Anaemic star carries mark of ancient ancestor

A newly discovered ancient star with record-low iron levels carries evidence of a class of even older stars long hypothesised but assumed to have vanished, researchers say.

Writing in the journal Monthly Notices of the Royal Astronomical Society: Letters, a team led by Australian Thomas Nordlander describes an ultra-metal-poor red giant star located in the halo of the Milky Way, about 35,000 light-years from Earth.

Nordlander, from the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), worked with colleagues from Australia, the US, Germany, Sweden and Italy. 

They located the star using the Australian National University’s SkyMapper Telescope. 

Spectroscopic analysis indicated it had an iron content of just one part per 50 billion.

“That’s like one drop of water in an Olympic swimming pool,” says Nordlander. “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.”

That’s enough to place the star, dubbed SMSS J160540.18–144323.1, into the record books, but it is what that low level implies about its origin that most interests them.

The first stars in the universe are thought to have consisted of only hydrogen and helium, 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. 

Stars like the Sun that are rich in heavy elements therefore contain material from many generations of stars exploding as supernovae.

As none of the first stars has yet been found, and may never be, their properties remain hypothetical. The hypothesis goes that they were huge – perhaps hundreds of times more massive than the Sun – and 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 new evidence.

Nordlander and colleagues suggest it 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, that has now been discovered.