'Anaemic' star carries link to ancient ancestor

A newly discovered ancient star, containing what is said to be a record-low amount of iron, carries evidence of a class of even older stars — long hypothesised but assumed to have vanished.

Researchers led by Dr Thomas Nordlander, from the ANU node of the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), have confirmed the existence of an ultrametal-poor red giant located in the halo of the Milky Way on the other side of the galaxy, about 35,000 light-years from Earth. Working with colleagues from Australia, the US and Europe, Dr Nordlander located the star, dubbed SMSS J160540.18–144323.1, using the ANU’s dedicated SkyMapper telescope at the Siding Springs Observatory in NSW.

Spectroscopic analysis, published in the journal Monthly Notices of the Royal Astronomical Society: Letters, indicated that the star had an iron content of just one part per 50 billion. According to Dr Nordlander, “That’s like one drop of water in an Olympic swimming pool.”

He continued, “This incredibly anaemic star, which likely formed just a few hundred million years after the Big Bang, has iron levels 1.5 million times lower than that of the Sun.” And while its low iron content is enough to place the star in the record books, astronomers are even more excited about what that low level implies.

The very first stars in the universe are thought to have consisted of only hydrogen and helium, along with traces of lithium. These elements were created in the immediate aftermath of the Big Bang, while all heavier elements have emerged from the heat and pressure of cataclysmic supernovae — titanic explosions of stars. Stars like the Sun that are rich in heavy element therefore contain material from many generations of stars exploding as supernovae.

As none of the first stars have yet been found, their properties remain hypothetical. They were long expected to have been incredibly massive, perhaps hundreds of times more massive than the Sun, and to have exploded in incredibly energetic supernovae known as hypernovae. The confirmation of the anaemic SMSS J160540.18–144323.1, although itself not one of the first stars, adds a powerful piece of evidence to this hypothesis.

Dr Nordlander and colleagues suggest that the star was formed after one of the first stars exploded. That exploding star is found to have been rather unimpressive — just 10 times more massive than the Sun — and to have exploded only feebly (by astronomical scales) so that most of the heavy elements created in the supernova fell back into the remnant neutron star left behind.

Only a small amount of newly forged iron escaped the remnant’s gravitational pull and went on, in concert with far larger amounts of lighter elements, to form a new star — one of the very first second-generation stars, like the one the researchers have discovered.

Co-researcher Professor Martin Asplund, a chief investigator of ASTRO 3D at ANU, said it is unlikely that any true first stars have survived to the present day. “The good news is that we can study the first stars through their children,” he said.

Image caption: A visualisation of the formation of the first stars. Image credit: Wise, Abel, Kaehler (KIPAC/SLAC).

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