The Dazzling Stellar Legacy Of The First Stars

When we look up at our sky on a clear night, we see a canvas of incredible blackness that is sprinkled with the distant fires of countless dazzling stars. How did these fiery stars come into being–and where did they come from? The first stars to shatter the primeval darkness of the ancient Universe were mysterious objects that were responsible for our very existence–we would not be here if the first stars had not forged literally all of the atomic elements heavier than helium in their searing-hot, fiery hearts. The iron in our blood, the calcium in our bones, the oxygen we breathe, the water that we drink, the sand beneath our feat, and the carbon that is the basis of life on Earth, were all created by stars–that shot their batches of freshly forged, heavy, life-sustaining elements screaming out into space when they “died,” after having burned up their necessary hydrogen fuel. In May 2019, astronomers at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, announced their new findings that, instead of inflating into spheres, as scientists once thought, ancient asymmetric supernova blasts may be responsible for seeding bright new baby stars that made life possible on Earth, and wherever else life may exist in the Cosmos.

Several hundred million years after the Big Bang birth of the Universe, that is thought to have occurred about 13.8 billion years ago, the very first generation of stars ignited, lighting up the Universe in the form of gigantic glaring globs of hydrogen and and helium gas. Within the hot cores of these first primeval stars, extreme thermonuclear reactions forged the first batch of heavier elements, including carbon, iron, and zinc.

It has been proposed that the first stars were probably giant fireballs that lived fast and died young. The bigger the star; the shorter its life. Massive stars burn their fuel faster than their smaller stellar siblings because they are much hotter. Hence, they live for only millions of years, while their less hefty kin shine brightly for billions–or even trillions–of years, on the hydrogen-burning main sequence of the Hertzsprung-Russell Diagram of Stellar Evolution. Astrophysicists have assumed for many years that these ancient, massive stars exploded as similarly spherical supernovae.

However, the team of astronomers at MIT and other institutions, have now found that these first stars may have blown themselves to smithereens in a much more powerful and asymmetric blast, hurling out jets howling into space that were sufficiently violent to eject heavy atomic elements into nearby galaxies. These newly forged elements–the first of their kind in the ancient Cosmos–served as the precious seeds for the second generation of stars, some of which can still be seen dancing brightly in our Universe today.

In a research paper published in the May 8, 2019 issue of the Astrophysical Journal, the sientists report a large amount of zinc in HE 1327-2326, which is an ancient stellar survivor that is among the Universe’s second generation of stars. They believe that the star could only have managed to get such an abundant quantity of zinc as a result of an asymmetric supernova blast that heralded the “death” of one of the very first stars to inhabit the primordial Cosmos. The now-vanished, short-lived, first generation star thus enriched the younger second-generation star’s natal cloud of gas with its freshly forged batch of heavier atomic elements.

“When a star explodes, some proportion of that star gets sucked into a black hole like a vacuum cleaner. Only when you have some kind of mechanism, like a jet that can yank out material, can you observe that material later in a next-generation star. And we believe that’s exactly what could have happened here,” Dr. Anna Frebel explained in a May 8, 2019 MIT Press Release. Dr. Frebel is an associate professor of physics at MIT and a member of MIT’s Kavli Institute for Astrophysics and Space Research.

“This is the first observational evidence that such an asymmetric supernova took place in the early Universe. This changes our understanding of how the first stars exploded,” commented Dr. Rana Ezzeddine, who is a postdoc at MIT, and the study’s lead author.

Stellar Generations

The first generation of stars were not like the stars we see today. This is because the first stellar generation was born directly from pristine hydrogen and helium–the two lightest atomic elements in the familiar Periodic Table. Both hydrogen and helium were born in the Big Bang (Big Bang nucleosynthesis). It is believed that the first stars were both gigantic and extremely brilliant, and their existence changed our Universe from what it was to what it now is.

There are three generations of stars. Our Sun is a member of Population I, meaning that it is a member of the youngest stellar generation. Population III stars are the most ancient, and they formed out the pristine gas that lingered after the Big Bang. In the jargon of astronomers, all atomic elements heavier than helium are called metals. Therefore, the term metal, as used by astronomers, is different from the same term when it is used by chemists. Population II stars are stars that are sandwiched between Populations I and III. These stars are older than our Population I Sun, but younger than the first stars of Population III. The first stars were depleted of metals, but the Population II stars show trace quantities of the metals forged in the hot hearts of the Population III stars. Population I stars, like our Sun, have the greatest metal content. However, this neat classification is somewhat misleading. This is because all stars, regardless of their generation, are roiling balls composed primarily of hydrogen gas.

Because metals can only be produced by way of the process of stellar nucleosynthesis, the existence of even trace quantities of metals indicates that an earlier Population of stars had to exist before the Population II stars were born. There had tp have been a population of stars that existed before them in order to create these metals. The Population III stars, which no longer exist in the visible Universe, left their chemical “footprints” behind in the generation of stars that came after them, and these stellar “footprints” tell of that now-vanished primordial population of the most ancient generation of stars.

Astronomers roughly categorize stars as either Population I (high metal content) or Population II (low metal content). But, because even the most metal-poor Population II stars sport a small quantity of metals, they reveal that their composition is composed of more than only the pristine primordial gas that formed in the Big Bang birth of the Universe. The Population III stellar giants were made up of only the lightest of pristine gases: hydrogen, helium, and scant amounts lithium. Therefore, the gas that composes Population III stars was not “polluted” by the heavy metals forged in the hot hearts of earlier stars. Population III stars triggered the gradual increase in stellar metallicity in increasingly younger and younger generations of stars.

Population III stars are generally thought to have been born in pure cradles of unpolluted gas. Numerical computer simulations have shed light on the very ancient and mysterious star-forming process, and the extremely short life-span of the first stars. The gigantic Population III stars did not go gentle into that good night, and they noisily blasted themselves to pieces in brilliant supernova explosions, that hurled their supply of newly-formed metals howling noisily into the space between stars. This made the newborn heavier atomic elements available to be incorporated into the giant cold, dark molecular clouds of gas and dust that served as the strange nurseries for later generations of more metal-rich stars.

Because the first stars were so massive, they rapidly used up their necessary supply of pristine hydrogen gas–and then blasted themselves to shreds in what were likely extraordinarily powerful, brilliant, and violent supernovae. Population III stars burned out at a comparatively youthful age by star-standards. These ancient supernovae were largely responsible for triggering a remarkable sea-change in the Universe. These stellar dazzlers changed utterly the dynamics of the Universe by heating it up. This new heat ionized the ambient gas.

The Lingering Legacy Of The First Stars

Dr. Frebel discovered the tattle-tale star, dubbed HE 1327-2326, in 2005. At the time, the star held the title of the most metal-deficient star known. This means that it sported extremely low concentrations of elements heavier than hydrogen and helium, indicating that it was a Population II star. HE 1327-2326 was born at a time when most of the Universe’s heavy metals had not yet been forged.

“The first stars were so massive that they had to explode almost immediately. The smaller stars that formed as the second generation are still available today, and they preserve the early material left behind by these first stars. Our star has just a sprinkle of elements heavier than hydrogen and helium, so we know it must have formed as part of the second generation of stars,” Dr. Frebel explained in the May 8, 2019 MIT Press Release.

“People thought from early observations that the first stars were not so bright or energetic, and when they exploded, they wouldn’t participate much in reionizing the Universe. We’re in some sense rectifying this picture and showing, maybe the first stars had enough oomph when they exploded, and maybe now they are strong contenders for contributing to reionization, and for wreaking havoc in their own little dwarf galaxies,” Dr. Frebel added.

The first supernovae that heralded the explosive death of the first stars may also have been sufficiently powerful to shoot their newly formed batch of heavy metals into nearby “virgin galaxies” that had yet to give birth to stars of their own.

Dr. Frebel continued to explain that “Once you have some heavy elements in a hydrogen and helium gas, you have a much easier time forming stars, especially little ones. The working hypothesis is, maybe second generation stars of this kind formed in these polluted virgin systems, and not in the same system as the supernova explosion itself, which is always what we had assumed, without thinking in any other way. So this is opening up a new channel for early star formation.”

Natasha M. McKnight

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