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Most descriptions of star ignition go something like… The star reaches a critical mass and ignites, blowing away the surrounding material. The most obvious question would be "Why are stars not all the same size?", given that the critical mass for ignition would appear to be the same, but I understand that stars can accumulate mass from collisions from other stars, planetary bodies etc.
So my question is also based on the fact that larger stars also burn more energetically, I assume blowing away more of the the lighter elements from the surrounding cloud after formation and gathering more heavy elements,… do larger stars have a significantly different composition from smaller stars? --- or is the simple description of star formation misleading?
The star reaches a critical mass and ignites, blowing away the surrounding material. (OP emphasis)
I think the misunderstanding is that it's not that explosive when the star "lights up". The process is in some sense continuous, so the nuclear reactions start slowly and gradually increase in strength as the star continues to contract, grow, and heat up inside. Once the amount of radiation from the surface becomes sufficiently hot, we can speak of "blowing away" material, but even then it's not violent. The absorption of light in the gas creates a pressure that pushes the gas away. It picks up pace as the star settles down (and depending on how massive the star is), but it's broadly a consistent stream. (Astronomers are probably unwise to use the word "ignite" because all we mean is that the nuclear reaction rates become non-negligible. It's not like a rocket taking off or something.)
Generally, star formation is a poorly-understood process and a hot topic for research. For stars larger than about 7 solar masses, we think that nuclear reactions begin before all the material has gathered in the star. i.e. while some is still flowing on. If this happens through a disk, then the light can stream out everywhere but the equator without much affecting the growth of the star.
That said, at very high masses, you do run into a problem of blowing mass away. For this reason, we don't really expect to see many stars over a few hundred solar masses, and those that are can be violently unstable (like Eta Carina, which threw off about 10 solar masses of material in the mid-19th century).
And as for stars genuinely "blowing up", this does happen in other circumstances. For example, in the core of a solar mass red giant, matter becomes nearly isothermal. It eventually gets hot enough to ignite helium, and it nearly all lights up at nearly the same time in a helium flash. This briefly produces as much light as a small galaxy! But only very briefly, and it doesn't make it to the surface for us to see. Incidentally, this is also the same mechanism as Type Ia supernovae: an isothermal carbon-oxygen white dwarf accretes matter until the carbon lights up, and the whole white dwarf basically ignites at the same time.
Chief Scientist Laboratories Star and Planet Formation Laboratory
Star and planet formation is one of the most fundamental structure-formation processes in the universe. By use of the state-of-the-art radio telescopes including ALMA, we are investigating when a disk structure is formed around a solar-type protostar, and how it is evolved into a protoplanetary disk and eventually to a planetary system. This is an essential question deeply related to the origin of the Solar system. We particularly focus on a relation between physical evolution and chemical evolution during star and planet formation. Related laboratory spectroscopic studies in the millimeter and submillimeter regimes are also planned.
Collapse or Collision: The Big Question in Star Formation
A young star that had been the poster child of massive star formation may have been lying about its weight.
The star is in its infant stages and is known as a protostar, with the designation M17-SO1. Its heft carries weight in an ongoing debate over how the most massive stars form.
A year ago, a group of astronomers published evidence showing that the mass of M17-SO1 was more than 15 times that of our Sun. The radiation from a star this big is thought to prevent matter from collapsing onto it. But if matter cannot fall - or accrete - onto a massive star, how does it grow to be so large?
"It's the big question in star formation at this moment," said Markus Nielbock of the Ruhr-Universitat Bochum in Germany, who along with Rolf Chini measured the mass of M17-SO1.
One theory states that stellar behemoths are built out of the collision of two medium-sized stars. But Chini and Nielbock found signs that M17-SO1 was chomping away on an accretion disk, which implies the same kind of slow feeding process that goes on in smaller stars.
"[Chini's group] proposed that high mass stars evolve in a similar manner to low/mid mass stars by going through a phase with a flared disk," said Shigeyuki Sako from the University of Tokyo. "This was a shock to the community studying the formation of high mass stars, especially to the theorists."
From their own data for M17-SO1, Sako and his colleagues claim the star has less than 8 solar masses, meaning radiation pressure is no longer a concern. Whether this is good news or bad news depends on how one thinks massive stars come into being.
The mass of M17-SO1 cannot be determined directly because the star is embedded inside the Omega Nebula in the constellation Sagittarius. In optical light, there is almost nothing to see, so astronomers use infrared instruments, like the Subaru telescope, to penetrate the dust.
"Determining the mass of a star surrounded by proto-stellar/proto-planetary disk is difficult since the surrounding material obscure the central star, especially when we are looking at the system edge-on," Sako said in an email message.
The edge-on view of M17-SO1 shows a butterfly-shaped envelope, 150 times the size of our solar system. Sako's group was able to map out some of the structure.
"The new observations show a circumstellar disk - the cradle of planets - in the process of forming," Sako said.
Some of the gas in the envelope is noticeably swirling around the star. By measuring the rotation speed of this gas with radio observations, astronomers can try to estimate the mass in the center.
In this way, Chini's group determined that M17-SO1 is 15 to 20 times the mass of our Sun and claimed that this agreed with how bright they estimated the star to be. But the recent work of Sako's team, reported in the 21 April issue of Nature, give a mass between 2.5 and 8 solar masses.
According to Sako, analyzing rotation velocities requires assumptions about the temperature and composition of the gas. This leads to large uncertainties in the mass estimate. Chini said they have new data - not yet fully compiled - that may resolve the issue.
But to support their lower mass estimate, Sako and company argue that M17-SO1 cannot be greater than 8 solar masses because, otherwise, its radiation would ionize the hydrogen gas in its neighborhood.
Although ionized hydrogen is not seen around M17-SO1, Chini said that this only implies that the star is very young, and therefore, it is too soon to see signs of stellar ionization.
Beyond this debate, though, the onset of ionizing radiation is one of the reasons that stars greater than 8 solar masses are thought to be so hard to form.
Small and medium sized stars are born through a collapse of gas, followed by a gradual accretion of matter through a stellar disk. But once a star reaches about 8 solar masses, its radiation is thought to blow away any remaining material - cutting off the gas supply.
The alternative to collapsing accretion is that stars in dense clusters collide and coalesce to form bigger stars. This would avoid any kind of radiation cutoff, but there is not much evidence that this scenario can work.
"The problem with the collision theory is that there is no observational tool to measure, i.e. to prove that such a collision has occurred," Chini said.
Chini also said that computer models have improved over the years, and it is no longer so hard to model an accretion history for a very massive star.
"Why should nature invent two mechanisms for the same process?" Chini asked. "[Accretion] works perfectly from 0.1 to 10 solar masses and - as theorists show - also beyond."
But if Sako's group is correct, Chini and others will have to look for another massive star feeding on an accretion disk. These are rare birds in the field of astronomy.
"The problem is we almost never get to see a young massive star because they form so quickly," said Debra Shepherd of the National Radio Astronomy Observatory, who was not involved with either group.
Shepherd explained that if massive stars form through accretion, they gain about a solar mass every 1000 years, so they're finished growing in as little as 20,000 years.
Molecular Clouds: Stellar Nurseries
As we saw in Between the Stars: Gas and Dust in Space, the most massive reservoirs of interstellar matter—and some of the most massive objects in the Milky Way Galaxy—are the giant molecular clouds. These clouds have cold interiors with characteristic temperatures of only 10–20 K most of their gas atoms are bound into molecules. These clouds turn out to be the birthplaces of most stars in our Galaxy.
The masses of molecular clouds range from a thousand times the mass of the Sun to about 3 million solar masses. Molecular clouds have a complex filamentary structure, similar to cirrus clouds in Earth’s atmosphere, but much less dense. The molecular cloud filaments can be up to 1000 light-years long. Within the clouds are cold, dense regions with typical masses of 50 to 500 times the mass of the Sun we give these regions the highly technical name clumps. Within these clumps, there are even denser, smaller regions called cores. The cores are the embryos of stars. The conditions in these cores—low temperature and high density—are just what is required to make stars. Remember that the essence of the life story of any star is the ongoing competition between two forces: gravity and pressure. The force of gravity, pulling inward, tries to make a star collapse. Internal pressure produced by the motions of the gas atoms, pushing outward, tries to force the star to expand. When a star is first forming, low temperature (and hence, low pressure) and high density (hence, greater gravitational attraction) both work to give gravity the advantage. In order to form a star—that is, a dense, hot ball of matter capable of starting nuclear reactions deep within—we need a typical core of interstellar atoms and molecules to shrink in radius and increase in density by a factor of nearly 10 20 . It is the force of gravity that produces this drastic collapse.
How were magnetars discovered?
On March 5, 1979, after sending satellites onto the surface of Venus, two Soviet spacecraft was sent drifting through the Solar System when they were all of a sudden blasted by an immense burst of gamma radiation. This contact raised the radiation readings on both the probes from a normal 100 counts per second to over 200,000 counts a second, in only a fraction of a millisecond.
This burst of gamma rays quickly continued to spread to other places also. Eleven seconds later, Helios 2, a NASA probe, which was orbiting the Sun, was saturated by the blast of radiation. It soon hit Venus, and the Pioneer Venus Orbiter‘s detectors were overcome by the wave.
Seconds later, Earth received the wave of radiation, where the highly intense output of gamma rays swamped the detectors of three U.S. Department of Defense Vela satellites, the Soviet Prognoz 7 satellite, and the Einstein Observatory. The blast also hit the International Sun-Earth Explorer.
This tremendous blast of gamma radiation constituted the strongest wave of extra-solar gamma rays ever detected. It was over 100 times more intense than any known previous extra-solar burst. Because gamma rays travel at the speed of light and the time of the pulse was recorded by several distant spacecraft as well as on Earth, the source of the gamma radiation could be calculated quite accurately.
It was found that this gamma radiation ejected from the remnants of a star that had gone supernova around 3000 B.C.E. It was situated in the Large Magellanic Cloud and the source was named SGR 0525-66 the event itself was named GRB 790305b, the first observed SGR megaflare.
Solar System Formation in Two Steps – Explains Composition and Features of Planets, Asteroids and Meteorites
The inner terrestrial protoplanets accrete early, inherit a substantial amount of radioactive 26Al, and hence melt, form iron cores, and degas their primordial volatile abundances rapidly. The outer Solar System planets start to accrete later and further out with less radiogenic heating, and hence retain the majority of their initially accreted volatiles. Credit: Mark A Garlick/markgarlick.com
An international team of researchers from the University of Oxford, LMU Munich, ETH Zurich, BGI Bayreuth, and the University of Zurich discovered that a two-step formation process of the early Solar System can explain the chronology and split in volatile and isotope content of the inner and outer Solar System.
Their findings will be published in Science on Friday, January 22, 2021.
The paper presents a new theoretical framework for the formation and structure of the Solar System that can explain several key features of the terrestrial planets (like Earth, Venus, and Mars), outer Solar System (like Jupiter), and composition of asteroids and meteorite families. The team’s work draws on and connects recent advances in astronomy (namely observations of other solar systems during their formation) and meteoritics — laboratory experiments and analyses on the isotope, iron, and water content in meteorites.
The suggested combination of astrophysical and geophysical phenomena during the earliest formation phase of the Sun and the Solar System itself can explain why the inner Solar System planets are small and dry with little water by mass, while the outer Solar System planets are larger and wet with lots of water. It explains the meteorite record by forming planets in two distinct steps. The inner terrestrial protoplanets accreted early and were internally heated by strong radioactive decay this dried them out and split the inner, dry from the outer, wet planetary population. This has several implications for the distribution and necessary formation conditions of planets like Earth in extrasolar planetary systems.
The numerical experiments performed by the interdisciplinary team showed that the relative chronologies of early onset and protracted finish of accretion in the inner Solar System, and a later onset and more rapid accretion of the outer Solar System planets can be explained by two distinct formation epochs of planetesimals, the building blocks of the planets. Recent observations of planet-forming disks showed that disk midplanes, where planets form, may have relatively low levels of turbulence. Under such conditions the interactions between the dust grains embedded in the disk gas and water around the orbital location where it transitions from gas to ice phase (the snow line) can trigger an early formation burst of planetesimals in the inner Solar System and another one later and further out.
The two distinct formation episodes of the planetesimal populations, which further accrete material from the surrounding disk and via mutual collisions, result in different geophysical modes of internal evolution for the forming protoplanets. Dr. Tim Lichtenberg from the Department of Atmospheric, Oceanic and Planetary Physics at the University of Oxford and lead-author of the study notes: “The different formation time intervals of these planetesimal populations mean that their internal heat engine from radioactive decay differed substantially.
“Inner Solar System planetesimals became very hot, developed internal magma oceans, quickly formed iron cores, and degassed their initial volatile content, which eventually resulted in dry planet compositions. In comparison, outer Solar System planetesimals formed later and therefore experienced substantially less internal heating and therefore limited iron core formation, and volatile release.
“The early-formed and dry inner Solar System and the later-formed and wet outer Solar System were therefore set on two different evolutionary paths very early on in their history. This opens new avenues to understand the origins of the earliest atmospheres of Earth-like planets and the place of the Solar System within the context of the exoplanetary census across the galaxy.”
This research was supported by funding from the Simons Collaboration on the Origins of Life, the Swiss National Science Foundation, and the European Research Council.
Stars – How They Are Formed
Do you want to know how stars are born? Well here goes. The birth of a star begins when massive clouds of dust and gas start to collapse and break down. How does this happen. It happens through the force of gravity.
Gravity bounds the elements together to create a Protostar. The protostar is basically a baby star, and grows into a star through its life cycle in the same way we are born and grow.
The main process of all stars birth is called ‘Nuclear Fusion’ nuclear fusion happens in very large clouds on gas called nebula. When nuclear fusion starts in a nebula and reaches a temperature of over 10,000 degrees the star begins to form. The new baby star that as we know now is called a protostar can star its very long life on the universe.
Gravity continues to play a part in the stars birth and continued life cycle. The mass of the star determines the colour of the star, which allows scientists categorise the type of star. Scientists can then also try and confirm the amount of energy the star can give off.
The star will then start its journey through its life cycle from birth to death. For more information and further reading on a stars life cycle, see TAB. Star Life-cycle-Supernovas.
Kids Fun Facts Corner
# 1. It is believed that almost every star has the same chemical composition. That basically means that all stars are made up of the same thing.
# 2. If a star is created really large after its birth, the star will live for less time as a smaller star.
Q. How many degrees does it need to get to for a star to star forming?
Q. What determines the color of a star?
Q. What is the name of the force that creates the birth of a star?
Download questions about how stars are formed here: how stars are formed (answers are on this page)
Teachers. For more in depth work sheets on Stars. Click on Kidskonnect Worksheets
SOFIA Observatory Indicates Star Eruptions Create and Scatter Elements with Earth-like Composition
An image from the Hubble Space Telescope showing debris expanding into space from a typical nova outburst that occurred decades before this picture. The featured research using SOFIA was focused on Nova Delphini 2013, which is too recent to allow a resolved picture of its debris cloud. Credits: NASA/ESA/STScI/AURA/NSF/Mike Shara, Bob Williams, David Zurek, Dina Prialnik
Observations made with NASA’s flying observatory, the Stratospheric Observatory for Infrared Astronomy (SOFIA) indicate that nova eruptions create elements that can form rocky planets, much like Earth.
Astronomers occasionally see a nova, which may appear as a “new” star that grows brighter and then fades away after a few weeks. In fact, “nova” (plural, novae) is the Latin word for “new.” We now know that novae are not actually new stars, but rather are associated with stellar old age: explosions occurring on the surfaces of burned-out stars. They are less violent and more common than the star-shattering explosions called supernovae that completely destroy an aging star.
NASA’s Stratospheric Observatory for Infrared Astronomy 747SP taking off just before sunset from Air Force Plant 42 in Palmdale, California on Sept. 15, 2015. Credits: NASA Photo / Greg Perryman
Principal investigator Bob Gehrz of the University of Minnesota Twin Cities, and collaborators have been using SOFIA to study novae as part of an ongoing research program to understand the role these objects play in creating and injecting elements into the material between the stars called the interstellar medium.
Gerhz and his team found high levels of elements such as carbon, nitrogen, oxygen, neon, magnesium, aluminum and silicon in the Nova Delphini, which erupted in 2013 in the constellation Delphinum (the Dolphin). Some of these elements can be found in living things, whereas others are important constituents of rocky planets such as Earth.
There is evidence that when the universe began in the Big Bang, only trace amounts of elements other than hydrogen and helium were created. Atoms of heavier elements were made later by processes inside stars, or during star death throes such as nova and supernova explosions.
The observations of the Nova Delphini debris cloud indicate that novae in general may be a major source of medium-weight elements in the universe. Their paper was published in the Astrophysical Journal.
SOFIA’s Program Scientist Pam Marcum noted that “these spectra of Nova Delphinum could only be obtained by SOFIA, not by any observatory on the ground or currently in space, because of SOFIA’s unique access to the specific range of infrared wavelengths needed for these measurements.” She continued, “this research is part of the broad, ongoing effort by astronomers to understand the life cycles of stars, and how the formation of planets like Earth fit into those cycles.”
What is star composition after formation - Astronomy
All stars, as far as we know, are born from the gravitational collapse of the core of a molecular cloud.
Molecular clouds are regions of relatively dense interstellar gas and dust that can shield their contents against the destructive ambient ultraviolet (UV) radiation field. In such a cold, protected environment the predominant form of matter, atomic hydrogen, preferentially associates into molecular hydrogen, or H2. However, owing to its simplifying symmetry, cold H2 has no emission line spectrum and therefore remains essentially invisible . However other atomic species, although far less abundant, also associate into common molecules like CO2, H2O, HCN, and so forth, and for the most part glow profusely at microwave radio frequencies and are used as "tracers", or "placeholders" for the otherwise invisible H2 molecule. In fact, over 118 molecules have been detected in dense molecular clouds, some as complicated as the amino acid glycine . This is a stunningly interesting point in regards to the development of life. Consider: if amino acids can form in "empty" space, and amino acids are the building blocks of proteins, and proteins are the building blocks of DNA, and DNA is the building block of life as we know it, is it so far-fetched to imagine the possibility of life elsewhere? Or, stated a different way, it appears that the most fundamental physical processes that serve as necessary conditions for the formation of life on Earth appear to happen elsewhere, and maybe everywhere.
A millimeter-wavelength spectrum of the core of the Orion giant molecular cloud, made at the Owens Valley Radio Observatory. The spectrum covers a small interval in the atmospheric window at 1.3 mm. Over 800 spectral features are seen! This type of data is needed to understand the chemistry of cold and dense interstellar regions.
Much data has been gathered on molecular clouds in relation to their ability to form stars, mostly via detection of the molecular emission lines at (sub)millimeter wavelengths. In these clouds, molecules radiate like little microwave radio transmitters as they spontaneously change rotational energy levels. For example, the carbon monoxide molecule (CO) is the most abundant molecule after H2. The first rotational excited state lies only 5 degrees Kelvin (using temperature as a synonym for energy) above the ground state, and therefore is readily excited by the ambient cosmic microwave background radiation or collisions with neighboring molecules (usually H2, since it's 10 4 times more abundant than even CO). When the CO molecule drops back to the ground state, it gives off a photon of light, in an effort to conserve energy. Because the difference in energy levels is so small, the photon emitted carries away a small amount of energy. For this particular transition in CO, the wavelength of the photon emitted is around 2.6 millimeters, or 115 GHz, in the microwave (radio) portion of the spectrum. This is 1000 times higher in frequency (energy) than what you receive with your FM radio.
Molecules can not only change electronic energy levels like atoms, but also can vibrate and rotate. Each of these new degrees of freedom complicates the spectrum substantially. A spectrum of a nearby star-forming cloud like the Orion Nebula can have thousands of detectable molecular spectral lines even within a narrow range of observed frequency! This is not a nightmare best forgotten, but rather provides a unique diagnostic tool. Each transition of each molecule probes diffeent physical conditions within the cloud -- each spectral line tells a different story, a different perspective. Putting the chorus together remains our best hope for disentangling the complicated physical and chemical structure of molecular clouds, and understanding the initial conditions, or "seeds" of star formation.
Stars observed in galaxies were originally divided into two populations by Walter Baade in the 1940s. Although a more refined means of classifying stellar populations has since been established (according to whether they are found in the thin disk, thick disk, halo or bulge of the galaxy), astronomers have continued to coarsely classify stars as either Population I (Pop I) or Population II (Pop II). They have even postulated a third population (Population III Pop III ), though stars of this type have yet to be observed.
The classification system is based on the metal content of the stars (their metallicity, usually given the symbol [Z/H]). Pop II stars are metal-poor, with metallicities ranging from approximately 1/1000th to 1/10th that of the Sun (i.e. from [Z/H]=-3.0 up to [Z/H]=-1.0). This means that the gas from which Pop II stars formed could only have been recycled (incorporated into, and then expelled) from previous generations of stars a few times at most, and that Pop II stars form very early in the star formation history of the galaxy.
Further evidence to support the early origins of Pop II stars comes from their abundance ratios, which show that the lighter elements (e.g. carbon and oxygen) dominate over the heavier elements (e.g. iron, nickel) in the chemical composition of the stars. Since light metals are produced primarily in Type II supernova explosions (the explosions of massive stars which have lifetimes of only a few million years), while the heavier elements can only be produced in Type Ia supernova explosions (the explosion of a much older white dwarf in a binary system), the relative lack of heavy metals in Pop II stars indicates that these stars formed in the first billion years or so of a galaxy’s star formation history.
Population II stars are mainly found in the bulge and halo of galaxies.
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