Most Ancient Star Lurks On Our Cosmic Doorstep

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Records for furthest, biggest and brightest are always being broken in astronomy, but one of the more intriguing records has just been smashed. Australian astronomers have found a Methuselah star that likely formed 13.6 billion years ago. As the Universe is 13.75 billion years old, this object is a rare cosmic jewel that truly stood the test of time.

But the best thing about this star is that it lurks inside our galaxy, a mere 6,000 light-years away, providing us with a close-up view into a time capsule from the dawn of our cosmos.

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The star was discovered by astronomers at the Mount Stromlo Observatory in Australia who analyzed spectral data of millions of stars collected by SkyMapper at the Siding Spring Observatory, New South Wales. SkyMapper is an automated telescope that surveys Southern Hemisphere skies for planets, stars and asteroids. A few ‘low metallicity’ candidates were then studied using high-resolution spectral observations by the Magellan Telescopes in Chile. When they focused on the ancient star, called SMSS J031300.36-670839.3, Stefan Keller (of the Australian National University in Canberra) and his team noticed something strange.

“The telltale sign that the star is so ancient is the complete absence of any detectable level of iron in the spectrum of light emerging from the star,” Keller told AFP.

The theory is that the star’s extremely low metallicity is because it’s a “second generation” star, one that formed only a couple of hundred million years after the Big Bang. Second generation stars formed from the material cast off from the “first generation” of stars after they went supernova. Recent research suggests that not all of the first stars — formed from a primordial soup of gases, primarily hydrogen, helium and trace amounts of lithium — exploded energetically, however, adding a twist to our understanding about how the earliest stars formed. This means that heavier elements (heavier than helium) formed inside the fusion cores of some of these first stars were not ejected throughout interstellar space.

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In the case of SMSS J031300.36-670839.3, it appears that it formed primarily from the hydrogen and helium from the post-supernova material — leaving it extremely anemic. What’s more, the researchers think that this particular object formed from the remnants of only one first generation star.

Usually when astronomers analyze the spectra of stars, the chemical fingerprint of iron can be spotted. The more iron in the star, the younger it is. Each subsequent generation of star fuses more and more iron in their cores. As each generation of star reaches the end of its life and explodes as a supernova, the iron (and other heavy elements) from its interior is blasted throughout space. This iron intermingles with other interstellar gases that clump together, collapse and ignite to create the next generation of stars.

The iron fingerprint can therefore be used to “age” any stellar object, much like the rings in a log can be used to age a tree.

“We can use the iron abundance of a star as a qualitative ‘clock’ telling us when the star was formed,” said Keller.

SMSS J031300.36-670839.3, however, has no detectable sign of iron. Even within the margins for error, and astronomers assume an upper limit on the quantity of iron it contains, the star is still dated 13.6 billion years old. Previous “oldest star” record breakers have been dated to 13.2 billion years old.

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“In the case of the star we have announced, the amount of iron present is less than one millionth that of the sun and a factor of at least 60 times less than any other known star. This indicates that our star is the most ancient yet found,” he added. This research as been published in the journal Nature.

Interestingly, the star is also rich in carbon. This factor provides us with a unique insight into stellar formation in the earliest epochs of our universe.

In computational models by co-author Anna Frebel, assistant professor of physics and a member of MIT’s Kavli Institute for Astrophysics and Space Research, carbon formation was simulated inside a first generation star. The carbon was then transported to the upper layers of stellar material. As the star (thought to be around 60 times the mass of our sun) went supernova in a low-energy explosion, the outer layers (light elements, laced with an abundance in carbon) were blown away, leaving the iron-rich core behind. The iron-rich core was then likely swallowed by a black hole that formed in the supernova’s wake. It was the carbon-rich material that went on to form SMSS J031300.36-670839.3 13.6 billion years ago.

“By zooming in on an early star and finding something slightly unusual that goes a bit against the mainstream view, we’ve sort of rattled theory a little in a good way to say, ‘Maybe we have to rethink how the first stars formed,’” Frebel said in a news release.

Publication: A single low-energy, iron-poor supernova as the source of metals in the star SMSS J031300.36−670839.3, Keller et al., Nature, 2014. doi:10.1038/nature12990

Sources: Physorg, AFP

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