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.

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