The deepest views of the cosmos from the Hubble Space Telescope yield clues that the very first stars may have burst into the universe as brilliantly and spectacularly as a fireworks finale. Except in this case, the finale came first, long before Earth, the Sun and the Milky Way Galaxy formed. Studies of Hubble's deepest views of the heavens lead to the preliminary conclusion that the universe made a significant portion of its stars in a torrential firestorm of star birth, which abruptly lit up the pitch-dark heavens just a few hundred million years after the big bang, the tremendous explosion that created the cosmos. Though stars continue to be born today in galaxies, the star birth rate could be a trickle compared to the predicted gusher of stars in those opulent early years.
Stars are formed, astronomers think, when such a cloud of gas and dust collapses gravitationally, first into clumps, then into dense cores, each of which can then begin to further collapse and form a young star. The details of how this happens are not well understood though. One difficulty is that most regions where this process is underway already have formed stars nearby. Those stars affect subsequent nearby star formation through their stellar winds and shock waves when they explode as supernovae. Scientists have found the first clear case of a clump of potentially star-forming gas that is on the verge of forming dense cores, and is unaffected by any nearby stars. Recently, astronomers found two immense clouds of pristine gas, nearly 12 billion light years away - clouds that astronomers suspect are the stuff from which the first stars were born.
While how the first stars formed from this dust and gas has been a burning question for years, a state-of-the-art computer simulation are offering the most detailed picture yet of how these first stars in the universe came into existence. The composition of the early universe was quite different from that of today, and the physics that governed the early universe were also somewhat simpler. When simulating those conditions what we get as the result is a detailed description of the formation of a protostar - the early stage of a massive primordial star of our universe. The researchers' computer simulation, which has been called a "cosmic Rosetta Stone", sets the bar for further investigation into the star formation process. The question of how the first stars evolved is so important because their formations and eventual explosions provided the seeds for subsequent stars to come into being. According to the simulation, gravity acted on minute density variations in matter, gases, and the mysterious "dark matter" of the universe after the Big Bang in order to form this early stage of a star - a protostar with a mass of just one percent of our Sun. The simulation reveals how pre-stellar gases would have actually evolved under the simpler physics of the early universe to form this protostar. Simulation also shows that the protostar would likely evolve into a massive star capable of synthesizing heavy elements, not just in later generations of stars, but soon after the Big Bang - which is pretty much on track with other findings suggesting the same. The abundance of elements in the universe has increased as stars have accumulated and the formation and destruction of stars continues to spread these elements further across the universe. So when you think about it, all of the elements in our bodies originally formed from nuclear reactions in the centers of stars, long long ago. More powerful computers, more physical data, and an even larger range will be needed for further calculations and simulations, but these researchers hope to eventually extend this simulation to the point of nuclear reaction initiation when a stellar object becomes a true star.
In March 2012 (4 days ago to be precise), astronomers using radio and infrared telescopes have obtained a first tantalizing look at a crucial early stage in star formation. The new observations promise to help scientists understand the early stages of a sequence of events through which a giant cloud of gas and dust collapses into dense cores that, in turn, form new stars. The scientists studied a giant cloud about 770 light-years from Earth in the constellation Perseus. They used the ESA's Herschel Space Observatory and the NSF's Green Bank Telescope (GBT) to make detailed observations of a clump, containing nearly 100 times the mass of the Sun, within that cloud. The far-infrared images revealed previously-unseen substructures within the clump that may be precursors to cores with the potential to form individual stars. The astronomers used the GBT to study the motions and temperatures of molecules, primarily ammonia, within these substructures. These GBT observations indicated that one of the substructures is likely to be gravitationally bound and thus farther along the path to condensing into a core than the others. The entire clump, the scientists say, could be expected to form about ten new stars.
Models indicate that the first stars were most likely quite massive and luminous and that their formation was an epochal event that fundamentally changed the universe and its subsequent evolution. These stars altered the dynamics of the cosmos by heating and ionizing the surrounding gases. The earliest stars also produced and dispersed the first heavy elements, paving the way for the eventual formation of solar systems like our own. And the collapse of some of the first stars may have seeded the growth of supermassive black holes that formed in the hearts of galaxies and became the spectacular power sources of quasars. In short, the earliest stars made possible the emergence of the universe that we see today - everything from galaxies and quasars to planets and people.
The simulations show that the primordial gas clouds would typically form at the nodes of a small-scale filamentary network and then begin to contract because of their gravity. Compression would heat the gas to temperatures above 1000 K. Some hydrogen atoms would pair up in the dense, hot gas, creating trace amounts of molecular hydrogen. The hydrogen molecules would then start to cool the densest parts of the gas by emitting infrared radiation after they collide with hydrogen atoms. The temperature in the densest parts would drop to about 200 to 300 K, reducing the gas pressure in these regions and hence allowing them to contract into gravitationally bound clumps. This cooling plays an essential role in allowing the ordinary matter in the primordial system to separate from the dark matter. The cooling hydrogen settles into a flattened rotating configuration that is clumpy and filamentary and possibly shaped like a disk. But because the dark matter particles would not emit radiation or lose energy, they would remain scattered in the primordial cloud. Thus, the star-forming system would come to resemble a miniature galaxy, with a disk of ordinary matter and a halo of dark matter. Inside the disk, the densest clumps of gas would continue to contract, and eventually some of them would undergo a runaway collapse and become stars.
The first star-forming clumps were much warmer than the molecular gas clouds in which most stars currently form. Dust grains and molecules containing heavy elements cool the present-day clouds much more efficiently to temperatures of only about 10 K. The minimum mass that a clump of gas must have to collapse under its gravity is called the Jeans mass, which is proportional to the square of the gas temperature and inversely proportional to the square root of the gas pressure. The first star-forming systems would have had pressures similar to those of present-day molecular clouds. But because the temperatures of the first collapsing gas clumps were almost 30 times higher than those of molecular clouds, their Jeans mass would have been almost 1000 times larger.
Calculations suggest that the predicted masses of the first star-forming clumps are not very sensitive to the assumed cosmological conditions (the exact nature of the initial density fluctuations). In fact, the predicted masses depend primarily on the physics of the hydrogen molecule and only secondarily on the cosmological model or simulation technique. One reason is that molecular hydrogen cannot cool the gas below 200 K, making this a lower limit to the temperature of the first star-forming clumps. Another is that the cooling from molecular hydrogen becomes inefficient at the higher densities encountered when the clumps begin to collapse. At these densities the hydrogen molecules collide with other atoms before they have time to emit an infrared photon; this raises the gas temperature and slows down the contraction until the clumps have built up to at least a few hundred solar masses. Observations and simulations show that the fragmentation of star-forming clumps is typically limited to the formation of binary systems (two stars orbiting around each other). Fragmentation seems even less likely to occur in the primordial clumps, because the inefficiency of molecular hydrogen cooling would keep the Jeans mass high. The simulations, however, have not yet determined the final outcome of collapse with certainty, and the formation of binary systems cannot be ruled out.
An important property of stars with no metals is that they have higher surface temperatures than stars with compositions like that of the sun. The production of nuclear energy at the center of a star is less efficient without metals, and the star would have to be hotter and more compact to produce enough energy to counteract gravity. Because of the more compact structure, the surface layers of the star would also be hotter. Models show that the stars had surface temperatures of about 100000 K - about 17 times higher than the Sun’s surface temperature. Therefore, the first starlight in the universe would have been mainly ultraviolet radiation from very hot stars, and it would have begun to heat and ionize the neutral hydrogen and helium gas around these stars soon after they formed. We call this event the cosmic renaissance. Although astronomers cannot yet estimate how much of the gas in the universe condensed into the first stars, even a fraction as small as one part in 100000 could have been enough for these stars to ionize much of the remaining gas. Once the first stars started shining, a growing bubble of ionized gas would have formed around each one. As more and more stars formed over hundreds of millions of years, the bubbles of ionized gas would have eventually merged, and the intergalactic gas would have become completely ionized. Sloan Digital Sky Survey has recently found evidence for the final stages of this ionization process from about 900 million years after the Big Bang. The results suggest that the last patches of neutral hydrogen gas were being ionized at that time. Helium requires more energy to ionize than hydrogen does, but if the first stars were as massive as predicted, they would have ionized helium at the same time. On the other hand, if the first stars were not quite so massive, the helium must have been ionized later by energetic radiation from sources such as quasars. Future observations of distant objects may help determine when the universe’s helium was ionized.
If the first stars were indeed very massive, they would also have had relatively short lifetimes—only a few million years. Some of the stars would have exploded as supernovae at the end of their lives, expelling the metals they produced by fusion reactions. Stars that are between 100 and 250 times as massive as the sun are predicted to blow up completely in energetic explosions, and some of the first stars most likely had masses in this range. Because metals are much more effective than hydrogen in cooling star-forming clouds and allowing them to collapse into stars, the production and dispersal of even a small amount could have had a major effect on star formation. When the abundance of metals in star-forming clouds rises above one thousandth of the metal abundance in the sun, the metals rapidly cool the gas to the temperature of the cosmic background radiation. This temperature declines as the universe expands, falling to 19 K a billion years after the big bang and to 2.7 K today. This efficient cooling allows the formation of stars with smaller masses and may also considerably boost the overall rate at which stars are born. In fact, it is possible that the pace of star formation did not accelerate until after the first metals had been produced. In this case, the second-generation stars might have been the ones primarily responsible for lighting up the universe and bringing about the cosmic renaissance.
At the start of this active period of star birth, the cosmic background temperature would have been higher than the temperature in present-day molecular clouds (10 K). Until the temperature dropped to that level - which happened about two billion years after the Big Bang - the process of star formation may still have favored massive stars. As a result, large numbers of such stars may have formed during the early stages of galaxy building by successive mergers of protogalaxies. A similar phenomenon may occur in the modern universe when two galaxies collide and trigger a starburst - a sudden increase in the rate of star formation. Such events are now fairly rare, but some evidence suggests that they may produce relatively large numbers of massive stars. This hypothesis about early star formation might help explain some puzzling features of the present universe. One unsolved problem is that galaxies contain fewer metal-poor stars than would be expected if metals were produced at a rate proportional to the star formation rate. This discrepancy might be resolved if early star formation had produced relatively more massive stars; on dying, these stars would have dispersed large amounts of metals, which would have then been incorporated into most of the low-mass stars that we now see. Another puzzling feature is the high metal abundance of the hot X-ray emitting intergalactic gas in clusters of galaxies. This observation could be accounted for most easily if there had been an early period of rapid formation of massive stars and a correspondingly high supernova rate that chemically enriched the intergalactic gas. The case for a high supernova rate at early times also dovetails with the recent evidence suggesting that most of the ordinary matter and metals in the universe lies in the diffuse intergalactic medium rather than in galaxies. To produce such a distribution of matter, galaxy formation must have been a spectacular process, involving intense bursts of massive star formation and barrages of supernovae that expelled most of the gas and metals out of the galaxies.
While today we like to say that galaxies give birth to stars, back in early days (or years if you want) - just as protostars were forming - we had protogalaxies forming too. As stated above, although the early universe was remarkably smooth, the background radiation shows evidence of small-scale density fluctuations - clumps in the primordial soup. The cosmological models predict that these clumps would gradually evolve into gravitationally bound structures. Smaller systems would form first and then merge into larger agglomerations. The denser regions would take the form of a network of filaments, and the first star-forming systems - small protogalaxies - would coalesce at the nodes of this network. In a similar way, the protogalaxies would then merge to form galaxies, and the galaxies would congregate into galaxy clusters. The process is ongoing: although galaxy formation is now mostly complete, galaxies are still assembling into clusters, which are in turn aggregating into a vast filamentary network that stretches across the universe.
According to majority of current cosmological models, the first small systems capable of forming stars should have appeared between 100 million and 250 million years after the Big Bang. These protogalaxies would have been 100000 to one million times more massive than the Sun and would have measured about 30 to 100 light-years across. These properties are similar to those of the molecular gas clouds in which stars are currently forming in the Milky Way, but the first protogalaxies would have differed in some fundamental ways. For one, they would have consisted mostly of dark matter. The second important difference is that the protogalaxies would have contained no significant amounts of any elements besides hydrogen and helium. The big bang produced hydrogen and helium, but most of the heavier elements are created only by the thermonuclear fusion reactions in stars, so they would not have been present before the first stars had formed. Astronomers use the term "metals" for all these heavier elements. The young metal-rich stars in the Milky Way are called Population I stars, and the old metal-poor stars are called Population II stars; following this terminology, the stars with no metals at all - the very first generation - are sometimes called Population III stars. As Population III stars are oldest ones, one may wonder if we can see those or at least the light from them? In 2005 Spitzer Space Telescope did just that. The light from the earliest stars is still detectable amidst the infrared light that makes up the background of the observable universe. Although this light was initially high energy, ultraviolet light, it has shifted to lower and lower energies and wavelengths over time as the universe has expanded (redshift). By subtracting out light from the camera itself, along with that from our solar system, interstellar gas and dust, and, finally, the estimated light from all the stars, galaxies and other light sources from the last 13 billion years or so, scientists isolated what it believes is the dawn of light. Perhaps we missed something that we are not aware of, but picture below is the best we can get.
Stars that are more than 250 times more massive than the sun do not explode at the end of their lives; instead they collapse into similarly massive black holes. Several of the computer simulations mentioned above predict that some of the first stars would have had masses this great. Because the first stars formed in the densest parts of the universe, any black holes resulting from their collapse would have become incorporated, via successive mergers, into systems of larger and larger size. It is possible that some of these black holes became concentrated in the inner part of large galaxies and seeded the growth of the supermassive black holes—millions of times more massive than the sun—that are now found in galactic nuclei. I will write more on that in next post. Furthermore, astronomers believe that the energy source for quasars is the gas whirling into the black holes at the centers of large galaxies. If smaller black holes had formed at the centers of some of the first protogalaxies, the accretion of matter into the holes might have generated mini quasars. Because these objects could have appeared soon after the first stars, they might have provided an additional source of light and ionizing radiation at early times.
What about star masses today? We differentiate several types of stars today based on their life cycle and masses. Actually, stellar classification is quite complex so I will try to simplify it here. It all starts with protostar which is gravitational collapse of a giant molecular cloud (GMC) and contain up to 6000000 solar masses (1.2x1037 kg). Then brown dwarf comes into the being which is protostars with masses less than 1.6x1029 kg and they never reach temperatures high enough for nuclear fusion of hydrogen. On the other hand sub-stellar objects are brown dwarfs heavier than 2.5x1028 kg and they fuse deuterium, and some astronomers prefer to call only these objects brown dwarfs, classifying anything larger than a planet but smaller than this to be a sub-stellar object. Both types, deuterium-burning or not, shine dimly and die away slowly, cooling gradually over hundreds of millions of years. Finally, for a more massive protostar, the core temperature will eventually reach 10 million K, initiating the proton-proton chain reaction and allowing hydrogen to fuse, first to deuterium and then to helium. Based on size, we can have red, orange, yellow, white and blue dwarfs (so called main-sequence start). As you can see below, our Sun is yellow dwarf.
Mid-sized stars, once they spend their fuel, may potentially build planetary nebula which at the center will have star core. Once it cools down it becomes white dwarf. Massive stars are way too fat to survive so mostly they end up as supernovas, Such beasts at the end will form either neutron star or black hole (more on that next time). History channel made a nice documentary on life cycle and star masses and mechanism behind. I'm embedding it below.
Astronomy is cool, but it is not all about physics. Far away from it. It is also about chemistry and later on about biology. While it will take a while until we hit biology, this may be good point to mention chemistry bit. Nearly 13.7 billion years ago, the universe was made of only hydrogen, helium and traces of lithium - byproducts of the Big Bang initial stages. Then, some hundreds million years later, the very first stars emerged, creating additional chemical elements throughout the universe. Since then, giant stellar explosions, or supernovas, have given rise to carbon, oxygen, iron and the rest of the 94 naturally occurring elements of the periodic table. Today, stars and planetary bodies bear traces of these elements, having formed from the gas enriched by these supernovas over time. For the past 50 years, scientists have been analyzing stars of various ages, looking to chart the evolution of chemical elements in the universe and to identify the astrophysical phenomena that created them.
In February 2012, MIT has detected the element tellurium for the first time in three ancient stars. The researchers found traces of this brittle, semiconducting element (which is very rare on Earth) in stars that are nearly 12 billion years old. The finding supports the theory that tellurium, along with even heavier elements in the periodic table, likely originated from a very rare type of supernova during a rapid process of nuclear fusion. The researchers also compared the abundance of tellurium to that of other heavy elements such as barium and strontium, finding that the ratio of elements was the same in all three stars. The matching ratios support a theory of chemical-element synthesis: namely, that a rare type of supernova may have created the heavier elements in the bottom half of the periodic table, including tellurium.
According to theoretical predictions, elements heavier than iron may have formed as part of the collapsing core of a supernova, when atomic nuclei collided with huge amounts of neutrons in a nuclear fusion process. For 50 years, astronomers and nuclear physicists have modeled this rapid process, named the r-process, in order to unravel the cosmic history of the elements. Now, it was found that the ratios of heavy elements observed in the three stars matched the ratios predicted by these theoretical models. The findings confirm the theory that heavier elements likely formed from a rare, extremely rapid supernova as you can make iron and nickel in any ordinary supernova, anywhere in the universe. But these heavy elements seem to only be made in specialized supernovas. Adding more elements to the observed elemental patterns will help us understand the astrophysical and environmental conditions needed for this process to operate.
Credits: Adolf Schaller, NASA, ESA, National Radio Astronomy Observatory, Bill Saxton, MIT, American Association for the Advancement of Science, Naoki Yoshida, Richard B. Larson, Volker Bromm, Wikipedia, History Channel