More recent data and simulations of “Milkomeda”, the collision of the Milky Way and Andromeda galaxies?

More recent data and simulations of “Milkomeda”, the collision of the Milky Way and Andromeda galaxies?

The headline Hubble Telescope Spots Two Galaxies in a Doomed (but Dazzling) Dance; The galaxies will ultimately crash into each other was probably overstated as seems to be policy in some popular press sites. The galaxies are not "doomed". However the paragraph below is interesting:

Our Milky Way, for example, is on an inevitable collision course with the neighboring behemoth galaxy - Andromeda. Individual star systems like ours will likely be largely undisrupted, but distant observers will see the two galaxies gradually become one in some four billion years. ESA nicknames this new merged galaxy "Milkomeda."

Question: How much more is predicted now about the "upcoming" collision and possible merger of the Milky Way and Andromeda galaxies than has already been described in this answer's citation of a 2012 Astrobites review The Fate of the Milky Way? What measurements (if any) have contributed to this additional level of prediction?

The current data could be improved by future additional HST observations. Also, soon it will be possible to compare them with independent water maser measurements for individual sources in M31 (see this Letter), which might allow measurement of other cool effects, such as the M31 proper motion rotation, and the increase in Andromeda's apparent size due to its motion towards us.

Since the second data release (DR2) of the European Space Agency's Gaia mission there has been a revolution in astrometry, including measuring the motion of the Andromeda Galaxy.

On February this year van der Marel et al (also in ArXiv) published interesting results on that matter by using Gaia's DR2 measurements. The results reveal that the collision is going to happen 600 million years later than the previous estimate (in 4.5 Gyr instead of 3.9 Gyr). Also Andromeda appears to have more tangential motion than previously thought and thus "the galaxy is likely to deliver more of a glancing blow to the Milky Way than a head-on collision".

Also interesting, M33 (Triangulum galaxy) is going to make its first infall towards Andromeda and might interact gravitationally quite a lot with it. The absence of stellar streams between Triangulum and Andromeda show that this is the first time they are going to meet each other.

Thanks to Gaia the measurements are now so precise that for the first time we are able to notice even the minute rotation of both Triangulum and Andromeda galaxies astrometrically (and not only by using doppler shifts).

We use an N-body/hydrodynamic simulation to forecast the future encounter between the Milky Way and the Andromeda galaxies, given present observational constraints on their relative distance, relative velocity, and masses. Allowing for a comparable amount of diffuse mass to fill the volume of the Local Group, we find that the two galaxies are likely to collide in a few billion years – within the Sun's lifetime. During the interaction, there is a chance that the Sun will be pulled away from its present orbital radius and reside in an extended tidal tail. The likelihood for this outcome increases as the merger progresses, and there is a remote possibility that our Sun will be more tightly bound to Andromeda than to the Milky Way before the final merger. Eventually, after the merger has completed, the Sun is most likely to be scattered to the outer halo and reside at much larger radii (>30 kpc). The density profiles of the stars, gas and dark matter in the merger product resemble those of elliptical galaxies. Our Local Group model therefore provides a prototype progenitor of late-forming elliptical galaxies.

It is well known that the Milky Way (MW) and Andromeda (M31) are the two largest members of the Local Group of galaxies. Together with their ∼40 smaller companions, the MW and Andromeda comprise our Galactic neighbourhood and, as such, represent the nearest laboratory, and therefore the most powerful tool, to study the formation and evolution of Galactic structure.

Like most extragalactic groups, the Local Group is very likely to be decoupled from the cosmological expansion and is now a gravitationally bound collection of galaxies. This notion is supported by the observed relative motion between its two largest galaxies namely, the MW and Andromeda are moving towards each other at ∼120 km s −1 ( Binney & Tremaine 1987). Unfortunately, this motion alone does not indicate whether the Local Group is bound or not. The unknown magnitude of Andromeda's transverse velocity adds uncertainty into the present-day orbital parameters and therefore the past and future evolution of the Local Group.

Barring the uncertain transverse velocity of Andromeda, a considerable amount of information can be inferred about the Local Group, provided a plausible set of assumptions. Nearly 50 yr ago, Kahn & Woltjer (1959) pioneered the ‘timing argument’, in which the MW and Andromeda are assumed to form within close proximity to each other, during the dense early stages of the Universe, before they were pulled apart by the general cosmological expansion. They have subsequently reversed their path and are approaching one another owing to their mutual gravitational attraction. According to the timing argument, the MW and Andromeda have now traced out nearly a full period of their orbital motion which is governed by Kepler's laws. By assuming that the system has no angular momentum and, given the present separation, velocity of approach, and the age of the Universe, the timing argument yields estimates for the mass of the Local Group (>3 × 10 12 M), the semimajor axis of the orbit (<580 kpc), and the time of the next close passage (>4 Gyr) (see section 10.2 of Binney & Tremaine 1987).

While the seminal results of Kahn & Woltjer (1959) were an early indication of the large mass-to-light ratio in the Local Group and therefore the presence of dark matter, they also began a nearly five decade long quest to understand the past, present, and future of our Local Group. In particular, a number of studies have extended the original timing argument by allowing for various angular momenta, by including more realistic or time-dependent mass distributions, by adding the effects of mass at scales beyond that of the Local Group, or testing its validity using numerical simulations (see e.g. Peebles et al. 1989 Fich & Tremaine 1991 Valtonen et al. 1993 Peebles 1994 Peebles et al. 2001 Loeb et al. 2005 Sawa & Fujimoto 2005 Li & White 2008 van der Marel & Guhathakurta 2008).

One of the most-intriguing developments stemming from the various studies of the Local Group is an estimate of the transverse velocity of Andromeda. By employing the action principle to the motions of galaxies within and near (<20 Mpc) the Local Group, Peebles et al. (2001) concluded that the transverse velocity of Andromeda is less than 200 km s −1 . Using the well-measured transverse velocity of M33 ( Brunthaler et al. 2005) and numerical simulations that tracked the potential tidal disruption during M33's past encounters with Andromeda, Loeb et al. (2005) found an even smaller estimate, ∼100 km s −1 , for the transverse velocity. While future astrometric observations using SIM 1 and GAIA 2 will be able to accurately measure the proper motion of Andromeda, the low values favoured by these papers suggest that the Local Group is indeed a gravitationally bound system.

Provided that the Local Group is gravitationally bound, and that the MW and Andromeda are heading towards each other, one must admit the possibility that they will eventually interact and merge. This outcome appears inevitable, given the massive haloes of dark matter that likely surround the MW and Andromeda. Numerical experiments have robustly concluded that dark matter haloes can exert significant dynamical friction, and are sponges that soak up energy and angular momentum leading to a rapid merger ( Barnes 1988).

Even though the eventual merger between the MW and Andromeda is common lore in Astronomy, the merger process has not been addressed by a comprehensive numerical study. The one exception is a paper by Dubinski, Mihos & Hernquist (1996) that presented a viable model for the Local Group and numerically simulated the eventual merger between the MW and Andromeda. However, Dubinski et al. (1996) utilized this Local Group model and its numerical evolution to study the production of tidal tails during such an encounter and the possibility to use the structure of this tidal material to probe the dark matter potential. While the study by Dubinski et al. (1996) provided the first enticing picture of the future encounter between the MW and Andromeda (for a more recent and higher resolution version of this simulation, see Dubinski 2006), it was designed neither to detail the merger dynamics including intergalactic material, nor to outline the possible outcomes for the dynamics of our Sun, nor to quantify properties of the merger remnant. In addition, the last decade has produced a number of improved models for the structure of the MW and Andromeda as well as the properties of the intragroup medium.

In this paper, we quantitatively predict when the interaction and merger of the MW and Andromeda will likely occur and forecast the probable dynamics of the Sun during this event. We achieve this goal by constructing a model for the Local Group in Section 2 that satisfies all observational constraints. We then evolve this model using a self-consistent N-body/hydrodynamic simulation, as described in Section 3. The generic properties of the merger, including the merger time-scale, the possible evolution of our Solar system, and properties of the merger remnant, are outlined in Section 4. Finally, we conclude in Section 5.


The "Milky Way" can be seen as a hazy band of white light some 30 degrees wide arcing across the sky. [36] Although all the individual naked-eye stars in the entire sky are part of the Milky Way, [37] the light in this band originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where light from distant stars is blocked by interstellar dust. The area of the sky obscured by the Milky Way is called the Zone of Avoidance.

The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light such as light pollution or stray light from the Moon. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be seen. [38] It should be visible when the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1. [39] This makes the Milky Way difficult to see from any brightly lit urban or suburban location, but very prominent when viewed from a rural area when the Moon is below the horizon. [nb 2]

As viewed from Earth, the visible region of the Milky Way's Galactic plane occupies an area of the sky that includes 30 constellations. [40] The center of the Galaxy lies in the direction of the constellation Sagittarius it is here that the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass around to the Galactic anticenter in Auriga. The band then continues the rest of the way around the sky, back to Sagittarius. The band divides the night sky into two roughly equal hemispheres.

The Galactic plane is inclined by about 60 degrees to the ecliptic (the plane of Earth's orbit). Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth’s equatorial plane and the plane of the ecliptic, relative to the Galactic plane. The north Galactic pole is situated at right ascension 12 h 49 m , declination +27.4° (B1950) near β Comae Berenices, and the south Galactic pole is near α Sculptoris. Because of this high inclination, depending on the time of night and year, the arc of Milky Way may appear relatively low or relatively high in the sky. For observers from approximately 65 degrees north to 65 degrees south on Earth's surface, the Milky Way passes directly overhead twice a day.

SCIENCE CHANNEL: Wonders of the Universe: Andromeda

Most of the galaxies that we observe appear to be moving away from us. This is determined by the redshift of the galaxy's light. Though this isn't the most physically correct explanation, you can kind of imagine that the wavelength of the light is stretched out as an object moves away.

The Andromeda Galaxy, however, appears to be moving towards us at a speed of about 70 miles per second. Astronomers are not sure, however, whether it will be a head-on collision or a glancing blow.

Research Box Title

NASA's Hubble Space Telescope has uncovered over 1,000 bright, young star clusters bursting to life in a brief, intense, brilliant "fireworks show" at the heart of a pair of colliding galaxies.

"The sheer number of these young star clusters is amazing," says Brad Whitmore of the Space Telescope Science Institute (STScI), Baltimore, Maryland. "The discovery will help us put together a chronological sequence of how colliding galaxies evolve. This will help us address one of the fundamental questions in astronomy: why some galaxies are spirals while others are elliptical in shape."

"These spectacular images are helping us understand how globular star clusters formed from giant hydrogen clouds in space," adds Francois Schweizer of the Carnegie Institution of Washington, Washington, D.C. "This galaxy is an excellent laboratory for studying the formation of stars and star clusters since it is the nearest and youngest example of a pair of colliding galaxies."

By probing the Antennae galaxies (called the Antennae because a pair of long tails of luminous matter formed by the encounter resembles an insect's antennae) and some of the other nearby galactic-scale collisions, Hubble is coming up with a variety of surprises:
Globular star clusters are not necessarily relics of the earliest generations of stars formed in a galaxy, as once commonly thought, but may also provide fossil records of more recent collisions.

  • The "seeds" for star clusters appear to be huge clouds (tens to hundreds of light-years across) of cold hydrogen gas, called giant molecular clouds, which are squeezed by surrounding hot gas heated during the collision and then collapse under their own gravity. Like a string of firecrackers being ignited by the collision, these reservoirs of gas light up in a great burst of star formation.
  • The ages of the resulting clusters provide a clock for estimating the age of a collision. This offers an unprecedented opportunity for understanding, step-by-step, the complex sequence of events which take place during a collision, and possibly even the evolution of spiral galaxies into elliptical galaxies.

Earlier Hubble pictures show that nearly a third of very distant galaxies, which existed early in the history of the universe, appear to be interacting galaxies, like the Antennae. In particular, the Hubble Deep Field (a "long-exposure" image from Hubble looking at galaxies far back into time), uncovered a plethora of odd-shaped, disrupted-looking galaxies. They offer direct visual evidence that galaxy collisions were more the rule than the exception in the early days of the universe.

These distant galaxy collisions are too faint and too small to study in much detail. Astronomers say we are fortunate to have such a nearby example as the Antennae to study, since collisions between galaxies are relatively rare today. "The degree of detail in these images is astounding, and represents both a dream come true and a nightmare when it comes to the analysis of such a large amount of data," Whitmore says.

In addition to providing a window into how stars and galaxies formed in the dim past, the Hubble views might also offer a glimpse of the future fate of Earth's home galaxy, the Milky Way, when it either sideswipes or plows head-on into the neighboring Andromeda galaxy billions of years from now.

The Hubble observations of the Antennae galaxies, as well as several other nearby colliding galaxies, were conducted by Whitmore (STScI) and co-investigators Francois Schweizer and Bryan Miller (Department of Terrestrial Magnetism, Carnegie Institution of Washington), and Michael Fall and Claus Leitherer (STScI) over the past several years.

Hubble's resolution and sensitivity allowed the team to uncover over 1,000 exceptionally bright young star clusters, sometimes called super star clusters, within the Antennae - the prototypical galaxy smashup. Ground-based telescopes were only able to see the brightest of these clusters, and even in these cases were not able to show that the clusters were very compact with the sizes of normal globular clusters.

Observing other galaxy collisions, the Hubble team discovered the presence of young star clusters which were very bright and blue in the case of ongoing collisions, but had faded to become fainter and redder for the older merger remnants. This allowed them to place the snapshots of galaxy collisions into a chronological sequence.


The Milky Way and the Andromeda galaxy are approaching each other with a speed of 300,000 miles per hour.

It's not certain yet whether we're in store for a head-on collision or a simple sideswiping by the massive galaxy, which is a near twin to the Milky Way. Astronomers will first need to use powerful new telescopes to precisely measure Andromeda's tangential motion across the sky. (Just as a baseball outfielder estimates whether a ball is heading directly toward him or is going to miss him by determining whether the ball is moving sideways.)

A direct collision would lead to a grand merger between the two behemoths, and the Milky Way would no longer be the pinwheel spiral we are familiar with, but would evolve into a huge elliptical galaxy.

It would happen no sooner than five billion years in the future. By then the Sun may have burned out, and the Earth reduced to a frigid, lifeless cinder. It's impossible to predict if there would be any vestige of humanity colonized among the stars, not to mention extraterrestrial civilizations around to witness this great collision.

The collision will take several billion years to fully run its course, so it will be hard for any one civilization, like ours, to fully understand the vast scale – both in time and space – of the collision.

However, by studying pairs of other colliding galaxies and using computer simulations, astronomers can assemble a series of snapshots of the collision process and get a preview of what might eventually happen to our galaxy.

Here is a scenario of how the Milky Way might change if it were to have a head-on collision with Andromeda:

    The Andromeda galaxy appears simply as a spindle-shaped smudge of light in the northern autumn sky. Because it is 2.2 million light-years away – or roughly 20 times the diameter of our Milky Way galaxy – it only appears four times the width of the full moon. As the two galaxies approach each other, Andromeda will grow ever larger in the sky, resembling an eerie glowing sword of light. When th e Andromeda galaxy and our Milky Way galaxy are close enough, huge clumps of cold, giant molecular clouds, each measuring tens to hundreds of light-years across, will be compressed. Like plugging in a string of Christmas light bulbs, these dark knots will light up as millions of stars burst into life. Most of these stars will be in brilliant blue clusters, many of them 100 times brighter than the original globular star clusters already present in the two galaxies. The disk of dust and stars that for billions of years marked the lanes of our galaxy and the Andromeda galaxy, will also begin to come apart under the gravitational pull of the two galaxies. As Andromeda swings past our galaxy, the sky will grow increasingly jumbled with tattered lanes of dust, gas, and brilliant young stars and star clusters.


Space between stars in a galaxy is nearly empty, except for a scattering of hydrogen atoms. The atoms are so far apart that, if an atom were an average- size person, each person would be separated by about 465 million miles, which is the distance between our Sun and Jupiter. These atoms are moving very fast because they are extremely hot, baked by ultraviolet radiation from stars. This makes it difficult for atoms to bond to form molecules. Those that do form don't last for long. If radiation doesn't break these molecules apart, a chance encounter with another atom will.

Some parts of space, however, are not wide open frontiers containing a few atoms. These cosmic spaces comprise dense clouds of dust and gas left over from galaxy formation. Since these clouds are cooler than most places, they are perfect breeding grounds for star birth. When the density is 1,000 times greater than what is found in normal interstellar space, many atoms combine into molecules, and the gas cloud becomes a molecular cloud. Like clouds in our sky, these molecular clouds are puffy and lumpy. Molecular clouds in our Milky Way Galaxy have diameters ranging from less than 1 light-year to about 300 light-years and contain enough gas to form from about 10 to 10 million stars like our Sun. Molecular clouds that exceed the mass of 100,000 suns are called Giant Molecular Clouds.

A typical full-grown spiral galaxy contains about 1,000 to 2,000 Giant Molecular Clouds and many more smaller ones. Such clouds were first discovered in our Milky Way Galaxy with radio telescopes about 25 years ago. Since the molecules in these clouds do not emit optical light, but do release light at radio wavelengths, radio telescopes are necessary to trace the molecular gas and study its physical properties. Most of this gas is very cold (about -440 degrees Fahrenheit) because it's shielded from ultraviolet light. Since gas is more compact in a colder climate, it is easier for gravity to collapse it to form new stars.

Ironically, the same climate that is conducive to star formation also may shut off the star birth process. The problem is heat. Young stars are very hot and can heat the molecular gas to more than 1,000 degrees Fahrenheit, which is an unfavorable climate for star birth. When the temperature exceeds about 3,000 degrees Fahrenheit, the gas molecules break down into atoms.

The density of the gas can increase considerably near the centers of some Giant Molecular Clouds: Gas as dense as 1 billion molecules per cubic inch has been observed. (Though dense by astronomical standards, such gas is still 100 billion times thinner than the air we breathe here on Earth at sea level!) In such dense regions, still denser blobs of gas can condense and create new stars. Although the star formation process is not fully understood, there is observational evidence that most stars are born in the densest parts of molecular clouds.

What happens when stars begin forming in Giant Molecular Clouds depends on the environment. Under normal conditions in the Milky Way and in most other present-day spiral galaxies, star birth will stop after a relatively small number of stars have been born. That's because the stellar nursery is blown away by some of the newly formed stars. The hottest of these heat the surrounding molecular gas, break up its molecules, and drive the gas away. As the celestial smog of gas and dust clears, the previously hidden young stars become visible, and the molecular cloud and its star-birthing capability cease to exist. Two years ago the Hubble Space Telescope revealed such an emerging stellar nursery in the three gaseous pillars of the Eagle Nebula.

Giant Molecular Clouds in colliding galaxies may experience a different fate. As the collision crunches the interstellar gas and stars form at an accelerating rate, the gas pressure around the surviving Giant Molecular Clouds increases one-hundred- to one-thousand-fold. Calculations suggest that the hot surrounding gas can trigger rapid star birth throughout the clouds by driving shock waves into them. The several hundred thousand stars that form from the cold molecular gas in such clouds use up most of the gas before it has time to be heated and dispersed. The result of such violent events is the nearly complete conversion of Giant Molecular Clouds into rich star clusters, each containing up to 1 million stars. Observations by the Hubble telescope suggest that many of these newly born star clusters remain bound by their own gravity and evolve into globular clusters, like those observed in the halo of our Milky Way.


For decades, many astronomers believed in a cookie cutter universe. Orderly, well-behaved, predictable. The mold for galaxies, the large systems where stars and planets reside, came in two shapes: spirals and ellipticals. They were "island universes" that evolved in "splendid isolation" just a few million years after the Big Bang. To these astronomers, colliding galaxies were merely an oddity, an anomaly.

But there was a group of astronomers who had a less kind view of the universe. They believed that the universe was a violent place, full of collisions, cannibalism, and mergers. Galaxies, they proposed, may not have been created in cookie-cutter fashion early in the universe. Maybe collisions between spirals spawned ellipticals. Primitive Computer Models

The debate over the role colliding galaxies play in galaxy evolution has continued for decades. In the 1940's, just a few years after American astronomer Edwin Hubble defined galaxy shapes, Swedish astronomer Erik Holmberg wondered what would happen if a couple of galaxies encountered one another. So he constructed an analog computer using about 200 light bulbs to simulate galaxy encounters. Based on this seemingly primitive computer simulation, Holmberg concluded that some galaxies may indeed collide, inducing tides or distortions that rob them of energy, thus causing them to slow down and eventually merge into a single galaxy. The Swedish astronomer's computer simulations also foreshadowed the important role that computers would play in studying galaxy interactions.

Snapping Images of Enigmas

The astronomical community largely ignored Holmberg's work. The snubbing, however, didn't stop some astronomers from pursuing these enigmatic galaxies. Swiss astrophysicist Fritz Zwicky at the California Institute of Technology was the first to systematically photograph interacting galaxies in the 1950's. He noticed wispy tails in these galaxies that were similar to those that Holmberg had discovered in his simulations, and concluded that they must stem from gravitational interaction. Zwicky also guessed that these tails must consist of stars.

Still, most astronomers paid little attention to the subject of colliding galaxies, mainly because they believed that the chance of galaxy encounters was relatively small. They didn't understand that galaxies, like stars, often orbit in double and multiple systems, creating a dense environment where collisions are more likely. Some astronomers proposed that the wispy tails were the remnants of gigantic explosions.

Peculiar or Symmetrical?

Many astronomers believed, as Hubble did, that most galaxies were orderly and symmetrical. Astronomer Allan Sandage emphasized those galaxies in his 1961 book "The Hubble Atlas of Galaxies." He also was among a group of astronomers who proposed that the blobby ellipticals were formed before the disk-shaped spirals.

But astronomer Halton Arp believed in a different kind of universe, one filled with violence. In 1966 he published a catalogue of 338 interesting systems called the "Atlas of Peculiar Galaxies." Arp was convinced that colliding galaxies were more than just oddball systems: He was the first to suggest that these galaxies could form stars in bursts.

Faster Computers Equal Better Models

Colliding galaxy research received a boost in the late 1960's when scientists made significant improvements in computer technology. Faster, more powerful computers meant more sophisticated simulations of galaxy interactions, which could furnish astronomers with details about these collisions.

Soon after, several astronomers using computer simulations to study colliding galaxies published scientific papers on their work. The paper with the most-developed theory was written in 1972 by the Toomre brothers, Alar and Juri. Instead of plugging in a couple of generic interacting galaxies into their computer to see the results, they also chose four well-known colliding spiral galaxies, including M51 and the Antennae. They wanted to know whether their computer results would match observational evidence. The brothers discovered that they did. Their models showed that galaxy collisions cause strong gravitational interactions, which produce features similar to the bridges and tails of dust and stars found in many of the galaxies in Arp's atlas of galaxies.

After colliding, these galaxies slow down and are drawn closer together until they eventually merge. The offspring of these mergers are star piles resembling elliptical galaxies. There must have been many more mergers in the past when the universe was younger and denser. Alar Toomre, in his classic 1977 paper, estimated that about 10 percent of all galaxies should be merger remnants, a percentage that roughly matches the number of ellipticals observed in the universe. Their conclusion was a salvo shot at a popular theory that ellipticals came before spirals.

The Toomres also were among the first astronomers to suggest that debris stirred up from galaxy interactions could provide fuel for black holes, which power quasars. They penned the phrases "stoking the furnace" and "feeding the monster," descriptions that are now indelibly linked with black holes and quasars.

A Puzzling Question

Although astronomers debated the Toomres' work, they began to take the study of colliding galaxies more seriously. But they still had objections. Among them was this puzzle. Spirals are full of gas, but contain relatively few globular clusters (dense spherical clusters of about 100,000 stars). Ellipticals, on the other hand, contain very little gas but possess many globular clusters. How, then, can two merging spiral galaxies produce an elliptical? It's almost like saying 2 plus 2 equals 8. Our Milky Way galaxy, a spiral, has about 150 globular clusters while an elliptical with the same brightness would contain about 600 globulars.

These dissenting astronomers did not consider the important role that gas plays in mergers. Most mergers involve gas being compressed, which triggers intense star formation. Perhaps this burst of star formation could produce new globular clusters?

A Burst of Infant Stars

Astronomers studying colliding galaxies hoped that a new infrared satellite would provide some clues. They weren't disappointed. The satellite, called the Infrared Astronomical Satellite (IRAS), was launched in 1983 to take an infrared survey of the sky. This survey revealed that the most luminous galaxies in the infrared part of the spectrum were always colliding galaxies, illuminated by dust surrounding a burst of infant stars. The images provided evidence that interacting galaxies showed signs of unusually vigorous star formation, a theory that originally had been proposed by Zwicky and Arp.

When two galaxies collide, their interstellar gas is compressed into thick clouds. These clouds of gas collapse even more under gravity's intense force to form new stars. The resulting star burst uses up nearly all of the interstellar gas and expels most of the remaining gas through supernovae explosions, leaving a gas-poor system similar to that of an elliptical galaxy.

Young Blue Star Clusters

Some astronomers believed this intense star formation might produce globular clusters. These clusters would shine with the blue light of hot stars. Other astronomers, however, argued that there was no such evidence for young globular clusters. They contended that globular clusters, such as the ones in our Milky Way, are old. But astronomer Francois Schweizer of the Carnegie Institution of Washington disagreed. Schweizer had teamed up with Alar Toomre to probe several interacting galaxies. In 1982 Schweizer studied the interacting galaxy NGC 7252 (the Atoms for Peace galaxy) using ground-based telescopes and observed six bluish knots of light near the galactic nucleus. He interpreted these knots as young star clusters formed during the merger. He and other astronomers (e.g. Keith Ashman at the Space Telescope Science Institute and Steve Zepf at Johns Hopkins University) suggested that the formation of young globular clusters by colliding spirals might explain why ellipticals have so many globular clusters.

Hubble Observations

But Schweizer and other astronomers couldn't provide solid evidence for the existence of new star clusters in interacting galaxies. Ground-based telescopes didn't have the resolution to completely define these clusters. Enter the Hubble Space Telescope, with its high-resolution capabilities and its great location above the Earth's atmosphere. In the Antennae, for example, one giant star- forming knot from a ground-based telescope often turns into 10 to 12 star clusters through the eyes of the Hubble telescope, each with the size of a normal globular cluster.

Even a Hubble telescope without corrective vision found plenty of young star clusters. Peering into the core of the interacting galaxy NGC 1275, the Hubble telescope's Wide Field and Planetary Camera found in 1992 what astronomer Jon Holtzman of Lowell Observatory described as 50 young clusters less than several hundred million years old. He concluded that the clusters were spawned by a merger.

In 1993 a team of astronomers, including Schweizer of Carnegie and led by Brad Whitmore of the Space Telescope Science Institute, provided conclusive evidence that mergers produce new star clusters. Using the Hubble telescope, the team identified 40 young clusters, mostly between 50 and 500 million years old, near the center of NGC 7252.

No Longer Oddball Galaxies

Since then, Whitmore and Schweizer and their collaborators, Miller of the Carnegie Institution of Washington, and Fall and Leitherer of the Space Telescope Science Institute, have continued to probe colliding galaxies. The Wide Field and Planetary Camera 2 with its corrective vision has penetrated more than 10 times deeper into the heart of colliding galaxies than earlier observations. Recent observations of NGC 7252, for example, have revealed more than 500 star clusters, compared with only 40 in 1993.

Whitmore now believes he can tell how long ago these collisions occurred by measuring the colors and brightness of young globular clusters. These clusters, many astronomers agree, may play an essential role in understanding how galaxies evolve.

From oddball galaxies to galaxy building blocks: The part colliding galaxies play in galaxy evolution has changed dramatically over the decades.


The average life span for us humans is about 75 years. But three-quarters of a century is just a blip on the evolutionary time scale, which stretches billions of years. So scientists are faced with the big job of establishing a clear evolutionary connection over time spans that are much longer than human lifetimes.

In the field of archaeology, for example, the classic problem is the search for the "missing link" between primates and humans. In a similar way, the search for the missing link between elliptical galaxies and colliding spiral galaxies has become one of the primary questions of extragalactic astronomy.

Since astronomers cannot watch an individual galaxy evolve in real time, they rely on a set of galaxy snapshots to tell about its life. The trouble is, astronomers don't know in which order they belong. What is needed is a tool to place them in the correct chronological sequence so that we can study their evolution.

Young star clusters formed in merging galaxies provide such a tool. When star clusters are born they are very blue and bright, because the stars forming in this group are extremely luminous and hot (blue stars mean hot stars).

During this stage the cluster may be more than 100 times brighter than it will appear in old age. After about 10 million years these super bright, blue stars die out, and the luster fades in brightness and becomes redder. Astronomers can use this fading and reddening to age-date the star clusters.

With this technique in mind, Hubble Space Telescope observations were obtained of a variety of merger remnants. The Hubble telescope photo collection ranges from ongoing interactions between two disk galaxies (i.e., NGC 4038/4039 - the Antennae galaxies – which are roughly 50 million years old), to recent merger remnants with characteristics of both mergers and elliptical galaxies (NGC 7252 – the Atoms for Peace galaxy – and NGC 3921, both of which are about 500 million years old), and on to dynamically young ellipticals, such as NGC 3610, where only the faint loops and shells reveal their merger history.

Scientists hope to use the information gleaned from these Hubble telescope observations to determine whether these galaxies can be linked together into an evolutionary sequence. The Hubble telescope observations, for example, provide evidence for the hypothesis that elliptical galaxies can be created by the merger of two spiral galaxies, as described in the "Illustration of Determining Stages of Interacting Galaxies ."

Based on the results from observing a small sample of galaxies to date, the technique of using the colors of the star clusters to age-date merger remnants looks promising. However, astronomers must obtain observations from a much larger sample before making any firm conclusions about whether elliptical galaxies can be created by the merger of two spiral galaxies.


The Milky Way consists of a bar-shaped core region surrounded by a disk of logarithmic spiral arm structures (see Spiral arms below). The mass distribution within the Milky Way closely resembles the type SBc in the Hubble classification, which represents spiral galaxies with relatively loosely wound arms. [1] Astronomers first began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1990s. [76] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005 [77] that showed the Milky Way's central bar to be larger than previously thought.

Galactic quadrants

A galactic quadrant, or quadrant of the Milky Way, refers to one of four circular sectors in the division of the Milky Way. In actual astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system, which places the Sun as the origin of the mapping system. [78]

Quadrants are described using ordinals—for example, "1st galactic quadrant", [79] "second galactic quadrant", [80] or "third quadrant of the Milky Way". [81] Viewing from the north galactic pole with 0 degrees (°) as the ray that runs starting from the Sun and through the Galactic Center, the quadrants are as follows:

  • 1st galactic quadrant – 0° ≤ longitude (ℓ) ≤ 90° [82]
  • 2nd galactic quadrant – 90° ≤ ℓ ≤ 180° [80]
  • 3rd galactic quadrant – 180° ≤ ℓ ≤ 270° [81]
  • 4th galactic quadrant – 270° ≤ ℓ ≤ 360° (0°) [79]

Galactic Center

The Sun is 26,000–28,000 ly (8.0–8.6 kpc) from the Galactic Center. This value is estimated using geometric-based methods or by measuring selected astronomical objects that serve as standard candles, with different techniques yielding various values within this approximate range. [14] [83] [84] [85] [86] In the inner few kpc (around 10,000 light-years radius) is a dense concentration of mostly old stars in a roughly spheroidal shape called the bulge. [87] It has been proposed that the Milky Way lacks a bulge formed due to a collision and merger between previous galaxies and that instead has a pseudobulge formed by its central bar. [88]

The Galactic Center is marked by an intense radio source named Sagittarius A* (pronounced Sagittarius A-star). The motion of material around the center indicates that Sagittarius A* harbors a massive, compact object. [89] This concentration of mass is best explained as a supermassive black hole [nb 3] [14] [83] (SMBH) with an estimated mass of 4.1–4.5 million times the mass of the Sun. [83] The rate of accretion of the SMBH is consistent with an inactive galactic nucleus, being estimated at around 6995100000000000000♠ 1 × 10 −5 M y −1 . [90] Observations indicate that there are SMBH located near the center of most normal galaxies. [91] [92]

The nature of the Milky Way's bar is actively debated, with estimates for its half-length and orientation spanning from 1 to 5 kpc (3,000–16,000 ly) and 10–50 degrees relative to the line of sight from Earth to the Galactic Center. [85] [86] [93] Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other. [94] However, RR Lyr variables do not trace a prominent Galactic bar. [86] [95] [96] The bar may be surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the Milky Way, as well as most of the Milky Way's star-formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way. [97] X-ray emission from the core is aligned with the massive stars surrounding the central bar [90] and the Galactic ridge. [98]

In 2010, two gigantic spherical bubbles of high energy emission were detected to the north and the south of the Milky Way core, using data of the Fermi Gamma-ray Space Telescope. The diameter of each of the bubbles is about 25,000 light-years (7.7 kpc) they stretch up to Grus and to Virgo on the night-sky of the southern hemisphere. [99] [100] Subsequently, observations with the Parkes Telescope at radio frequencies identified polarized emission that is associated with the Fermi bubbles. These observations are best interpreted as a magnetized outflow driven by star formation in the central 640 ly (200 pc) of the Milky Way. [101]

Later, on January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sagittarius A*, a black hole in the center of the Milky Way. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A*, according to astronomers. [75]

Spiral arms

Outside the gravitational influence of the Galactic bars, astronomers generally organize the structure of the interstellar medium and stars in the disk of the Milky Way into four spiral arms. [102] Spiral arms typically contain a higher density of interstellar gas and dust than the Galactic average as well as a greater concentration of star formation, as traced by H II regions [103] [104] and molecular clouds. [105]

Maps of the Milky Way's spiral structure are notoriously uncertain and exhibit striking differences. [74] [102] [104] [106] [107] [108] [109] [110] Some 150 years after Alexander (1852) [111] first suggested that the Milky Way is a spiral, there is currently no consensus on the nature of the Milky Way's spiral arms. Perfect logarithmic spiral patterns only crudely describe features near the Sun, [104] [109] because galaxies commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity. [86] [109] [110] The possible scenario of the Sun within a spur / Local arm [104] emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way. [109]

As in most spiral galaxies, each spiral arm can be described as a logarithmic spiral. Estimates of the pitch angle of the arms range from about 7° to 25°. [112] [113] There are thought to be four spiral arms that all start near the Milky Way's center. These are named as follows, with the positions of the arms shown in the image at right:

Color Arm(s)
cyan 3-kpc Arm (Near 3 kpc Arm and Far 3 kpc Arm) and Perseus Arm
purple Norma and Outer arm (Along with extension discovered in 2004 [114] )
green Scutum–Centaurus Arm
pink Carina–Sagittarius Arm
There are at least two smaller arms or spurs, including:
orange Orion–Cygnus Arm (which contains the Sun and Solar System)

Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giants and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm: the Scutum-Centaurus Arm contains approximately 30% more red giants than would be expected in the absence of a spiral arm. [112] [115] In 2008, Robert Benjamin of the University of Wisconsin–Whitewater used this observation to suggest that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess old stars. [74] In December 2013, astronomers found that the distribution of young stars and star-forming regions matches the four-arm spiral description of the Milky Way. [116] [117] [118] Thus, the Milky Way appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars. The explanation for this apparent discrepancy is unclear. [118]

The Near 3 kpc Arm (also called Expanding 3 kpc Arm or simply 3 kpc Arm) was discovered in the 1950s by astronomer van Woerden and collaborators through 21-centimeter radio measurements of HI (atomic hydrogen). [119] [120] It was found to be expanding away from the central bulge at more than 50 km/s. It is located in the fourth galactic quadrant at a distance of about 5.2 kpc from the Sun and 3.3 kpc from the Galactic Center. The Far 3 kpc Arm was discovered in 2008 by astronomer Tom Dame (Harvard-Smithsonian CfA). It is located in the first galactic quadrant at a distance of 3 kpc (about 10,000 ly) from the Galactic Center. [120] [121]

A simulation published in 2011 suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy. [122]

It has been suggested that the Milky Way contains two different spiral patterns: an inner one, formed by the Sagittarius arm, that rotates fast and an outer one, formed by the Carina and Perseus arms, whose rotation velocity is slower and whose arms are tightly wound. In this scenario, suggested by numerical simulations of the dynamics of the different spiral arms, the outer pattern would form an outer pseudoring [123] and the two patterns would be connected by the Cygnus arm. [124]

Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago. However, several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped thick disk of the Milky Way. [125]

The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center. [126] However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years from the Galactic Center. About 40% of the Milky Way's clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation. [127] The globular clusters can follow rosette orbits about the Milky Way, in contrast to the elliptical orbit of a planet around a star. [128]

Although the disk contains dust that obscures the view in some wavelengths, the halo component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo, as there is little gas cool enough to collapse into stars. [15] Open clusters are also located primarily in the disk. [129]

Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much further than previously thought, [130] the possibility of the disk of the Milky Way extending further is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the Cygnus Arm [114] [131] and of a similar extension of the Scutum-Centaurus Arm. [132] With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the Galactic disk.

On January 9, 2006, Mario Jurić and others of Princeton University announced that the Sloan Digital Sky Survey of the northern sky found a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Milky Way. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years (9 kpc) away. [133]

Gaseous halo

In addition to the stellar halo, the Chandra X-ray Observatory, XMM-Newton, and Suzaku have provided evidence that there is a gaseous halo with a large amount of hot gas. The halo extends for hundreds of thousand of light years, much further than the stellar halo and close to the distance of the Large and Small Magellanic Clouds. The mass of this hot halo is nearly equivalent to the mass of the Milky Way itself. [134] [135] [136] The temperature of this halo gas is between 1 million and 2.5 million kelvin. [137]

Observations of distant galaxies indicate that the Universe had about one-sixth as much baryonic (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way. [138] If the finding that the mass of the halo is comparable to the mass of the Milky Way is confirmed, it could be the identity of the missing baryons around the Milky Way. [138]

Sun’s location and neighborhood

The Sun is near the inner rim of the Orion Arm, within the Local Fluff of the Local Bubble, and in the Gould Belt, at a distance of 8.33 ± 0.35 kiloparsecs (27,200 ± 1,100 ly) from the Galactic Center. [14] [83] [139] The Sun is currently 5–30 parsecs (16–98 ly) from the central plane of the Galactic disk. [140] The distance between the local arm and the next arm out, the Perseus Arm, is about 2,000 parsecs (6,500 ly). [141] The Sun, and thus the Solar System, is located in the Milky Way's galactic habitable zone.

There are about 208 stars brighter than absolute magnitude 8.5 within a sphere with a radius of 15 parsecs (49 ly) from the Sun, giving a density of one star per 69 cubic parsec, or one star per 2,360 cubic light-year (from List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs) within 5 parsecs (16 ly) of the Sun, giving a density of about one star per 8.2 cubic parsec, or one per 284 cubic light-year (from List of nearest stars). This illustrates the fact that there are far more faint stars than bright stars: in the entire sky, there are about 500 stars brighter than apparent magnitude 4 but 15.5 million stars brighter than apparent magnitude 14. [142]

The apex of the Sun's way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun's orbit about the Milky Way is expected to be roughly elliptical with the addition of perturbations due to the Galactic spiral arms and non-uniform mass distributions. In addition, the Sun oscillates up and down relative to the Galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. These oscillations were until recently thought to coincide with mass lifeform extinction periods on Earth. [143] However, a reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find a correlation. [144]

It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a galactic year), [15] so the Sun is thought to have completed 18–20 orbits during its lifetime and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Milky Way is approximately 220 km/s or 0.073% of the speed of light. The Sun moves through the heliosphere at 84,000 km/h (52,000 mph). At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit). [145] The Solar System is headed in the direction of the zodiacal constellation Scorpius, which follows the ecliptic. [146]

Galactic rotation

The stars and gas in the Milky Way rotate about its center differentially, meaning that the rotation period varies with location. As is typical for spiral galaxies, the orbital speed of most stars in the Milky Way does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 and 240 km/s. [149] Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike the situation within the Solar System, where two-body gravitational dynamics dominate and different orbits have significantly different velocities associated with them. The rotation curve (shown in the figure) describes this rotation. Toward the center of the Milky Way the orbit speeds are too low, whereas beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation.

If the Milky Way contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotation speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter. [32] The rotation curve of the Milky Way agrees with the universal rotation curve of spiral galaxies, the best evidence for the existence of dark matter in galaxies. Alternatively, a minority of astronomers propose that a modification of the law of gravity may explain the observed rotation curve. [150]

Speeding bullet galaxy observed slamming into galaxy cluster

XMM-Newton flux image of RXCJ2359.3-6042. All three detectors were combined after the exposure correction in the [0.5-2.0] keV band. A scale of three arcmin is shown with a bar. Credit: arXiv:1501.02371 [astro-ph.GA]

(—Researchers working at Europe's XMM-Newton X-ray space telescope have observed a speeding galaxy smashing its way through a galaxy cluster called Abell 4067. They have reported their observations and findings in a paper uploaded to the arXiv server, soon to be published in the journal Astronomy & Astrophysics.

Scientists have observed a galaxy smashing into another one before—back in 2008 such an event was seen and provided what many have called proof of the existence of dark matter. The speeding galaxy left behind a gas trail, while evidence of something else clearly left a trail veering off in a different direction. The observation also added credence to the theory that dark matter exists in a ring around most galaxies.

The more recent collision occurred approximately 1.4 billion light years away—the bullet, the team reports, was traveling at approximately 814 miles per second and survived. Such impacts allow researchers both beautiful imagery and a trove of data—they allow for the bullet galaxy to have its mass measured, for example. This latest one weighed 200 trillion time as much as planet Earth. It also allows space scientists to view firsthand what goes on when such events occur, letting them see what actually happens to both the bullet and the cluster. The researchers note that because this most recent collision was much slower than the prior event, and because it was smaller, it was much more difficult to calculate the mass of the bullet. The researchers came across the collision as they were conducting a survey of distant galaxy clusters. It has been noted that the Milky Way galaxy will be one day serving as a similar bullet, smashing into the Andromeda galaxy approximately four billion years from now.

The researchers plan to continue studying the bullet and collision (they have just been given the go-ahead to engage in a deeper observation event by the telescope operators) hoping to learn more about how galaxies behave in general, particularly when under stress. More observations should reveal, for example, how much gas surrounds the speeding galaxy along with data about the shock wave. There is also of course, a chance that it could lead to a better understanding of dark matter.

We report the discovery of the merging cluster, RXCJ2359.3-6042, from the REFLEX II cluster survey and present our results from all three detectors combined in the imaging and spectral analysis of the XMM-Newton data. Also known as Abell 4067, this is a unique system, where a compact bullet penetrates an extended, low density cluster at redshift z=0.099 clearly seen from our follow-up XMM-Newton observation. The bullet goes right through the central region of the cluster without being disrupted and we can clearly watch the process how the bullet component is stripped of its layers outside the core. There is an indication of a shock heated region in the East of the cluster with a higher temperature. The bulk temperature of the cluster is about 3.12 keV implying a lower mass system. Spearheading the bullet is a cool core centred by a massive early type galaxy. The temperatures and metallicities of a few regions in the cluster derived from the spectral analysis supports our conjecture based on the surface brightness image that a much colder compact component at 1.55 keV with large metallicity (0.75 Zsol) penetrates the main cluster, where the core of the infalling component survived the merger leaving stripped gas behind at the centre of the main cluster. We also give an estimate of the total mass within r500, which is about 2e14Msol from the deprojected spherical-beta modelling of the cluster in good agreement with other mass estimates from the M—Tx and M-sigma_v relations.

Proposed move

Milky Way to Milky Way Galaxy. A lot of people typing in Milky Way will be looking for the candy bar. Milky Way should be a disambiguation page pointing to the candy bar and the galaxy. Voortle 20:26, 29 August 2006 (UTC)


Add "* Support" or "* Oppose" followed by an optional short explanation, then sign your opinion with

  • Oppose. I think it should be left as is -- the galaxy should trump a candy bar named after the galaxy, especially in an encyclopedia. I'd guess (though I can't prove it) that far more people looking for "Milky Way" on an encyclopedia are looking for the galaxy than are looking for the candy bar. Most of what people want to know about a candy bar is included on the label or can be discovered by eating one. If they want to know something else, they can find it through the disambiguation wikilink at the top of the galaxy article. -- Moondigger 21:00, 29 August 2006 (UTC)
  • While I'll admit it is a very well known candy bar, I'm inclined to agree that the dominant interest of encyclopedia users will be the galaxy, and so that should stay here. Dragons flight 04:27, 30 August 2006 (UTC)
  • Oppose -- prefer to leave as is, for all of the reasons cited above. Polaris999 13:44, 30 August 2006 (UTC)
  • Support - The Milky Way is traditionally not the galaxy, but just the visible concentration of stars in the night sky. An article on what you see with the naked eye should sit at Milky Way. It'd also be useful to amateur astronomers on sights in the night sky in the river of stars132.205.45.148 17:36, 31 August 2006 (UTC)
    • It isn't just the visible concentration of stars in the night sky it's also the actual name of the galaxy. I don't think an article exists that only discusses what you see when you look up, and even if one were created I believe it would be quickly merged into this article. -- Moondigger 18:39, 31 August 2006 (UTC)
      • However, the name "Milky Way" was named for the visible concentration. The current article is so long that a good treatment of the visible aspect in the sky would be lost. We should have a separate article for the night sky, since that is clearly a different subject from the galaxy. The "Milky Way" is the visible concentration of stars and such in the night sky, and it is a part of the "Milky Way Galaxy". You can't see the barred spiral even with Keck II, you need to have computers process it out. 05:07, 4 September 2006 (UTC)
        • Well, your proposal is a bit different than what this survey covers, which is to create a disambiguation page because of confusion with the candy bar(s). If you have the inclination to create an article that talks about the Milky Way as it appears when you look up on a dark, clear night, then I would suggest writing it -- but I believe there would be a strong desire by most editors to merge such information into this article. Also, I'm wondering what information it would contain that's not already included in the current article. -- Moondigger 05:26, 4 September 2006 (UTC)


        Add any additional comments

        The above discussion is preserved as an archive of the debate. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

        Note: This article has a small number of in-line citations for an article of its size and currently would not pass criteria 2b.
        Members of the Wikipedia:WikiProject Good articles are in the process of doing a re-review of current Good Article listings to ensure compliance with the standards of the Good Article Criteria. (Discussion of the changes and re-review can be found here). A significant change to the GA criteria is the mandatory use of some sort of in-line citation (In accordance to WP:CITE) to be used in order for an article to pass the verification and reference criteria. It is recommended that the article's editors take a look at the inclusion of in-line citations as well as how the article stacks up against the rest of the Good Article criteria. GA reviewers will give you at least a week's time from the date of this notice to work on the in-line citations before doing a full re-review and deciding if the article still merits being considered a Good Article or would need to be de-listed. If you have any questions, please don't hesitate to contact us on the Good Article project talk page or you may contact me personally. On behalf of the Good Articles Project, I want to thank you for all the time and effort that you have put into working on this article and improving the overall quality of the Wikipedia project. Agne 01:00, 26 September 2006 (UTC)

        Is it just me or does 'Although the Milky Way is but one of billions of galaxies in the universe, the Galaxy has special significance to humanity as it is the home of the Solar System.' sound like it was written by some sort of alien? Seeing as how most people reading Wikipedia will be humble human beings, is this pompous phrasing really necessary? Auspiciously 17:37, 27 September 2006 (UTC)

        I added a paragraph to the section Speed through space explaining the speed through CMB. Maybe this should be explained in the CMB article, but given the presence of that section, I believe it was needed for completeness. Any comments are welcome. Franjesus 16:02, 23 October 2006 (UTC)

        According to a recent study: "The chemistry we see in the stars in these dwarf galaxies is just not consistent with current cosmological models," said Amina Helmi of the Kapteyn Astronomical Institute. [2]

        There's a large gap between "Structure" and its paragraph. I tried to fix it but I'm pretty poor at changing coding. Anybody wanna take a go at closing that gap?

        (1sttomars 21:18, 19 December 2006 (UTC))

        The article has been vandalized

        THe article has been vandalized. "IN THE CENTER OF THE GALAXY IS A GIANT FOOTBALL." was included. I have already removed them.

        The result of the debate was No Merger

        Let your Xbox blast some Mars into your lonely face tonight!

        Tonight, as you're wondering if your level 81 dark elf mage and her sweet double-enchanted dragonscale armor makes up for the girlfriend you lost playing Skyrim, turn your xbox over to the live streaming of the landing of the new Mars rover, Curiosity. Because it's right there on your console's dashboard!

        It's really a win-win situation. Arborea Darkshadow can wait a few minutes, I'm sure, and you'll either get to witness an action-packed landing of a big new Mars rover, or you'll get to see the hopes and dreams of hundreds of scientists and engineers crash and disintegrate on the cold surface of a dead planet millions of miles away!

        You know what JGordon will be doing approximately 12 hours from now? Definitely not watching the Curiosity landing! But that's only because I don't have an internet connection at my apartment. No, I'll probably be forcing the cat to participate in the St. Paul Cat Olympics. As far as I know, there will be only one contestant, but it promises to be hilarious! Why? Because she probably can't swim very well.

        Astronomical history

        In Meteorologica (DK 59 A80), Aristotle (384&ndash322 BC) wrote that the Greek philosophers Anaxagoras (c. &thinsp500 &ndash428 BC) and Democritus (460&ndash370 BC) proposed that the Milky Way might consist of distant stars. [206] However, Aristotle himself believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" [207] and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions." [208] [209] The Neoplatonist philosopher Olympiodorus the Younger (c. &thinsp495 &ndash570 A.D.) criticized this view, arguing that if the Milky Way were sublunary, it should appear different at different times and places on Earth, and that it should have parallax, which it does not. In his view, the Milky Way is celestial. This idea would be influential later in the Islamic world. [210]

        The Persian astronomer Abū Rayhān al-Bīrūnī (973&ndash1048) proposed that the Milky Way is "a collection of countless fragments of the nature of nebulous stars". [211] The Andalusian astronomer Avempace (d 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image due to the effect of refraction in Earth's atmosphere, citing his observation of a conjunction of Jupiter and Mars in 1106 or 1107 as evidence. [209] Ibn Qayyim Al-Jawziyya (1292&ndash1350) proposed that the Milky Way is "a myriad of tiny stars packed together in the sphere of the fixed stars" and that these stars are larger than planets. [212]

        According to Jamil Ragep, the Persian astronomer Naṣīr al-Dīn al-Ṭūsī (1201&ndash1274) in his Tadhkira writes: "The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color." [213]

        Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars. [214] [215] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, [216] speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much larger scales. [217] The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own. Kant referred to both the Milky Way and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s. [218] [219] [220]

        The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the Milky Way with the Solar System close to the center. [221]

        In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture. [222]

        In 1904, studying the proper motions of stars, Jacobus Kapteyn reported that these were not random, as it was believed in that time stars could be divided into two streams, moving in nearly opposite directions. It was later realized that Kapteyn's data had been the first evidence of the rotation of our Galaxy, which ultimately led to the finding of galactic rotation by Bertil Lindblad and Jan Oort.

        In 1917, Heber Curtis had observed the nova S Andromedae within the Great Andromeda Nebula (Messier object 31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within the Milky Way. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were actually independent galaxies. [223] In 1920 the Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift. [224]

        The controversy was conclusively settled by Edwin Hubble in the early 1920s using the Mount Wilson observatory 2.5 m (100 in) Hooker telescope. With the light-gathering power of this new telescope, he was able to produce astronomical photographs that resolved the outer parts of some spiral nebulae as collections of individual stars. He was also able to identify some Cepheid variables that he could use as a benchmark to estimate the distance to the nebulae. He found that the Andromeda Nebula is 275,000 parsecs from the Sun, far too distant to be part of the Milky Way. [225] [226]


        The ESA spacecraft Gaia provides distance estimates by determining the parallax of a billion stars and is mapping the Milky Way with four planned releases of maps in 2022. [227] [228]