When we look up at the dark sky at night, we see a vast swath of blackness that has been set on fire by the distant, furious flames of billions and billions of incandescent stars. But where did the first stars come from, and when did they appear on this vast stage of blackness to brighten up a dismal scene?
early cosmos NASA
Indeed, the birth of the first stars in our Universe is one of the most intriguing mysteries haunting today's astronomers. The most ancient stars are thought to have caught fire as early as 100 million years after the inflationary Big Bang birth of the Universe. In January 2013, astronomers announced that they had discovered the oldest star seen so far to be bouncing around in our Universe. It is a mere 186 light-years from our own Solar System, making it a near neighbor, as stars go--and it is estimated to be at least 13.2 billion years old. The Universe itself is about 13.77 billion years old, and so this oldest of all known stars is almost as old as the Universe!
Astronomers now think that the first stars inhabiting the Cosmos were unlike the stars we know and love today. This is because they were born directly from primordial gases churned out in the Big Bang itself. The primordial gases were primarily hydrogen and helium, and these two lightest of all atomic elements are believed to have pulled themselves together to form ever tighter and tighter knots. The cores of the very first protostars to dwell in our Universe first started to ignite within the mysterious dark and very cold hearts of these extremely dense knots of pristine primordial hydrogen and helium--which collapsed under their own heavy gravitational weight. It is thought that the first stars were enormous (compared to the star's dwelling in the Cosmos today), because they did not form in the same way, or from the same elements, as stars do now. The first generation of stars are called Population III stars, and they were likely gigantic megastars. Our Sun is a lovely member of the most youthful generation of stars, and is a so-called Population I star. In between the first and most recent generations of stars are, of course, the Population II stars.
Extremely heavy Population III stars were also dazzlingly bright, and their existence is largely responsible for causing the sea-change of our Universe from what it was to what it now is! These enormous and brilliant stars changed the dynamics of our Universe by heating and thus ionizing the ambient gases.
The metallicity of a star refers to the percentage of its material that is made up of atomic elements heavier than the primordial hydrogen and helium. Because stars, which compose the lion's share of the visible (atomic) matter in the Universe, are composed mainly of hydrogen and helium, astronomers use (for convenience) the all-encompassing designation of metal when describing all of the elements of the Periodic Table that are heavier than hydrogen and helium. Both hydrogen and helium formed in the inflationary Big Bang --the heavier elements, however, were all born in the nuclear-fusing, searing-hot cores of our Universe's vast multitude of incandescent stars--or in their ultimate explosive deaths. Therefore the term metal, in astronomical terminology, possesses a different meaning than the same term has in chemistry. This term should not be confused with the chemist's definition of metal. Metallic bonds are impossible in the extremely hot cores of stars, and the very strongest of chemical bonds are only possible in the outer layers of cool "stars", such as brown dwarfs, which are not even stars in the strictest sense because, even though it is thought that they are born in the same way as normal stars, they are far too small for their nuclear-fusing fires to catch flame.
The metallicity of a star provides a valuable tool for astronomers to use, because its determination can reveal the star's age. When the Universe came into being, its "normal" atomic matter was almost entirely hydrogen which, through primordial nucleosynthesis, manufactured a large quantity of helium and small quantities of lithium and beryllium--and no heavier elements. Therefore, older stars (Populations II and III) show lowermetallicities than younger stars (Population I), like our lovely bouncing baby of a Sun. Nucleosynthesis refers to the process by which heavier elements are formed out of lighter ones, by way of nuclear fusion--the fusing of atomic nuclei.
The stellar Populations I, II, and III, reveal to astronomers a decreasing metal content with increasing age. Therefore, Population I stars, like our Sun, display the greatest metal content. The three stellar populations were named in this somewhat confusing way because they were designated in the order that they were discovered, which is the reverse of the order in which they formed. Therefore, the first stars to catch fire in our Universe (Population III) were depleted of metals. The stars bearing the highest metal content are the Population I stars, the youngest in our Universe.
Population II Stars
Population II stars are very ancient, but not as old as the Population III stars. Population II stars carry the metals manufactured in the searing-hot hearts of the first generation of stars, but they do not possess the higher metal content of stars like our Sun, which contain the metals forged in the hearts of the more ancient Population II stars.
Even though the most ancient stars contain fewer heavy elements than younger stars, the fact that all stars carry at least some scant quantity of metals presents a puzzle. The currently favored explanation for this puzzling observation is that Population III stars must have existed--even though not one Population III star has ever been observed. This line of reasoning suggests that in order for the ancient Population II stars to carry the small quantity of metals that they possess, their metals must have been created in the nuclear-fusing hearts of an earlier generation of stars.
Population II stars also possess very low metallicities, and are the oldest stars to be directly observed by astronomers. However, this must be kept in its proper perspective. Even metal-rich stars, the Population I stars like our Sun, contain only minute quantities of any element heavier than hydrogen or helium. In fact, metals (in the astronomical sense of the term), make up only an extremely small percentage of the overall chemical composition of the Universe. The elderly Population II stars were born during an ancient, remote epoch. So-called Intermedicate Population II stars are most common in the bulge near the center of the Milky Way; whereas Population II stars dwelling in the Galactic halo are considerably older and hence even more metal-poor. Globular clusters also harbor a large number of Population II stars.
The Oldest Star
The star, HD 140283, is a Population II star. It dwells near our Solar System, and it is the oldest star ever spotted by astronomers. HD 140283 is at least 13.2 billion years old--but it could be much older!
"We believe this star is the oldest known in the Universe with a well determined age," Dr. Howard Bond told the press in January 2013. Dr. Bond, of Pennsylvania State University in University Park, and his colleagues, announced the discovery of this ancient star on January 10, 2013, at the winter meeting of the American Astronomical Society (AAS) in Long Beach, California.
This very old star dwells a mere 186 light-years from our Solar System, and its close proximity made it a choice target for determining a precise age measurement. The star has been scrutinized by astronomers for over a century.
Astronomers have known for a very long time that HD 140283 is made up almost entirely of hydrogen and helium--the lower the metal content, the older the star. Therefore, it has long been suspected that HD 140283 is quite antiquated--but its precise venerable age had not previously been calculated.
Dr. Bond's team determined that the star is 13.9 billion years old--plus or minus 700 hundred million years. This does not cause a conflict with the 13.77 billion year age of the Universe itself, because the calculation lies within the experimental error bars.
The discovery places some constraints on ancient star formation, however. Population III stars coalesced from primordial hydrogen and helium, and did not contain sizable quantities of elements heavier than helium. This means that as antiquated as the elderly HD 140283 most assuredly is, its composition--which contains scant amounts of metals--means that it must have formed after the first generation of stars in our Universe--the Population III stars.
Therefore, conditions for the formation of Population II stars must have existed very early in the history of the Universe. Astronomers generally think that the first stars were born a few hundred million years after our Universe was born--but that they were massive, lived fast and furiously, and died young, in wild and gigantic supernova blasts that heated the ambient gas and blessed it with all of the elements heavier than hydrogen and helium.
However, before Population II stars could be born, that ambient gas had to cool off. The very old age of HD 140283 suggests that this cooling-off time, that existed between the first and second generations of stars, might have been brief by cosmological standards--a mere few tens of millions of years.
This research was published on January 10, 2013 in the journal Nature.
author: Judith E. Braffman-Miller