The Mystery Hidden In Ancient Stardust

Where did the stars come from? These shimmering spheres of glittering silvery light are generally thought to have ignited, soon after the Universe was born, in the exponential, wild inflation of the Big Bang about 13.8 billion years ago–lighting up with their newborn stellar flames what had been a strange, eerie, featureless swath of unimaginable blackness. The first generation of stars to be born in the primordial Cosmos were not like the stars we are familiar with today, because they were born directly from the lightest of all pristine gases–hydrogen and helium–formed in the fireball of the Big Bang. In March 2017, a team of astronomers announced that they have used the Atacama Large Millimeter/submillimeter Array (ALMA), located in the Atacama desert of northern Chile, to observe an enormous mass of glowing stardust in a galaxy far, far away–seen as it was long ago when the baby Universe was only four percent of its current age. This distant galaxy was observed as it was soon after its formation, and it is the most remote galaxy in which dust has been detected by astronomers–thus shedding new light into the mysterious birth and explosive deaths of the very first stars.

In astronomy, long ago is the same as far away. The more distant a celestial object is in Space, the more ancient it is in Time. This is because its traveling light has taken a longer time, wandering its beaming way through the Universe, to reach the prying eyes of curious astronomers on Earth. No known signal can travel faster than light in a vacuum, and this “universal speed limit” means that the light flowing out into Space from distant objects takes a longer time to reach Earth than objects that are closer to us, because of the expansion of the Universe. Therefore, we see distant objects as they were long ago when they first hurled their fabulous light out into the mysterious expanse of Spacetime.

The new results, derived from ALMA, are also important because they reveal the most remote detection of oxygen in the Universe. An international team of astronomers, led by Dr. Nicolas Laporte of University College London (UK), used ALMA to observe A2744_YD4–the most remote and youngest galaxy ever seen by ALMA. The astronomers were surprised to find that this young galaxy contained an abundant amount of interstellar dust. Interstellar dust is produced by the explosive deaths of an earlier generation of stars.

Additional observations were performed using the X-shooter instrument on the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT). X-shooter confirmed the great distance to A2744_YD4. Indeed, this galaxy appears to us as it was when the Universe was a mere baby at only 600 million years old–the ancient era when the first generation of stars and galaxies were forming.

“Not only is A2744_YD4 the most distant galaxy yet observed by ALMA, but the detection of so much dust indicates early supernovae must have already polluted this galaxy,” explained Dr. Laporte in a March 8, 2017 ESO Press Release.

Cosmic dust is primarily made up of carbon, aluminum, and silicon–composed of tiny motes as small as a millionth of a centimeter across. The chemical elements in these tiny grains are formed in the searing-hot furnaces of stars, and they are hurled across Space and Time when the stars perish–most dramatically in the fiery fury of supernovae blasts, which herald the demise of massive stars that live fast and die “young”. The more massive the star, the shorter its stellar “life.” In the Cosmos today, dust is plentiful and serves as an important building block in the formation of more recent generations of stars, planets, and complex molecules. In contrast, in the ancient Universe–before the first generations of stars had yet perished–cosmic dust was rare.

Stellar Generations

In 1944, Walter Baade (1893-1960) categorized collections of stars in our Galaxy from their spectra. Two primary divisions were defined as Populations I and II–with another division named Population III added in 1978. Walter Baade was a German astronomer who did his work in the United States at Mount Wilson, Palomar Observatories in California. The differences between the three stellar populations were ultimately shown to be extremely significant, separating the populations of stars into classes based on their chemical composition–or metallicity. Each of the three stellar population groups reveal that there is a decreasing metal content with increasing age. Therefore, the first generation of stars in the Universe (low metal content) were named Population III, and younger stars (high metallicity)–such as our Sun–were categorized as Population I. In between the most ancient (Population III) stars, and the most recent (Population I) stars, are the Population II stars.

In the terminology that astronomers use, all of the atomic elements heavier than helium are termed metals. The Big Bang manufactured only the lightest of atomic elements–hydrogen, helium, and small amounts of lithium and beryllium. Literally, all of the heavier atomic elements, listed in the familiar Periodic Table, were created within the searing-hot, nuclear-fusing furnaces of the Universe’s myriad of fiery, brilliant stars–or, alternatively, in the terrible fury of the beautiful supernovae that heralded the deaths of the most massive stars inhabiting the Cosmos. This means that the Population III stars were pristine spheres of mostly hydrogen gas produced in the Big Bang. There had been no earlier generation of fiery stars to create the “metals”.

Population II stars are ancient, but not as ancient as the first stars–the Population III stars. However, a problem with this classification arose because even the most metal-poor Population II stars contain some trace amounts of metals. This means that there had to be an earlier generation of stars that existed before Population II. A slow increase in stellar metallicity occurred with progressively younger generations of stars. However, at present, no Population III star has definitely been directly observed.

The mysterious Population III stars are generally believed to have been born in pure primeval nurseries composed of only the lightest gases produced in the Big Bang. The first stars were likely very massive and, as a result, short-lived. These heavy primordial stars died dramatically in supernova explosions that shot their newly manufactured batch of heavy atomic metals out into space. In this way, the freshly fused heavy metals were made available to younger stellar generations–“polluting” their natal dark, giant, frigid molecular clouds with ancient stardust.

Small stars like our Sun can linger in the Universe for billions of years before they finally come to the inevitable end of that long stellar road. Very ancient, relatively small stars, that may have been born in the ancient Universe, may still be hanging around–and ready to tell their story. Our own Milky Way Galaxy hosts a population of low-mass, ancient, long-lived little stars.

Stars that are similar to our Sun, and denizens of the relatively youthful Population I generation, possess the largest content of metals. The primordial Population III stars, in contrast, would have been born metal-free. The ancient Population II stars contain small quantities of metals manufactured by the Population III stars, as a result of stellar nucleosynthesis–a process that happens within a star’s intensely hot furnace, as light atomic elements are fused into heavier ones. Low metallicity Population II stars are the oldest stars to be observed directly by astronomers. Indeed, the primordial existence of the extremely massive Population III stars is generally thought to be responsible for causing a sea-change in the Universe. These giant first stars dramatically changed the dynamics of the Universe by heating and, as a result, ionizing the ambient gas.

Although they are commonly thought to have been spread out everywhere in the ancient Universe, metal-poor stars today are rarities both within our own Galaxy and other nearby galaxies. As time passed, younger generations of stars were born in cradles composed of increasingly metal-rich stardust. In contrast, stars that are metal-poor were born in “unpolluted” cradles that existed shortly after the Big Bang.

The Mystery Hidden In Ancient Stardust

The observations of the dusty galaxy A2744_YD4 were possible because this remote galaxy is fortuitously situated behind a massive cluster of galaxies dubbed Abell 2744. This is because of a phenomenon termed gravitational lensing. The foreground galaxy cluster behaved like an enormous cosmic “telescope”, magnifying the more distant background galaxy approximately 1.8 times. This enabled the team of astronomers to look far back in time to a mysterious era, very long ago, when the Universe was young.

Gravitational lensing is a prediction of Albert Eistein’s Theory of General Relativity (1915), which proposed that light traveling from a background object can be warped, bent, and magnified by a massive object in the foreground–thus having lens-like effects.

The ALMA observations also made the important discovery of ionized oxygen from A2744_YD4. This is the most remote and, therefore, earliest detection of oxygen in the Universe. The previous record-holder was another ALMA discovery made in 2016.

The discovery of cosmic dust in the ancient Universe provides important new information about when the first supernovae exploded–and, therefore, the time when the first searing-hot stars sent their brilliant, dancing light out into space. Determining the timing of this “cosmic dawn” is one of modern astronomy’s long sought holy grails, and it can be indirectly observed through the study of ancient interstellar dust.

The team of astronomers has estimated that A2744_YD4 contains a quantity of cosmic dust that amounts to approximately 6 million times solar-mass. For comparison, the galaxy’s total mass–the mass of all its stars–is equivalent to 2 billion times the mass of our Star. The astronomers also measured the stellar birth rate of A2744_YD4 and found that stars are being born at the rate of 20 solar masses annually–compared to just one solar-mass per year in our Milky Way Galaxy.

“This rate is not unusual for such a distant galaxy, but it does shed light on how quickly the dust in A2744_YD4 formed. Remarkably, the required time is only about 200 million years–so we are witnessing this galaxy shortly after its formation,” noted Dr. Richard Ellis in the March 8, 2017 ESO Press Release. Dr. Ellis is of the ESO and University College London, and a co-author of the study.

Essentially, this suggests that significant star-birth began about 200 million years before the era at which the galaxy is being observed. This provides an important opportunity for ALMA to help study the ancient era when the first stars and galaxies ignited–the earliest epoch yet probed. Our Sun, our Earth, and our very existence resulted from the stardust created in the nuclear-fusing hearts of the Universe’s myriad of sparkling, dancing stars. We are here, 13 billion years later, because of the ancient existence of stars. By studying stellar birth, stellar “lives”, and stellar deaths, astronomers are really exploring our origins.

“With ALMA, the prospects for performing deeper and more extensive observations of similar galaxies at these early times are very promising,” Dr. Ellis added.

As Dr. Laporte concluded in the March 8, 2017 ESO Press Release: “Further measurements of this kind offer the exciting prospect of tracing early star formation and the creation of the heavier chemical elements even further back into the early Universe.”

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