The birth of the primeval first generation of fiery stars is cloaked in alluring mystery–their attributes remain undetermined. The oldest stars are believed to have ignited as early as 100 million years after the Big Bang birth of our Universe about 13.8 billion years ago, casting their lovely, brilliant, raging fires into the swath of incredible, featureless darkness that was our primordial Universe before the stars were born. For decades, astronomers have theorized about the existence of this first ancient generation of stars–known as Population III stars–that emerged from the pristine material formed in the Big Bang. In June 2015, astronomers using the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT), announced that they may have solved this mystery when they discovered what is by far the most brilliant galaxy yet seen in the ancient Universe, finding strong evidence that some members of the mysterious and elusive first generation of stars may be haunting it. These sparkling, brilliant, fiery ancient stars–purely hypothetical objects–may have at last been found hiding in that distant galaxy, which is three times brighter than the brightest distant galaxy known up to now.
Population III stars were the creators of the very first batch of heavy elements in the Cosmos. After the Big Bang, the Universe knew only hydrogen, helium, and trace quantities of lithium–all of the atomic elements heavier than helium, which are termed metals by astronomers, were cooked up in the searing hot, nuclear-fusing cores of the stars, their stellar furnaces progressively fusing lighter atomic elements into heavier ones. Hydrogen is the lightest and most abundant atomic element in the Universe, and helium is the second-lightest. The existence of these heavy metals, cooked up in the hot hearts of ancient stars, were necessary to give birth to the kind of stars that we see today, the planets that circle them, and life as we know it. The oxygen we breathe, the carbon that is the basis for life on Earth, the dirt beneath our feet, the iron in our blood, all exist because ancient stars were there to create them within their secretive, seething, roiling hot hearts.
Astronomers have long hypothesized the existence of Population III stars, born from the very light primeval material of the Big Bang. Because all metals were created in the nuclear-fusing cores of stars, this means that the first stars must have formed out of the only atomic elements that existed before the stars were there–hydrogen, helium, and a pinch of lithium.
It is commonly thought that these Population III stars would have been behemoths–several hundred or even a thousand times more massive than our own fiery Sun. The first stars were likely searing-hot, and short-lived–blasting themselves to shreds in the rage of supernovae after only about two million years of nuclear-fusing stellar existence. However, until now, the hunt for direct, physical proof of the existence of these ancient, enormous stars, has been inconclusive.
Population III stars were unlike the stars we know, love, and wish upon today. Pristine hydrogen and helium are believed to have somehow pulled themselves together to create increasingly tighter and tighter knots. The first stars did not form in the same way, or from the same elements, as stars do now. Population III stars were likely dazzling, monster-size giants. Our Sun is a glittering, glaring member of the youngest generation of stars, and is designated a Population I star. In between the first and most recent generations of stars is the stellar “sandwich generation”, appropriately dubbed Population II stars.
An Ancient Stellar Story
Back in the 1940s, the German astronomer Walter Baade (1893-1960), who did his work in the United States from 1931 until 1959, divided the stars observed in galaxies into two populations (I and II). Even though a more sophisticated method of classifying stellar populations has since been devised, astronomers have continued to classify stars as Populations I, II, and III. More refined modern methods, however, classify them according to whether they are found in the galactic thin disk, thick disk, halo or bulge. However, astronomers still continue to broadly define stellar populations as either Population I (metal-rich) or Population II (metal-poor). However, even the most metal poor Population II stars show metallicities (Z/H) considerably greater than that of the relic gas left over from the Big Bang.
It was for this reason that astronomers proposed the existence of a third class of star: Population III. Because Population III stars are composed entirely of pristine primordial gas, the gas from which Population III stars were born had not been incorporated into–and then ejected from–earlier generations of stars. The earliest generation of stars were formed out of the pure, unpolluted material left over from the very beginning of the Universe, and were also the very first generation of stars to be born within a galactic host. These Population III stars are thought to have manufactured the metals observed in Population II stars and start the gradual increase in metallicity across subsequent stellar generations.
The metallicity of a star provides a valuable tool for astronomers to use because its determination can reveal a star’s true age. When the Universe was born, its “ordinary” atomic matter was almost entirely hydrogen–along with lesser amounts of helium, and only trace quantities of lithium and beryllium–and no heavier atomic elements at all (Big Bang nucleosynthesis). Therefore, older generations of stars (Populations II and III) show lower metallicities than younger stars (Population I), like our Sun, that show the highest metal content. The three populations of stars were named in this backward way because they were classified in the order that they were first discovered, which is the reverse of the order in which they were born. Therefore, Population III stars were depleted of heavy metals.
Even though older stars carry fewer heavy metals than younger stars, the fact that Population II stars contain at least some small quantity of metals is a mystery. The currently most popular explanation for this puzzle is that Population III stars must have existed–even though not one Population III star has ever been seen. In order for the ancient Population II stars to carry a small amount of metals, their metals must have been formed in the nuclear-fusing, hot hearts of an earlier generation of stars. Population II stars are the oldest stars to be directly observed by astronomers.
A number of explanations have been suggested to explain why no Population III stars have ever been observed. One explanation states that the majority of Population III stars would have used up their necessary supply of fuel long ago and would now be observed only as stellar corpses-white dwarfs, neutron stars, or black holes.Therefore, the original composition of the oldest generation of stars is almost impossible to determine. However, this explanation alone cannot account for the absence of Population III stars because those with the lightest masses should still be around–although they would be difficult to detect because of their very low luminosities.
A second explanation put forth is that stars accrete gas from interstellar space as they travel through it, and this may contaminate the outer layers of the otherwise pristine Population III stars. This would cause Population III stars to masquerade as metal-poor Population II stars.
Another explanation is that the metals forged in the fiery cores of Population III have been dredged up to the surface by convection. Such “self-contaminated” Population III stars would also most likely be misclassfied as metal-poor Population II stars.
The currently most popular explanation for the lack of observed Population III stars is that they were all extremely massive, with almost unimaginable masses ranging from 60 to 300 times that of our Sun. This would mean that no low mass Population III stars were born. Indeed, many theoretical models indicate that primordial Population III stars carried considerably greater masses than the stars observed in the Universe today. If this is the case, then all Population III stars would have used up their necessary supply of nuclear fuel very long ago and would now be present only as relics like black holes.
The problem is that Population III stars are currently entirely hypothetical objects. Despite a dedicated hunt for them by astronomers, these most ancient of stars have never been observed directly.
Primordial Stars Haunt A Bright And Ancient Galaxy!
A team of astronomers led by Dr. David Sobral of the Institute of Astrophysics and Space Sciences, the Faculty of Sciences of the University of Lisbon in Portugal, and Leiden Observatory in the Netherlands, has now used ESO’s VLT to look very far back in time to that remote, dark, and mysterious era known as reionization. Peering back into the ancient Universe, to a “mere” 800 million years after its Big Bang beginning, the astronomers believe that they have spotted signs of the elusive Population III stars. Rather than conducting a narrow and deep observation of a small portion of the sky, they broadened their scope to produce the widest survey of extremely distant, ancient galaxies ever attempted.
Their careful and exhaustive study was made using the VLT, along with the aid of the W.M. Keck Observatory and the Subaru Telescope, as well as the NASA/European Space Agency (ESA) Hubble Space Telescope (HST). The team discovered, and confirmed, a significant number of extremely remote and very bright young galaxies inhabiting the early Universe. One of these galaxies, named CR7 was an exceptionally rare object, by far the most brilliant galaxy ever seen at this ancient stage of our Universe’s history. With the discovery of CR7 and other bright galaxies, the study was already highly productive, but additional study provided even more exciting news.
The X-shooter and SINFONI instruments on the VLT spotted strong ionized helium emission from CR7 but–surprisingly–no indication of heavy metals in an especially brilliant pocket within that very distant galaxy. This meant that the team had spotted strong evidence for clusters of the elusive first generation of stars that had ionized gas within the galaxy in the very early Universe.
“The discovery challenged our expectations from the start, as we didn’t expect to find such a bright galaxy. Then, by unveiling the nature of CR7 piece by piece, we understood that not only had we found by far the most luminous distant galaxy, but also started to realize that it had every single characteristic expected of Population III stars. Those stars were the ones that formed the first heavy atoms that ultimately allowed us to be here. It doesn’t really get more exciting than this,” Dr. Sobel explained in a June 17, 2015 ESO Press Release.
Both bluer and somewhat redder stellar clusters were observed within the CR7 galaxy, suggesting that the birth of Population III stars had occurred in waves–as predicted by some theories. Therefore, the team has directly observed the final wave of Population III stars. This means that such stars should be easier to spot than previously believed because they dwell among later generations of stars in brighter galaxies–and not exclusively within the smallest, faintest, and earliest galaxies, which are so dim that their observation presents quite a challenge.
The second author of the new study, Dr. Jorryt Matthee, told the press on June 17, 2015 that, “I have always wondered where we come from. Even as a child I wanted to know where the elements came from: the calcium in my bones, the carbon in my muscles, the iron in my blood. I found out that these were first formed at the very beginning of the Universe, by the first generation of stars. With this discovery, remarkably, we are starting to actually see such objects for the first time.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.