Artist's rendition of an ancient star (inset) divulges the explosion of one of the universe's first stars; cluster (main image). Image via National Astronomical Observatory of Japan

New research reveals that the stars that populated our early universe were enormous beasts, massive monstrosities that dwarfed our own sun. What's more, this discovery comes from our own backyard. Astronomers recently announced that they uncovered an ancient star, one that is floating through the cosmos a mere thousand light-years from Earth, which has chemical elements that appear to have been forged by the death of a supermassive star that was one of the first to appear after the Big Bang. Ultimately, if this new discovery is confirmed, it will alter our understanding of the formation of the universe's earliest galaxies, as the violent deaths of stars of this magnitude would greatly impact the growth of early galaxies. Moreover, the discovery could confirm our understanding of the nature of the universe's earliest stars.

Finding the Size of Early Stars:

Ultimately, in order for us to understand the science behind our theories regarding the size and formation of the earliest stars, we need to go all the way back to the Big Bang. The Big Bang was the violent event that birthed the universe as we know it. All of spacetime, and all the matter that is spread throughout it, was created by this amazingly powerful blast. However, a majority of the elements in the cosmos formed over time. Most elements were not thrust into existence at the moment of the Big Bang. Rather, they were formed under the intense heat and pressure found inside of massive stars. Indeed, the Big Bang itself produced only hydrogen, helium, and a little lithium. The rest of the elements in the universe only came to be after the earliest stars went supernovae.

This fact is important because gas clouds that only contain hydrogen, helium, and lithium can't cool. This, in turn, means that the gas clouds in the primordial universe were exceedingly large, much larger than they are today.

SDSS J0018-0939, a small, second-generation star bearing the chemical imprint of one of the universe's first stars. Image via SDSS/NAOJ

Because they could not cool, the early clouds could not split into smaller parts (cooler objects require less gravity to collapse, so cooling clouds tend to collapse at different rates, forming smaller stars of varying sizes). Because the primordial gas clouds stayed warm, they did not split apart, which resulted in the creation of larger stars.  These massive stars, those that were born 140 to 300 times as massive as the sun, exploded in a way unseen in the Milky Way today. These explosions took the form of a "pair-instability explosion," an event that is 10 to 100 time more powerful than an ordinary supernova. A pair-instability explosion heralds the death of a star that is so luminous that photons hold up its weight. However, because the star is so hot, the photons can convert themselves into pairs of electrons and antielectrons. These pairs exert very little outward pressure, so the star begins to collapse. This induces a chain reaction, where collapsing causes heating, which causes more pair formation, more collapsing, and then more heating. , and so on. Eventually, this runaway reaction leads to an epic stellar explosion. During the explosion, helium nuclei bombard one another, creating heavier elements. Because helium is atomic number 2, elements with even atomic numbers vastly outnumber odd-numbered ones, which is exactly the pattern that Wako Aoki, an astronomer at the National Astronomical Observatory of Japan in Tokyo, and his colleagues have discovered  in the star in Cetus.

The Nature Of The Beast:

In a recent report online in Science, it was announced that Aoki has discovered a star bearing signs of a pair-instability explosion. "This is a unique example," Aoki says. "We were very surprised by the chemical composition." In order to conduct this research, the team searched for 18 chemical elements in SDSS J0018-0939, a dim orange star in the constellation Cetus that emits less light than the sun. AAAS reports that, "the star belongs to the Milky Way's stellar halo, the ancient population that surrounds the galaxy's bright disk. Like other halo stars, it has little iron, because it arose before most of the stellar explosions that spewed the element into space." Notably, as previously mentioned, the scientists discovered that the star has more atomic elements with an even number, indicating that it was created by a pair-instability explosion (which could only come form the death of a massive star). Volker Bromm, an astronomer at the University of Texas, Austin, calls the discovery very important. "It really is a new window into star and element formation in the early universe," he says. "It's always interesting to see a star with abundances like no other," says Stan Woosley, an astronomer at the University of California, Santa Cruz. But he's not fully convinced those abundances signify a pair-instability supernova rather than an ordinary one. To distinguish between the two, he'd like observations of additional elements.


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