Thursday, August 21, 2014

Subaru Telescope: Traces of One of Universe's First Stars Detected

The most massive stars in the early universe would eject material high in iron when they exploded. 

Astronomers can read the composition of the next generation of stars to determine what made up their ancestors.

Credit: National Astronomical Observatory of Japan

An ancient star in the halo surrounding the Milky Way galaxy appears to contain traces of material released by the death of one of the universe's first stars, a new study reports.

The chemical signature of the ancient star suggests that it incorporated material blasted into space by a supernova explosion that marked the death of a huge star in the early universe, one that may have been 200 times more massive than the sun.

"The impact of very-massive stars and their explosions on subsequent star formation and galaxy formation should be significant," lead author Wako Aoki, of the National Astronomical Observatory of Japan, told by email.

Hidden giants
The first stars in the cosmos, known as Population III stars, formed from the hydrogen and helium that dominated the early universe.

Through nuclear fusion, other elements were forged in their hearts. At the end of their lifetimes, supernovas scattered these elements into the space around them, where the material was folded into the next generation of stars.

The universe's first massive stars would have been short-lived, so to determine their composition, scientists must examine the makeup of their offspring, stars that formed from the material distributed by their explosive deaths.

While numerical simulations have suggested that at least some of the first stars should have reached enormous proportions, no previous observational evidence had managed to confirm their existence.

Aoki and a team of scientists used the Subaru Telescope in Hawaii to perform follow-up observations of a large sample of low-mass stars with low quantities of what astronomers term "metals," elements other than hydrogen and helium. 

They identified SDS J0018-0939, an ancient star only 1,000 light-years from Earth.

"The low abundance of heavy elements suggests that this star is quite old — as old as 13 billion years," Aoki said.
(Scientists think the Big Bang that created the universe occurred approximately 13.8 billion years ago.)

The chemical composition of SDS J0018-0939 suggests it gobbled up the material blown off of a single massive ancient star, rather than several smaller bodies.

If multiple supernovas had provided the material that constructed the star, the "peculiar abundance ratios" in its interior would have been erased, Aoki said.

Volker Bromm of the University of Texas, Austin agrees, saying that SDS J0018 likely evolved from the material from a single star, which could have been more than 200 times as massive as the sun.

Bromm, who has performed theoretical studies on the properties of the first generation of stars and their supernova explosions, did not participate in the new study.

He authored a corresponding "News & Views" article that appeared with the research online today (Aug. 21) in the journal Science.

Signs of low-mass first-generation stars have appeared to be more plentiful in their descendants, which contain large amounts of carbon and other light elements, but until these results, scientists had detected no traces of their very massive siblings.

The scarcity suggested that low-mass stars were more numerous in the early universe.

"We have come to understand that the first stars had a range of masses, from a few solar masses, all the way up to 100 solar masses, or even more," Bromm told reporters.

"The typical, or average, mass is predicted to be somewhere close to a few tens of solar masses.".

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