of angular momentum), which prevents gas from raining directly down into the protostar, instead settling into a protostellar disks. Once the processes of star ignition described above take place, these protostellar disks become solar nebula, which consist mostly of hydrogen, helium, carbon, silicon, oxygen and iron, and the rest of the elements on the periodic table that result from the final chain reaction of a dying high-mass star. This is how the stardust that results from the life and death of high-mass stars (along with hydrogen and helium already present from the big bang) becomes fuel for a solar nebula. [vii]

## Pluto and the Kuiper Belt

In 2006, the International Astronomical Union (IAU) passed two resolutions that resonated strongly with the public and had an immediate impact on elementary school curricula. The first was a change in the definition of what it means for an object in our Solar System to be a “planet”. The second introduced and defined the term “dwarf planets”. The notorious revision of these definitions stripped Pluto of its planetary status and took the world by storm. It is important to understand why the definition of a planet had to be changed and why Pluto, despite being reclassified, is still essential to our understanding of our solar system.

The discovery of the Kuiper Belt and an increase in our understanding of the objects within the Kuiper Belt led to the re-evaluation of planetary classification. The Kuiper Belt is a zone at the edge of our solar system stretching over 20 astronomical units (AU)—that is, 20 times the distance from the Earth to the Sun—which contains thousands of astronomical objects composed mostly of ice and ranging vastly in size. Pluto is the largest known dwarf planet and is the second closest dwarf planet to the Sun. 1 It is suspected that hundreds of dwarf planets exist within the Kuiper Belt.

Though Pluto is no longer technically classified as a planet, it is essential to continue to study Pluto and keep it in school curricula. Pluto and the Kuiper Belt hold valuable information about the creation of our solar system, including how planetary orbits and locations vary with time. The massive size of the Kuiper Belt itself, along with its recent discovery, demonstrates how little is known about our solar system—how much there still is to explore.

By studying the history of Pluto and the Kuiper Belt, society can achieve a greater understanding of our solar system, its creation, and how the objects within it are classified. They may also learn to appreciate the importance of Pluto and the Kuiper Belt and why these astronomical topics should remain in the school curricula at all ages.

To incorporate these topics into your classroom, please check out some of our useful tools for teachers!

#### IN THE BEGINING

##### PLANETARY CREATION:

When a star is created, a large and dense rotating disk of gas and dust surrounds the star. Substances from the disk clump together and accumulate other materials and objects as they orbit around the star. Eventually these clumps become massive enough to either engulf everything in their orbital path or knock them out of their way. They then become spherical planets orbiting a star. 2

##### PLUTO’S CREATION:

In the outer regions of the star’s disk, the gas and dust is less dense. In our solar system, this area is known as the Kuiper Belt. This region is less affected by solar winds, which blow away lighter particles in developing regions nearer to the star. These lighter particles circulate in the outer-disk to create objects like asteroids, comets, dwarf planets, and moons. Due to the length of its orbital path, and its small orbital speed, Pluto along with the other Kuiper Belt objects never managed to coalesce into a large planet.

Another point that differentiates Pluto from the planets is that it is a part of a binary system. This is when two masses rotate around a common centre of mass rather than the smaller body rotating around the larger one. The Earth’s moon rotates around a centre of mass that is within the Earth’s surface because the moon is much smaller than the Earth. However, the size difference is not as great between Pluto and its largest moon Charon. Charon and Pluto circulate around a centre of mass that lies between two bodies while they orbit around the sun as a unit. Pluto’s other moons also orbit this centre of mass because their orbits are beyond both Pluto’s and Charon’s orbits. 1

#### PLUTO

Pluto is extremely cold and covered with ice. Its temperature ranges from about 33 to 55 Kelvin. Pluto’s mass is just 0.2% that of the Earth and 18% that of the moon”. 3 Its gravity is about one-fifteenth the Earth’s gravity and its diameter is about 2,300 km. Pluto has five moons: Charon, Kerberos, Nix, Hydra, and Styx.

Pluto orbits the Sun at a distance of about 5.8 billion kilometers on average and its orbital period is 248 years. Its orbit is found in the Kuiper Belt and it is notable that Pluto’s mass is less than 1% of the total mass of all the objects found in its orbit. 4 Pluto’s orbit is inclined relative to the orbits of the planets in our solar system and is also more oval-like.

In July 2015, NASA’s New Horizons spacecraft arrived at Pluto. It discovered that Pluto’s surface appears to be mostly nitrogen ice with a mixture of carbon methane. 4 Pluto’s surface is very varied and is comprised of craters,

Measurements made by the New Horizons spacecraft showed that Pluto’s atmosphere has a density about 2,000 times smaller than Earth’s. Only the largest KBOs are expected to have atmospheres. The combination of Pluto’s large size, its high reflectivity, and its comparatively close proximity to the Sun, makes it the brightest known KBO. That is why Pluto is the easiest KBO to see from Earth and why it was discovered in 1930, almost 50 years prior to any other KBO. Since it was discovered so many decades before any other member of the Kuiper Belt, Pluto is the only KBO to ever have been given the status of “planet.”

“Pluto Through Time”. Copyright 2016 Christopher Altrogge.

##### PLUTO THROUGH TIME
• Approximately 4.6 Billion years ago, rock ice and gasses form the mass of Pluto. 1
• 1905: After studying the orbit of Uranus, astronomer Percival Lowell (1855-1916) attributes the cause of irregularities in the planet’s orbit to another unknown planet beyond Neptune. He initiates a systematic search for the mystery planet. 6
• 1930: Clyde Tombaugh officially discovers Pluto at the Lowell Observatory where it is given its name. 7
• 1992: David Jewitt and Jane Luu successfully detect the first object in the Kuiper Belt since Pluto, QB1. 8
• 2005: Eris, a Pluto-sized Kuiper Belt Object, is observed and named Xena then later changed to Eris. This discovery calls the current planet classification criteria into question. 9
• 2006: NASA’s New Horizons satellite is launched. 7
• 2006: IAU ratifies a resolution that defines Planets and Dwarf Planets Pluto is classified as a dwarf planet. 10
• 2015: New Horizons arrives at Pluto. 4

#### KUIPER BELT

Facts on the Kuiper Belt.
Courtesy of NASA/JPL-Caltech/Bill Dunford and Katie McKissick.

The Kuiper Belt marked its debut with the discovery of Pluto in 1930 and has since sparked the interest of astronomers, however the Kuiper Belt had otherwise evaded the eyes of astronomers for centuries. New information and discoveries are found regularly within the Kuiper Belt, yet there is still much to learn about this recently discovered realm. The Kuiper Belt itself is a disc-shaped region of icy bodies, dwarf planets, and comets located beyond Neptune’s orbit. It extends from about 30 – 55 AU and is populated with hundreds of thousands of icy bodies larger than 100 km across plus an estimated trillion comets. Astronomers can find Kuiper Belt Objects (KBO) among the myriad of faint stars because KBOs move slowly over time. To observe these slight shifts in position, very detailed photographs of the sky are taken in given time intervals to reveal the motion of these little ‘stars’. A very small change in position indicates that the object is very far from the Sun and most likely located in the Kuiper Belt. Large changes indicate objects closer to the sun and are more easily discovered.

The two largest Kuiper Belt Objects are Pluto and Eris, each with a diameter of about 2,380 kilometers. Scientists believe additional KBOs in the 1000-2000 km size range will be found, but most KBOs are much smaller. Roughly 80 KBOs have companions, and more are being discovered all the time. They are called “binary KBOs” because the two objects would be of a similar size, so they orbit around a common centre of mass opposed to a smaller moon orbiting around a larger object. The best-known pair is Pluto and Charon, which orbit each other every six days at a distance of about 17,000 kilometers. 7

##### TIMELINE OF KBO DISCOVERIES

1930: Clyde Tombaugh discovers Pluto, which later turns out to be one of many KBOs.

1943: Astronomer Kenneth E. Edgeworth speculated that there could exist smaller planets beyond Pluto’s orbit.

1951: Gerard Kuiper expanded on this theory through a mass-distribution model that predicted the existence of large icy masses formed during the creation of our solar system. However, Kuiper believed that the gravitational forces of Pluto had since relocated these masses. At the time, Pluto was believed to be much larger, so naturally, he predicted that it would have cleared the entire surrounding area of debris.

1992: David Jewitt and Jane Luu discovered what was believed to be the first Kuiper Belt object: an icy mass about 250 km in diameter that was attributed the name “1992 QB1”.

2002: Quaoar, a large Kuiper Belt object, about five times the diameter of 1992 QB1, was discovered. Ironically, Quaoar had been photographed as early as 1980, but was not recognized as a Kuiper Belt Object until 2002.

2004: Haumea, an egg-shaped dwarf planet, was discovered in the Kuiper Belt located just beyond Neptune’s orbit. It is thought to have collided with an object about half its size, which would account for the fast spin and unique shape.

2005: The dwarf planet Eris was discovered. It is the second largest known dwarf planet at about 2300 km in diameter, and is still considered the most massive Dwarf Planet. The discovery of an object in the Kuiper Belt more massive than Pluto sparked the debate on Pluto’s planetary status and the definition of a planet.

2005: The Kuiper Belt Object Makemake, also known as 2005FY9, was discovered. Makemake was later classified as a dwarf planet. This was made official by the International Astronomical Union in 2008.

2006: NASA launched a space probe called ‘New Horizons’. Its mission is to fly by Pluto and other Kuiper Belt Objects and return critical data to further our understanding of the solar system and in turn, the universe. 7

##### TYPES OF OBJECTS IN THE KUIPER BELT

The Kuiper Belt. Courtesy of NASA/JHUAPL/SwRI.

Millions of objects are estimated to lie within the Kuiper Belt, and with new objects continuously being discovered, it is important to categorize and monitor the different kinds of objects in the Belt. Different types of Kuiper Belt Objects exist due to the different ways KBOs have gravitationally interacted with Neptune. Kuiper Belt Objects are divided into five major categories: Cold Classical KBO’s, Hot Classical KBO’s, Resonant KBO’s, Scattered KBO’s and a newly created, unnamed group composed of only two unique objects.

‘Cold’ Classical KBOs do not refer to temperature at all. Though these KBOs are located between 42 and 48 AU from the Sun, their temperature is not significantly different from other KBO since everything is cold by Earth standards at such a great distance. ‘Cold’ simply refers to objects that have consistent and unperturbed orbits. Cold Classical KBOs occupy a narrow region about 6 AU wide and about 7 AU thick, and they tend to be smaller and redder than other KBOs.

Hot Classical KBOs refer to objects with orbits that are inconsistent, irregular, or eccentric. Again, the term ‘hot’ in its name does not refer to temperature, as their eccentric orbits cause them to stray much farther from the Sun than many Cold Classical KBOs. Their sizes and colors vary, but Hot Classical KBOs tend to appear much larger and grayer than Cold Classical KBOs.

Resonant KBOs orbit in resonance with Neptune, either due to the influence of Neptune or simply by chance. This explains why these objects have not been cleared away by Neptune’s gravitational force. Resonant KBOs are divided into many subsections that define the interval of resonance a particular object has. Plutinos, for example, have a 2:3 resonance and make up the largest subsection of Resonant KBOs with 92 confirmed plutino objects. Twotinos have a resonance of 1:2 and are located near the very edge of the Kuiper Belt, occurring at 47.8 AU.

Scattered KBOs are objects that have wandered too close to Neptune in the past, resulting in an irregular orbit. Some Scattered KBOs have orbits that stretch hundreds of AU away from the Sun at the most distant point in their orbit and then return even closer to the Sun than Neptune. Therefore, their orbits may intersect Neptune’s orbit, but this does not mean that they are likely to collide.

The last category is so new that it only has two known members to date: Sedna and 2012 VP11. This category does not have a name yet. It is controversial whether these objects belong to the Kuiper Belt at all. Sedna orbits farther from the Sun than any other known KBO, never comes closer than 76 AU, and reaches out to approximately 1,000 AU at the most distant point of its 12,000-year orbit. Sedna is at about half the size of Pluto, and is suspected to be one of the largest members of a huge population of undiscovered objects. 7

#### PLANET VS DWARF PLANET

##### DEFINITION: DWARF PLANET

Dwarf Planet locations in the Kuiper Belt. Courtesy of NASA/JHUAPL/SwRI.

The IAU has officially designated five objects as dwarf planets: Pluto, Eris, Makemake, Haumea, and Ceres. This is only a small fraction of the hundreds of other probable candidates. But what is a dwarf planet?

According to the definition adopted by the IAU in 2006, a dwarf planet is a celestial body orbiting a star that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to overcome its compressive strength and achieve hydrostatic equilibrium. 11

In other words, a dwarf planet:

• Must be round and massive enough so that its own gravity is the dominant force affecting it. The size of a dwarf planet depends on its composition: more rigid materials require a larger diameter and mass to become spherical.
• Must achieve hydrostatic equilibrium. This is where the outward pressure equals the inward force of gravity of a massive object such as a planet or star.
• Must be is in direct orbit around the sun. It cannot revolve around another celestial body like a moon or satellite does.
• Cannot have obtained orbital dominance. This means that the object has not cleared the other matter in its orbit (or “neighbourhood”) like the planets have. There are still ice and dust of varying size in its orbit that it could encounter. Dwarf planets will be deflected in their orbits by objects in their way. 11
##### DEFINITION: PLANET

The three main qualifications (from IAU 2006) that need to be present to be considered a Plant are

1. Orbits the sun
2. Needs to be large enough to have its own gravity which makes it mostly round
3. Must have cleared its neighbourhood around its orbit (Orbital dominance)

As noted in the above section In the Beginning, Pluto failed to become massive enough to clear all the objects in orbit during its creation. Regular planets are the only objects in their orbital paths. Pluto meets the first two criteria but the Kuiper Belt was not dense enough and Pluto did not have enough energy to engulf or knock out the other objects in its orbit during its creation. In sum, Pluto does not clear the neighbourhood around its orbit—it is one of perhaps thousands of icy bodies that have avoided Neptune—and for this reason it is not classified as a planet. 12

#### THE SIGNIFICANCE OF THESE OBJECTS

When Pluto was “demoted” to dwarf planet, the topic was debatable because many did not understand the reason for change. Some were in this stage of denial while others discredited the ex-planet.

Now that the dust has had time to settle, the question remains: Should Pluto still be taught in schools? Yes. Probably more than ever, as Pluto is our gateway to the Outer Solar System, which has only recently been discovered.

Pluto is still an important part of our solar system, not just a fallen planet to forget about. In fact, other objects in the Kuiper Belt should be taught at the same time as it could increase our understanding of other astronomical objects and the origins of our solar system.

The recently discovered KBO, 2014 MU69, is an excellent example of how the Kuiper Belt Objects are furthering our knowledge of the solar system. MU69 is a Cold Classical KBO, which means that it is believed to be relatively unchanged from its origin. Comparing any findings from these unchanged bodies to the more evolved parts of the solar system, such as the planets, tells the story of how the planets and solar system have changed over time.

The New Horizons probe recently completed its flyby of Pluto and is now on course to fly past 2014 MU69 in 2019. This will be an exciting time for scientists to discover more about the origins of the solar system. It would be like having a snapshot in time to see what the objects in our solar system might have been like before the planets formed. Every new discovery from the Kuiper Belt is a contributing piece to the puzzle of understanding the evolution and nature of our solar system and beyond.

It is important to communicate these new discoveries to students not only so that they are up to date on the topic but also to help them understand that science is fluid. Many students believe that there is a right and final answer when it comes to science class, but research is continuous and ever changing. It can be a tough concept to grasp but you have to approach science and astronomy with an open mind to see the different ways science can be interpreted.

## Contents

The word theory means a contemplation or speculation, as opposed to action. It is a statement of how and why particular facts are related. Theory is especially often contrasted to "practice". While theories may address ideas and empirical phenomena which are not easily measurable, scientific theory is generally understood to refer to a proposed explanation of empirical phenomena, made in a way consistent with scientific method. Such theories are preferably described in such a way that any scientist in the field is in a position to understand and either provide empirical support ("verify") or empirically contradict ("falsify") it. A common distinction made in science is between theories and hypotheses. Hypotheses are individual empirically testable conjectures while theories are collections of hypotheses that are logically linked together into a coherent explanation of some aspect of reality and which have individually or jointly received some empirical support.

1. a coherentstatement or set of ideas that explainsobservedfacts or phenomena, or which sets out the laws and principles of something known or observed a hypothesis confirmed by observation, experiment etc.
2. the underlying principles or methods of a given technical skill, art etc., as opposed to its practice
3. a field of study attempting to exhaustivelydescribe a particular class of constructs
4. a hypothesis or conjecture
5. a set of axioms together with all statements derivable from them. Equivalently, a formal language plus a set of axioms (from which can then be derived theorems) is called a theory.

The nomology and any effort to acquire a system of laws or knowledge focusing on any natural body in the sky especially at night constitutes the theory of astronomy.

The overall theory of astronomy consists of three fundamental parts:

1. the derivation of logical laws,
2. the definitions of natural bodies (entities, sources, or objects), and
3. the definition of the sky (and associated realms).

Def. "the expanse of space that seems to be over the earth like a dome" [2] is called the sky, or sometimes the heavens.

This definition applies especially well to an individual on top of the Earth's solid crust looking around at what lies above and off to the horizon in all directions. Similarly, it applies to an individual's visual view while floating on a large body of water, where off on the horizon is still water.

The image at right shows the horizon marking the lower edge of the sky and the upper edge of the Atlantic Ocean, with a layer of cumulus clouds just above.

A more general definition of 'sky' allows for skies as seen on other worlds. At left is a 360° panarama of the horizon on Mars as perceived in the visual true-color range of the NASA Mars Exploration Rover 'Spirit' on November 23-8, 2005.

Def. an "expanse of space that seems to be [overhead] like a dome" [2] is called a sky.

Even in day light, the sky may seem absent of objects if a nearby source tends to overwhelm other luminous objects.

At top is a view of the horizon on the Moon's solid surface taken by an Apollo 16 astronaut. The image shows a black sky without stars because the sunlight coming from the left is overwhelming.

1.a: an "independent, separate, or self-contained existence", 1.b: "the existence of a thing as contrasted with its attributes", or 2. "something that has separate and distinct existence and objective or conceptual reality",

1.a: "something that is or is capable of being seen, touched, or otherwise sensed", 1.b: "something physical or mental of which a subject is cognitively aware", 2. "something that arouses an emotion in an observer", or 3. "a thing that forms an element of or constitutes the subject matter of an investigation or science"

1.a: "a mass of matter distinct from other masses" or 2.b: "something that embodies or gives concrete reality to a thing [specifically] : a sensible object in physical space"

1.a: "a separate and distinct individual quality, fact, idea, or [usually] entity", 1.b: "the concrete entity as distinguished from its appearances", 1.c: "a spatial entity", or 1.d: "an inanimate object distinguished from a living being"

1: "an observable fact or event", 2.a: "an object or aspect known through the senses rather than by though or intuition", 2.b: "an object of experience in space and time as distinguished from a thing-in-itself", or 2.c: "a fact or event of scientific interest susceptible of scientific description and explanation"

is called a phenomenon. [2]

Such words as "entity", "object", "thing", and perhaps "body", words "connoting universal properties, constitute the very highest genus or "summum genus"" of a classification of universals. [3] To propose a definition for say a plant whose flowers open at dawn on a warm day to be pollinated during the day time using the word "thing", "entity", "object", or "body" seems too general and is. But, astronomy deals with the universe, sometimes only the very local universe just above the Earth's atmosphere. These "universals" may be just the words to use.

On the basis of dictionary definitions, what is the difference between a 'body', an 'entity', an 'object', a 'thing', and a 'phenomenon'?

The categories for synonymy and most common usage place 'body' in "3. SUBSTANTIALITY" [4] , 'entity' in the same, 'object' in "651. INTENTION" [4] , 'thing' in "3. SUBSTANTIALITY" [4] , and 'phenomenon' in "918. WONDER" [4] . The more common uses of the words 'object' and 'phenomenon' are not exactly the same as may be used in a specialized endeavor like a science such as astronomy. A slightly less common use of 'phenomenon' is in category "150. EVENTUALITY" [4] . For the word 'object' a slightly less common or popular meaning is in category "543. MEANING" [4] . The closest category of meaning or synonymy for 'object' to category 1. is category "375. MATERIALITY" [4] .

Of each of these words, 'entity' uses the word 'existence', category "1. EXISTENCE" [4] in each definition. 'Entity' may be thought of as the most general of these terms because its meanings are the closest to category 1. The farthest from category 1. on the basis of conceptual meaning and synonymy is the word 'object' in category 375. A tentative order is 'entity' > 'phenomenon' > 'object' by generalness, or by preciseness (perhaps more description is needed beyond only existence) 'object' > 'phenomenon' > 'entity'.

'Thing' (category 3.) has the word 'entity' in three of four meanings and 'object' in the fourth. The second most popular meaning of 'thing' is in category 375.

'Body' (category 3.) has 'mass' and is closer to 'substantiality' in common usage than 'thing', and neither word has a synonym closer in meaning to 'existence'. The second most common meaning of 'body' is category "203. BREADTH, THICKNESS" [4] .

This suggests a hierarchy such as 'entity' > 'body' > 'thing' > 'phenomenon' > 'object' by generalness, where 'existence' is the most general word or, 'object' > 'phenomenon' > 'thing' > 'body' > 'entity' by preciseness. An 'astronomical object' may be expected to require a more detailed description in its definition to indicate meaning than an 'astronomical entity'. In astronomy, the concept of an 'astronomical body' may suggest a meaning closer to category 203. rather than a 'thing' or 'entity'.

The choice of general order is 'entity' > 'source' > 'object' > 'phenomena'. The term 'astronomical body' has much less popularity per Google scholar than 'object'. The body of astronomers in the International Astronomical Union is auspicious and here is considered an astronomical entity.

Def. the theory of the science of the biological, chemical, physical, and logical laws (or principles) with respect to any natural being, body, thing, entity, source, object, or phenomena in the sky especially at night is called theoretical astronomy.

1.a: an "independent, separate, or self-contained [astronomical] existence", 1.b: "the [astronomical] existence of a [person, place, or] thing as contrasted with its attributes", or 2. "some [astronomical] thing that has separate and distinct existence and objective or conceptual reality", [2]

is called an astronomical entity.

By generalness, 'being' > 'entity' > 'phenomenon' > 'object'. Further, 'being' > 'body' > 'something' or 'thing' > 'entity'. [4]

What are some astronomical entities?

"[V]oids [are] now considered as regular astronomical entities in their own rights, [and] are clustered." [5]

There are "a plethora of observations from heavenly bodies which did not agree with each other despite being from the same astronomical entities." [6] The observations themselves, media of recording, and the heavenly bodies are all astronomical entities. So are the observers and astronomers who make or made the records. Constellations are astronomical entities. 'Sky' is an astronomical entity. [7]

Included as astronomical entities are 'astronomical objects' and 'astronomical sources', even those with large error regions of whole degrees. Diffuse background radiations, whether gamma ray or radio, are astronomical entities.

"Astronomical named entities":

1. "Names of telescopes and other measurement instruments,"
2. "Names of celestial objects,"
3. "Types of objects," and
4. "Features that can be pointed to on a spectrum". [8]

"Gazetteers are useful for finding commonly referenced names of people, places or organisations" [9] associated with astronomy. These are astronomical entities that can be used for information processing.

Astronomical entities include some journals (such as The Astrophysical Journal, the Monthly Notices of the Royal Astronomical Society, and Astronomy & Astrphysics), articles in journals and magazines, books on astronomy that may be references or be cited for astronomy information or facts.

Named entity recognition (NER) for astronomy literature: [10] NER "involves assigning broad semantic categories to entity references in text." [10]

Types of entities for Natural Language Processing (NLP):

1. names - person, location, organization
2. temporal expressions - date, time
3. numeric expressions - money, percent
4. instrument name
5. source name
6. source type
7. spectral feature and
8. text and scientific databases. [10]

"Astronomy is a broad scientific domain combining theoretical, observational and computational research, which all differ in conventions and jargon." [10] "There is a major effort in astronomy to move towards integrated databases, software and telescopes." [10] ("The Virtual Observatory" [10] ).

Entity categories include 'galaxy', 'nebula', 'star', 'star cluster', 'supernova', 'planet', 'frequency', 'duration', 'luminosity', 'position', 'telescope', 'ion', 'survey', and 'date'. [10]

### Dominant groups Edit

The term "dominant group" is used in astronomy to identify other astronomical entities of importance. The genera differentia for possible definitions of "dominant group" fall into the following set of orderable pairs:

Genera differentia for "dominant group" [4]
Synonym for "dominant" Category Number Category Title Synonym for "group" Category Number Catgeory Title
“superior” 36 SUPERIORITY "arrangement" 60 ARRANGEMENT
“influential” 171 INFLUENCE "class" 61 CLASSIFICATION
“musical note” 462 HARMONICS "assembly" 74 ASSEMBLAGE
“most important” 670 IMPORTANCE "size" 194 SIZE
“governing” 739 GOVERNMENT "painting", "grouping" 572 ART
"master" 747 MASTER "association", "set" 786 ASSOCIATION
----- --- ------- "sect" 1018 RELIGIONS, CULTS, SECTS

'Orderable' means that any synonym from within the first category can be ordered with any synonym from the second category to form an alternate term for "dominant group" for example, "superior class", "influential sect", "master assembly", "most important group", and "dominant painting". "Dominant" falls into category 171. "Group" is in category 61. Further, any word which has its most or much more common usage within these categories may also form an alternate term, such as "ruling group", where "ruling" has its most common usage in category 739, or "dominant party", where "party" is in category 74.

"A particular subject of interest is the cluster ion series (NH3)nNH4 + , since it is the dominant group of ions over the whole investigated temperature range." [11] For astrochemisty, "[t]hese studies are expected to throw light on the sputtering from planetary and interstellar ices and the possible formation of new organic molecules in CO--NH3–H2O ice by megaelectronvolt ion bombardment." [11]

All alternate terms for dominant group [relative synonyms] used in astronomy are astronomical entities. Here are some examples from the literature:

1. "Once created, device class objects are registered with an instance of the master class." [12]
2. "For ATIC, a possible set of defined classes would be a master class event, and sub-classes header, silicon, scintillator, bgo and track." [13]
1. "The superior size and albedo of Venus completely turn the scale, with the result that Venus at her brightest is about 12 times brighter than Mercury at his brightest." [14]
2. "There is no reason to question but that they are simply ordinary meteors, which from their superior size and unusually slow speed have survived to reach the earth's surface." [15]
1. "Together with Leonard Searle, he wrote an influential set of papers which established that stellar disks are truncated at about four exponential scale-lengths, and that the vertical scale-height of disks is constant with radius." [16]
2. "Until now Themo has been best known for an influential set of questions on Aristotle's Meteorologica, which is closely related to similar sets by Nicole Oresme and, putatively, Simon Tunsted." [17]

Def. a natural source usually of radiation in the sky especially at night is called an astronomical source.

An astronomical source may have generated or be capable of generating electromagnetic radiation, a star, or a galaxy, for example. A source reflects, generates, transmits, or fluoresces that which may be detectable.

A celestial source is any astronomical source except the Earth.

An astronomical source usually has intensity often as a spatial, temporal, or spectral profile. Such a profile may be continuous, intermittent, transient, fluctuating, aperiodic, or unpredictable.

Some astronomical objects are not detectable directly as a source but instead may be absorbers of a portion of a signal from a source further away.

The image at right is a celestial map of the astronomical sources within the original detected error circle around the first apparent astronomical X-ray source discovered in the constellation Serpens Cauda (Serpens XR-1, or Serpens X-1). The other sources within this error circle are stars, other X-ray sources, a gamma-ray burst source, and a dark nebula.

In the theory of source astronomy comes at least an attempt to answer "Where did it come from?" Is there a causality? Is it modal? Or, is it of uncountable origin?

The science of astronomy consists of three fundamental parts:

The SIMBAD reference database "contains identifications, 'basic data', bibliography, and selected observational measurements for several million astronomical objects." [18] "The specificity of the SIMBAD database is to organize the information per astronomical object". [18] "Building a reference database for . all astronomical objects outside the Solar System – has been the first goal of the CDS". [18] "The only astronomical objects specifically excluded from SIMBAD are the Sun and Solar System bodies." [18]

Def. a natural object in the sky especially at night is called an astronomical object.

As indicated above about the astronomical objects in the SIMBAD database and in the learning reference astronomy, there are many objects between the observer on the ground atop some portion of the Earth's crust and astronomical objects other than the Sun and Solar System bodies. Further, for those observers looking toward the Earth from another location such as near the Moon in the photograph at above right, it seems that the Earth is a natural object. On the Earth 384,000 km away, the sunset terminator bisects Africa.

A closer view of Earth shows some of the astronomical objects near the Earth and apparently just above the surface, where an observer may be. Some of these objects such as clouds probably by convention are more likely to be studied by planetary observers, or weather observers, rather than astronomical observers.

With perspectives other than upwards from the Earth's crustal surface, the word "sky" may seem insufficient or inappropriate, although studying the Earth as part of planetary science may leave interesting astronomical objects near the Earth that are occasionally "in the sky". The idea being that the Earth cannot be in its own sky, or can it? Perhaps, it is more a matter of whether other observers agree that what an observer is observing is astronomy or planetary science, or both.

### Star by dictionary Edit

For the object, "star", a dictionary definition is

1.a: "any natural luminous body visible in the sky [especially] at night", 1.b: "a self-luminous gaseous celestial body of great mass whose shape is [usually] spheroidal and whose size may be as small as the earth or larger than the earth's orbit". [2]

This definition seems okay for a dictionary, but is it adequate for a science, and especially, astronomy?

Def. the study of the chemical composition of stars and outer space is called astrochemistry.

"An important goal for theoretical astrochemistry is to elucidate which organics are of true interstellar origin, and to identify possible interstellar precursors and reaction pathways for those molecules which are the result of aqueous alterations." [19]

### Atmospheres Edit

Def. a layer of gases that may surround a material body of sufficient mass,[3] and that is held in place by the gravity of the body is called an atmosphere.

Def. the gases surrounding the Earth or any astronomical body is called an atmosphere.

### Interplanetary medium Edit

Def. that part of outer space between the planets of a solar system and its star is called interplanetary space.

Def. the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move is called an interplanetary medium.

### Interstellar medium Edit

Def. the matter that exists in the space between the star systems in a galaxy is called an interstellar medium.

### Intergalactic medium Edit

Def. a rarefied plasma [20] that is organized in a cosmic filamentary structure [21] is called the intergalactic medium (IGM).

### Ions Edit

Def. an atom or group of atoms bearing an electrical charge such as the sodium and chlorine atoms in a salt solution is called an ion.

### Materials Edit

Def. matter which may be shaped or manipulated, particularly in making something is called a material.

Def. any instrument used in astronomy for observing distant objects is called a telescope.

Def. an object, usually made of glass, that focuses or defocuses the light or an electron beam that passes through it is called a lens.

### Meteorites Edit

Def. a metallic or stony object that is the remains of a meteor is called a meteorite.

### Shelters Edit

Def. a refuge, haven or other cover or protection from something is called a shelter.

Def. anything shaped like a common structural element of architecture that resembles the hollow upper half of a sphere, a cupola], often used as a cover is called a dome.

Astrognosy deals with the materials of celestial objects and their general exterior and interior constitution.

The theoretical constitution of the Earth is illustrated using the one-dimensional Preliminary Reference Earth Model [22] (PREM) at right. The density in kg-m -3 of radial layers is plotted against radius in km.

The geography applicable to astronomy may be designated 'astrogeography'. But, this term is often more restricted. "[T]he relationship between outer-space geography and geographic position (astrogeography), and the evolution of current and future military space strategy" [23] has been identified and evaluated. [23]

Def. the art of describing or delineating the stars a description or mapping of the heavens is called astrography.

Def. a place where stars, planets and other celestial bodies are observed is called an observatory.

From the Ebers Papyrus, a year has 360 days of twelve months of thirty days each. [24]

"A period of 360 days, comprising 12 months of 30 days each, was assigned by the Mesopotamians to the year in days and months at least by the third millennium BC." [25]

In ancient Iran (Persia), the year was 360 days with 12 months of 30 days each. [26] [27]

"All Veda [India] texts speak uniformly and exclusively of a year of 360 days [12 months of 30 days each]. Passages in which this length of the year is directly stated are found in all the Brahmanas." [28] This period dates to approximately the third millennium (

An ancient Chinese calendar had a 360 day year of twelve months of thirty days each. [30]

The Mayans had an old tradition that the year had twelve months of thirty days each, 360 days in a year. [31]

"The Peruvian year was divided into twelve Quilla, or moons, of thirty days." [32]

Apparently, with each of these locations around the globe and several others near to the Mediterranean Sea, the year had exactly 360 days of 12 months of 30 days each, then at some point near 2700 b2k the year became lengthened to today's year.

Def. (from 1945) "those parts of human interest in celestial phenomena which are amenable to mathematical treatment" are called astronomy. [33] This is Neugebauer's definition of astronomy. [34]

Def. "the search for astronomical strata whose contents could be classified" is called astronomy. [35] This is Herschel's definition of astronomy. [36]

"Mere "star ordering" is not "astronomy", so far as the modern usage of the term implies, regardless of the word's etymology". [25]

Def. (from 2000) "the accurate mapping of the heavens in order to make possible the accurate prediction of phenomena" is called astronomy. [25]

Def. "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties" is called astronomy. [37]

### Electrical Sun Edit

"[T]he solar corona is eminently variable, and therefore like our aurora borealis, which is known to be electric." [38] "This vast electric mass must have a great electric repulsion through vacant space, and it lends probability to my position that it drives away from the sun the tails of comets and our zodiacal light and aurora borealis." [38] "Electricity alone can repel electricity." [38] "[T]he direction of the comets' tails is but the interaction between the sun and the comets, the same as the action between a charged prime conductor and a charged pith ball of an electric machine." [38]

"[A] variety of geophysical and astrophysical phenomena can be explained by a net charge on the Sun of -1.5 x 10 28 e.s.u." [39] This figure was later reduced by a factor of five. [40]

"There appears to be considerable misunderstanding on the part of physicists of the nature and degree of the observational support of gravitational theory. For example, it appears to be commonly believed that the observations of planetary motion agree with computed orbits to the accuracy of the observations. On the other hand, it has long been known by the astronomers that there are sizable systematical discrepancies between computed and observed orbits". [41]

### Milky Way Edit

Democritus "lived at Abdère 300 years before the Christian era [2300 b2k]. In a short fragment quoted by Plutarch, he declares that the Milky Way is an agglomeration of small stars too far away to be perceived singly." [42]

### Coronal clouds Edit

"Beginning with the daguerreotype of the corona of 1851, the Reverend Lecturer had thrown on the screen pictures illustrating the form of the corona in different years. The drawings of those of 1867, 1878, and 1900 all showed long equatorial extensions with openings at the solar poles filled with beautiful rays." [43] "The intermediate years, as, for example, 1883, 1886, and 1896 showed the four groups of synclinals which mainly constitute the corona gradually descending towards the equator of the sun, with a corresponding opening of the polar regions." [43]

"Some of the theories of the solar corona were then illustrated and discussed." [43]

1. "The corona is not of the nature of an atmosphere round the sun, for the pressure at the sun's limb would be enormous, while the thinness of the chromospheric lines show that it is not." [43]
2. "comets, such as that of 1843, have approached the sun with enormous velocities within the region of the prominences without suffering disruption or retardation." [43]
3. "If not an atmosphere of particles of gas, still less is it an atmosphere of solid stones or meteorites." [43]
4. "Meteor streams do circle round the sun, but there is no reason why the positions of the orbits, or the intrinsic brightness of such streams should vary with the sun-spot period." [43]
5. "the appearance of the corona does not seem to be such as the projection of meteor streams upon the celestial vault would give." [43]
6. "Prof. Schaeberle has proposed a mechanical origin of the solar corona, due to the forces of ejection of particles from the solar limb, as evidenced by the prominences, and the force of gravity under the particular conditions of the solar rotation and the inclination of its axis to the earth's orbit." [43]
7. "The electrical theory of the corona does not negative the mechanical theory, but supplements it. In addition to the forces of gravity and ejection, it takes account of the repulsive force which the sun exerts on matter which has the same electrical sign as itself, and which has been ejected from it." [43]
8. "it would seem that the solar corona is of the nature of an electrical aurora round the sun." [43]
9. "the coronoidal discharges in poor vacua obtained by Prof. Pupin about an insulated metal ball are exceedingly like the rays and streamers of the solar corona." [43]

### Zodiacal lights Edit

"According to Gruson and Brugsch the Egyptians were acquainted with, and even worshipped, the zodiacal light from the very earliest times, as a phenomenon visible throughout the East before sunrise and after sunset. It was described as a glowing sheaf or luminous pyramid perpendicular to the horizon in summer, and inclined more or less during the winter. Indeed the Egyptians represented the zodiacal light under the form of a triangle which sometimes stood upright and at other times was inclined." [44]

The simplest description of the paths astronomical objects may take when passing each other includes a hyperbolic and parabolic pass. When capture occurs it usually produces an elliptical orbit.

Def. mathematics used in the study of astronomy, astrophysics and cosmology is called astromathematics.

The planet Mercury is especially susceptible to Jupiter's influence because of a small celestial coincidence: Mercury's perihelion, the point where it gets closest to the Sun, precesses at a rate of about 1.5 degrees every 1000 years, and Jupiter's perihelion precesses only a little slower. One day, the two may fall into sync, at which time Jupiter's constant gravitational tugs could accumulate and pull Mercury off course. This could eject it from the Solar System altogether [45] or send it on a collision course with Venus or Earth. [46]

### Orbital theory Edit

Orbits come in many shapes and motions. The simplest forms are a circle or an ellipse.

The foci of an ellipse are two special points F1 and F2 on the ellipse's major axis and are equidistant from the center point. The sum of the distances from any point P on the ellipse to those two foci is constant and equal to the major axis ( PF1 + PF2 = 2a ). Each of these two points is called a focus of the ellipse.

In the gravitational two-body problem, if the two bodies are bound to each other (i.e., the total energy is negative), their orbits are similar ellipses with the common barycenter being one of the foci of each ellipse. The other focus of either ellipse has no known physical significance. Interestingly, the orbit of either body in the reference frame of the other is also an ellipse, with the other body at one focus.

Ideally, the motion of two oppositely charged particles in empty space would also be an ellipse.

A real orbit (and its elements) changes over time due to gravitational perturbations by other objects and the effects of relativity. A Keplerian orbit is merely an idealized, mathematical approximation at a particular time.

### Eccentricities Edit

"Mercury's orbit eccentricity [e] varies between about 0.11 and 0.24 with the shortest time lapse between the extremes being about 4 x 10 5 yr". [47] "Smaller amplitude variations occur with about a 10 5 yr period." [47]

### Inclinations Edit

"The orbital inclination [i] [of Mercury] varies between 5° and 10° with a 10 6 yr period with smaller amplitude variations with a period of about 10 5 yr." [47]

### Obliquities Edit

In axial tilt, axial tilt (also called obliquity) is the angle between an object's rotational axis, and a line perpendicular to its orbital plane. The planet Venus has an axial tilt of 177.3° because it is rotating in retrograde direction, opposite to other planets like Earth. The planet Uranus is rotating on its side in such a way that its rotational axis, and hence its north pole, is pointed almost in the direction of its orbit around the Sun. Hence the axial tilt of Uranus is 97°. [48]

The obliquity of the Earth's axis has a period of about 41,000 years. [49]

### Precessions Edit

The equinoxes of Earth precess with a period of about 21,000 years. [49]

### Orbital poles Edit

An orbital pole is either end of an imaginary line running through the center of an orbit perpendicular to the orbital plane, projected onto the celestial sphere. It is similar in concept to a celestial pole but based on the planet's orbit instead of the planet's rotation.

### Resonances Edit

An orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. The physics principle behind orbital resonance is similar in concept to pushing a child on a swing, where the orbit and the swing both have a natural frequency, and the other body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies, i.e., their ability to alter or constrain each other's orbits. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be stable and self-correcting, so that the bodies remain in resonance. Examples are the 1:2:4 resonance of Jupiter's moons Ganymede, Europa and Io, and the 2:3 resonance between Pluto and Neptune. Unstable resonances with Saturn's inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance (between bodies with similar orbital radii) causes large Solar System bodies to eject most other bodies sharing their orbits this is part of the much more extensive process of clearing the neighbourhood, an effect that is used in the current definition of a planet.

### Orbital decays Edit

Orbital decay is the process of prolonged reduction in the altitude of a satellite's orbit. This can be due to drag produced by an atmosphere [frequent collisions between the satellite and surrounding air molecules]. The drag experienced by the object is larger in the case of increased solar activity, because it heats and expands the upper atmosphere.

“Contrary to the belief generally held by laboratory physicists, astronomy has contributed to the growth of our understanding of physics.” [1]

Def. the physical properties of celestial bodies and with the interaction between matter and radiation in celestial bodies and in the space between them is called astrophysics.

Theoretical astronomy seeks to understand what is behind cosmic events by taking the physics from the laboratory and testing it in models against the data obtained from observations. This is usually referred to as Draft:astrophysics. But, often the observations seem more than just what the physics can describe. Adding in extra tidbits may help to describe and help to produce better agreement. If these extra tidbits are physical in nature, they are part of theoretical astrophysics, if astronomical in nature, then theoretical astronomy.

### Astronomical units Edit

Def. "1 day (d)" is called the astronomical unit of time. [50]

Def. "365.25 days" is called a Julian year. [50]

Def. "36,525 days" is called a Julian century. [50]

Def. "the distance from the centre of the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.2568983 days" is called the Astronomical Unit (AU). [50]

Def. "149,597,870,700 meters" is called the Astronomical Unit. [51]

Def. "the mass of the Sun" is called the astronomical unit of mass. [50]

Def. the rate at which a star radiates energy in all directions is called luminosity.

Def. "the distance at which one Astronomical Unit subtends an angle of one arcsecond" is called the parsec (pc). [50]

Def. "the distance traveled by light in one Julian year in a vacuum" is called the light-year (ly). [50]

### Auroras Edit

Computer simulations are usually used to represent auroras. The image at right shows a terrella in a laboratory experiment to produce auroras.

### Fluctuating visible source Edit

Consider only that portion of the emission of the visible source at right that is a level maximum. If this is the first observation received, a reasonable theoretical explanation from physics is a constant black body visible source, like a light bulb. In a physics laboratory, a steady voltage/current power supply produces a steady intensity.

Now consider the full length observation indicated by the moving green circle. From a physics perspective, it appears the power supply is not steady. Using alternating current (AC) to power the light bulb at 60 cycles per second may trigger the detector to yield an oscillatory intensity curve if its response time is short enough to resolve the use of AC. This is a possible theoretical physics hence, theoretical astrophysics additional explanation of what may be happening.

A theoretical astronomy explanation is indicated in the colorful figure above as two visible sources, unresolved by the detector (seen only as a point source), but possibly responsible for the changes in the visible light received at the detector. Which do you think is more likely: a fluctuating power supply or an eclipsing binary?

Physics deals with forces, fields, energy, kinetics, and radiation. Astronomy has its own laws with respect to entities or bodies in motion. Application of a field to an astronomical phenomenon may clarify what is happening. That's the focus of astrophysics. Theory is needed to bring the physics in line with the magnitude of the situation and its complexity.

### Luminosity Edit

The luminosity of stars is measured in two forms: apparent (visible light only) and bolometric (total radiant energy). (A bolometer is an instrument that measures radiant energy over a wide band by absorption and measurement of heating.) When not qualified, "luminosity" means bolometric luminosity, which is measured either in the SI units, watts or in terms of solar luminosities, L ⊙ > , that is, how many times as much energy the object radiates as the Sun.

Luminosity is an intrinsic measurable property independent of distance, and is appraised as absolute magnitude, corresponding to the apparent luminosity in visible light of a star as seen at the interstellar distance of 10 parsecs, or bolometric magnitude corresponding to bolometric luminosity. In contrast, apparent brightness is related to the distance by an inverse square law. In addition to this brightness decrease from increased distance there is an extra linear decrease of brightness for interstellar "extinction" from intervening interstellar dust. Visible brightness is usually measured by apparent magnitude. Both absolute and apparent magnitudes are on an inverse logarithmic scale, where 5 magnitudes increase counterparts a 100th part decrease in nonlogarithmic luminosity.

By measuring the width of certain absorption lines in the stellar spectrum, it is often possible to assign a certain luminosity class to a star without knowing its distance. Thus a fair measure of its absolute magnitude can be determined without knowing its distance nor the interstellar extinction, and instead the distance and extinction can be determined without measuring it directly through the yearly parallax. Since the stellar parallax is usually too small to be measured for many far away stars, this is a common method of determining distances.

Given a visible luminosity (not total luminosity), one can calculate the apparent magnitude of a star from a given distance:

mstar is the apparent magnitude of the star (a pure number) msun is the apparent magnitude of the Sun (also a pure number) Lstar is the visible luminosity of the star L ⊙ > is the solar visible luminosity dstar is the distance to the star dsun is the distance to the Sun

### Nucleosynthesis Edit

"Our calculations show that production of [lithium] in low-energy flares [by nucleosynthesis], taking place in the surfaces of T Tauri-like stars, is energetically possible and can give the observed excesses over the interstellar abundance." [52]

"[T]here is evidence of lithium production in some stars due to some undefined mechanism. The observations show that the Li abundance on some red giants . and young stars exceeds the average abundance in the universe by 2 orders of magnitude". [53] It is "suggested that Li produced in the helium envelopes of red giants comes to the surface of the stars as the result of a strong convection." [53] For young stars, "the production of the light elements in nonthermal nuclear reactions seems the most appropriate mechanism that can be responsible for enrichment of stellar photospheres by Li." [53] "At least 0.3 metric tons of excited Li and Be nuclei were produced during the solar flare of 1991 November 15. One can estimate the equilibrium concentration of 7 Li nuclei in the solar atmosphere by assuming that they are produced only in solar flares and that a leakage of Li nuclei occurs with the solar wind." [53]

Although 7 Be is usually assumed to have been produced by the Big Bang nuclear fusion, excesses (100x) of the isotope on the leading edge [54] of the Long Duration Exposure Facility (LDEF) relative to the trailing edge suggest that "most of the sun's fusion must occur near the surface rather than the core." [55] The particular reaction

3 He + 4 He → 7 Be + γ (429 keV)

and the associated reaction chains

generate 14% and 0.1% of the α-particles, respectively, and 10.7% of the present-epoch luminosity of the Sun. [56] Usually, the 7 Be produced is assumed to be deep within the core of the Sun however, such 7 Be would not escape to reach the leading edge of the LDEF.

### Stars Edit

Def. "any object forming on a dynamical timescale, by gravitational instability", is called a star. [57]

### Starspots Edit

"[T]here have been three possible periods of marked solar anomaly during the last 1000 years: the Maunder Minimum, another minimum [the Spörer Minimum] in the early 16th century, and a period of anomalously high activity in the 12th and early 13th centuries." [58]

The basic causes of the solar variability and solar cycles are still under debate, with some researchers suggesting a link with the tidal forces due to the gas giants Jupiter and Saturn, [59] [60] or due to the solar inertial motion. [61] [62]

### Weak equivalence principle Edit

All test particles at the alike spacetime point in a given gravitational field will undergo the same acceleration, independent of their properties, including their rest mass. [63]

"The observation of a neutrino burst within 3 h of the associated optical burst from supernova 1987A in the Large Magellanic Cloud provides a new test of the weak equivalence principle, by demonstrating that neutrinos and photons follow the same trajectories in the gravitational field of the galaxy." [64]

The astrolabe was effectively an analog calculator capable of working out several different kinds of problems in spherical astronomy.

Some form of an "astrolabe" may have been in use by the third millennium BC. [25]

Hipparcos is the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects.

These measurements allow "the accurate determination of proper motions and parallaxes of stars, their distance and tangential velocity.

Planetary science (rarely planetology) is the scientific study of planets (including Earth), moons, and planetary systems, in particular those of the solar system and the processes that form them. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation, interrelations and history. It is a strongly interdisciplinary field, originally growing from astronomy and earth science, [65] but which now incorporates many disciplines, including planetary astronomy, planetary geology (together with geochemistry and geophysics), atmospheric science, oceanography, hydrology, theoretical planetary science, glaciology, and the study of extrasolar planets. [65] Allied disciplines include space physics, when concerned with the effects of the Sun on the bodies of the Solar System, and astrobiology.

### Planets Edit

Def. "a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

(c) has cleared the neighbourhood around its orbit" is called a planet. [66]

The proposed more general definition for a planet in orbit around another star substitutes "a star" for "the Sun" in part (a), keeps part (b), does not contain part (c), and adds "is neither a star nor a satellite of a planet." [67]

Def. "a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid forces so that it assumes a hydrostatic equilibrium (nearly round) shape,

(c) has not cleared the neighbourhood around its orbit, and

(d) is not a satellite" is called a dwarf planet. [66]

Def. "[a]ll other objects [not a planet or dwarf planet], except satellites, orbiting the Sun" are called collectively Small Solar-System Bodies. [66]

Def. a wanderer that is a moving light in the sky is called a planet. [66] This is the original description meant by the word "planet". [66]

From a theoretical planetary physics perspective: "The shape of objects with mass above 5 x 10 20 kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation." [67]

Def. a celestial body "formed by accumulation of a rocky core, on a much longer timescale, ≳ 10 7 yr, with subsequent acquisition of a gaseous envelope if the circumstances allow this, and with an initially fractionated elemental composition" is called a planet. [57]

Meta-astronomy, or metaastronomy, is the collection of approaches to theoretical astronomy that may be considered when seeking to understand an astronomical phenomenon.

In the model shown at right the Sun and regions around it are labeled. "The core of the Sun is considered to extend from the center to about 0.2 to 0.25 solar radius. [68] It is the hottest part of the Sun and of the Solar System. It has a density of up to 150 g/cm³ (150 times the density of liquid water) and a temperature of close to 15,000,000 kelvin [15 MK] The core is made of hot, dense gas in the plasmic state. The core, inside 0.24 solar radius, generates 99% of the fusion power of the Sun. It is in the core region that solar neutrinos may be produced.

The radiation zone or radiative zone is a layer of a star's interior where energy is primarily transported toward the exterior by means of radiative diffusion, rather than by convection. [69] Energy travels through the radiation zone in the form of electromagnetic radiation as photons. Within the Sun, the radiation zone is located in the intermediate zone between the solar core at .2 of the Sun's radius and the outer convection zone at .71 of the Sun's radius. [69]

The convection zone of a star is the range of radii in which energy is transported primarily by convection. Stellar convection consists of mass movement of plasma within the star which usually forms a circular convection current with the heated plasma ascending and the cooled plasma descending. This is the granular zone in the outer layer of a star.

The standard solar model (SSM) is a mathematical treatment of the Sun as a spherical ball of gas (in varying states of ionisation, with the hydrogen in the deep interior being a completely ionised plasma). This model, technically the spherically symmetric quasi-static model of a star, has stellar structure described by several differential equations derived from basic physical principles. The model is constrained by boundary conditions, namely the luminosity, radius, age and composition of the Sun, which are well determined.

As the Sun consists of a plasma and is not solid, it rotates faster at its equator than at its poles. This behavior is known as differential rotation, and is caused by convection in the Sun and the movement of mass, due to steep temperature gradients from the core outwards. This mass carries a portion of the Sun’s counter-clockwise angular momentum, as viewed from the ecliptic north pole, thus redistributing the angular velocity.

"Solar rotation is able to vary with latitude because the Sun is composed of a gaseous plasma. The rate of rotation is observed to be fastest at the equator (latitude φ=0 deg), and to decrease as latitude increases. The differential rotation rate is usually described by the equation:

where ω is the angular velocity in degrees per day, φ is the solar latitude and A, B, and C are constants. The values of A, B, and C differ depending on the techniques used to make the measurement, as well as the time period studied. [70] A current set of accepted average values [71] is:

A= 14.713 deg/day (± 0.0491) B= –2.396 deg/day (± 0.188) C= –1.787 deg/day (± 0.253)

"[B]y assuming a harmonic variation having a period of 11.13 years . the phases of such a variation [in polar diameter minus equatorial diameter of the Sun] coincide to within one-fifth of a year with the phases of the sun-spot fluctuations that, at times corresponding to minimum of sun-spottedness, the polar diameter is relatively larger that, at times of maximum sun-spottedness, the equatorial diameter is relatively larger. The amplitude of the variation is extremely small, but its reality would seem to be established. [This] at least renders the existence of such periodic fluctuations in the shape of the sun more probable than their non-existence." [72]

"Solar oblateness, the difference between the equatorial and polar diameters, reflects certain fundamental properties of the Sun. . the oblateness reflects properties of the Sun's interior, . [There is] a time varying, excess equatorial brightness [producing] a difference between the equatorial and polar limb darkening functions . at times when the excess brightness is reduced, the intrinsic visual oblateness can be obtained from the observations without detailed knowledge of the excess brightness. A period of reduced excess brightness occurred in 1973 September." [73] The period of reduced excess equatorial brightness occurred between solar cycle maximum around 1970 and minimum around 1975. Considering excess equatorial brightness and seeking to perform measurements at opportunities of reduced excess equatorial brightness has the effect of reducing solar oblateness from some 86.6 ± 6.6 milli-arcsec to 18.4 ± 12.5 milli-arcsec. [73]

The Babcock Model describes a mechanism which can explain magnetic and sunspot patterns observed on the Sun.

1. The start of the 22-year cycle begins with a well-established dipole field component aligned along the solar rotational axis. The field lines tend to be held by the highly conductive solar plasma of the solar surface.
2. The solar surface plasma rotation rate is different at different latitudes, and the rotation rate is 20 percent faster at the equator than at the poles (one rotation every 27 days). Consequently, the magnetic field lines are wrapped by 20 percent every 27 days.
3. After many rotations, the field lines become highly twisted and bundled, increasing their intensity, and the resulting buoyancy lifts the bundle to the solar surface, forming a bipolar field that appears as two spots, being kinks in the field lines.
4. The sunspots result from the strong local magnetic fields in the solar surface that exclude the light-emitting solar plasma and appear as darkened spots on the solar surface.
5. The leading spot of the bipolar field has the same polarity as the solar hemisphere, and the trailing spot is of opposite polarity. The leading spot of the bipolar field tends to migrate towards the equator, while the trailing spot of opposite polarity migrates towards the solar pole of the respective hemisphere with a resultant reduction of the solar dipole moment. This process of sunspot formation and migration continues until the solar dipole field reverses (after about 11 years).
6. The solar dipole field, through similar processes, reverses again at the end of the 22-year cycle.
7. The magnetic field of the spot at the equator sometimes weakens, allowing an influx of coronal plasma that increases the internal pressure and forms a magnetic bubble which may burst and produce an ejection of coronal mass, leaving a coronal hole with open field lines. Such a coronal mass ejections are a source of the high-speed solar wind.
8. The fluctuations in the bundled fields convert magnetic field energy into plasma heating, producing emission of electromagnetic radiation as intense ultraviolet (UV) and X-rays.

Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe. All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main sequence star.

Nuclear fusion powers a star for most of its life. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main sequence star. Later, as the preponderance of atoms at the core becomes helium, stars like the Sun begin to fuse hydrogen along a spherical shell surrounding the core. This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red giant phase. Stars with at least half the mass of the Sun can also begin to generate energy through the fusion of helium at their core, while more massive stars can fuse heavier elements along a series of concentric shells. Once a star like the Sun has exhausted its nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula. Stars with around ten or more times the mass of the Sun can explode in a supernova as their inert iron cores collapse into an extremely dense neutron star or black hole.

The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. It is one of the most active research areas in astrophysics.

Galaxy formation is hypothesized to occur, from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model for this that is in general agreement with observed phenomena is the Λ Cold Dark Matter cosmology that is to say that clustering and merging is how galaxies gain in mass, and can also determine their shape and structure.

## Topics similar to or like Commensurability (astronomy)

Time a given astronomical object takes to complete one orbit around another object, and applies in astronomy usually to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars. Often referred to as the sidereal period, determined by a 360° revolution of one celestial body around another, e.g. the Earth orbiting the Sun. Wikipedia

Object in the direction opposite the rotation of its primary, that is, the central object . Object's rotational axis. Wikipedia

Set of gravitationally bound objects in orbit around a planetary mass object or minor planet, or its barycenter. Set of natural satellites , although such systems may also consist of bodies such as circumplanetary disks, ring systems, moonlets, minor-planet moons and artificial satellites any of which may themselves have satellite systems of their own. Wikipedia

Measure of the mass of a planet-like object. Solar mass, the mass of the Sun. Wikipedia

Orbiting astronomical body always has the same face toward the object it is orbiting. Known as synchronous rotation: the tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. Wikipedia

List of types of orbits: Less commonly used Wikipedia

Orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit refers to a regularly repeating trajectory, although it may also refer to a non-repeating trajectory. Wikipedia

"Clearing the neighbourhood" around a celestial body's orbit describes the body becoming gravitationally dominant that there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence. One of three necessary criteria for a celestial body to be considered a planet in the Solar System, according to the definition adopted in 2006 by the International Astronomical Union . Wikipedia

Either point at the ends of an imaginary line segment that runs through the center of an orbit and is perpendicular to the orbital plane. Projected onto the celestial sphere, orbital poles are similar in concept to celestial poles, but are based on the body's orbit instead of its equator. Wikipedia

Recent variation of the Nice model that begins with five giant planets, the four plus an additional ice giant in a chain of mean-motion resonances. Broken, the five giant planets undergo a period of planetesimal-driven migration, followed by an instability with gravitational encounters between planets similar to that in the original Nice model. Wikipedia

Astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and – according to the International Astronomical Union but not all planetary scientists – has cleared its neighbouring region of planetesimals. Ancient, with ties to history, astrology, science, mythology, and religion. Wikipedia

Astronomical body or object is the speed at which it orbits around either the barycenter or, if one object is much more more massive than the other bodies in the system, its speed relative to the center of mass of the most massive body.. Entire orbit, or its instantaneous speed at a particular point in its orbit. Wikipedia

Scientific study of planets , moons, and planetary systems (in particular those of the Solar System) and the processes that form them. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation, interrelations and history. Wikipedia

Torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars, that is occupied by a great many solid, irregularly shaped bodies, of many sizes but much smaller than planets, called asteroids or minor planets. Also called the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such as near-Earth asteroids and trojan asteroids. Wikipedia

Angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. It differs from orbital inclination. Wikipedia

## Criticism

#### Substance

There continues to be criticism regarding the wording of the final draft of the definition. Notably, the lead scientist on NASA's robotic mission to Pluto, Alan Stern, contends that, like Pluto, Earth, Mars, Jupiter and Neptune have not fully cleared their orbital zones either. Earth orbits with 10,000 near-Earth asteroids. Jupiter, meanwhile, is accompanied by 100,000 Trojan asteroids on its orbital path. "If Neptune had cleared its zone, Pluto wouldn't be there," he added. However, his own earlier work on neighborhood clearing supported the distinction between the largest eight planets and the rest of the solar system. There is a substantial difference in the extent to which the neighborhood has been cleared between Pluto and the eight planets. Also, Pluto's position is due to the gravitational effects of Neptune as they are in orbital resonance.

The debates have clarified that "clearing its orbit" refers to the process that happened during the formation of the planets. It does not talk about the presence of bodies that later strayed into the orbit after the accretions took place.

The definition may be difficult to apply outside our solar system. Techniques for identifying extrasolar objects generally cannot determine if an object has "cleared its orbit," except indirectly via Stern and Levison's Λ parameter, and provide limited information about when the objects were formed. The wording of the new definition is heliocentric in its use of the word Sun instead of star or stars, and is thus not applicable to the numerous objects that have been identified in orbit around other stars.

#### Process

The final vote has come under criticism because of the relatively small percentage of the 9000-strong membership who participated. Besides the fact that most members do not attend the General Assemblies, this lack was also due to the timing of the vote: the final vote was taken on the last day of the 10-day event, after many participants had left or were preparing to leave. Of over 2,700 astronomers attending the conference, about 800 were present on the day for the significant resolutions on a vote on a subsidiary resolution, the first that required a count, only 424 votes were cast.There is also the issue of the many astronomers who were unable or who chose not to make the trip to Prague and, thus, cast a vote. Astronomer Marla Geha has clarified that not all members of the Union were needed to vote on the classification issue: only those whose work is directly related to planetary studies.

#### Other

Some astrologers have chosen not to follow the new definiton.

#### Petition for a "better definition"

Within five days of the new IAU Planet Definition over 300 scientists signed a petition that opposes the new definition. The full text of the petition says: “We, as planetary scientists and astronomers, do not agree with the IAU's definition of a planet, nor will we use it. A better definition is needed.” The petition does not list any specific complaint with the definition, so it is unknown whether all the signatories opposed it for the same reasons.