Recently astronomers discovered one of the oldest stars of our universe, likely belonging to the second generation of stars after the Universe burst into existence 13.8 billion years ago.
Another ancient star in the Milky Way was discovered lurking. A red giant star called SMSS J160540.18–144323.1 was discovered to have the lowest iron concentrations of any star yet analyzed in the galaxy about 35,000 light-years away.
This implies that it is one of the oldest stars in the Universe, likely belonging to the second generation of stars after the Universe burst into existence 13.8 billion years ago.
“This extremely anaemic star, which probably formed just a few hundred million years after the Big Bang, has iron levels 1.5 million times lower than that of the Sun,” stated astronomer Thomas Nordlander of the ARC Center of Excellence for All-Sky Astrophysics in 3-Dimensional and the Australian National University.
“It’s like a drop of water in an Olympic pool.”
And that’s how we can say how old the star is because there were no metals in the very early Universe. The first stars were mainly made up of hydrogen and helium and believed to be very large, very hot, and very short-lived. These stars are called, Population III and we have never seen them.
Stars are powered by nuclear fusion, where they combine the atomic nuclei of lighter elements to generate heavier ones. This is primarily the fusion of hydrogen into helium in smaller stars. But elements up to and including silicon and iron can be forged in bigger stars-such as the Population III stars are believed to have been.
When such stars end their lives in spectacular explosions of supernova, they burst these elements into the Universe. The components are captured in them as new stars form-and thus, how much metal a star contains is a reliable indicator of when it was formed.
For example, we know the Sun is about 100,000 generations from the Big Bang based on the metallicity of our star.
But in the Milky Way, we have discovered other stars with low metallicity, suggesting an early origin of the Universe. One such object is 2MASS J18082002–5104378 B, the former record holder for the lowest iron content of [Fe/H] = −4.07 ± 0.07 — about 11,750 times less metallic than the Sun.
But SMSS J160540.18–144323.1 is at [Fe/H] = −6.2 ± 0.2. As Nordlander said, that’s around 1.5 million times less metallic.
It is unlikely that any Population III stars have survived long enough for us to study them. But their tales can be unravelled through the stars that followed.
The scientists believe that for the early Universe, the star that gave SMSS J160540.18–144323.1 its iron was comparatively low mass, only about ten times the Sun’s mass. This is large enough to generate a neutron star, and the team believes that this is what it did after a relatively fragile supernova.
An explosion of supernova can cause a rapid process of neutron-capture or r-process. This is a series of nuclear reactions where atomic nuclei collide with neutrons to synthesize elements heavier than iron.
There was no significant proof of these elements in the star, which might imply that the newly dead neutron star captured these elements back. But enough iron escaped incorporating it into the formation of SMSS J160540.18–144323.1.
It was probably one of the very first members of that second generation of stars. And it’s going to die. It is a red giant, meaning that the star is at the very end of its lifetime, using the last of its hydrogen before switching to helium fusion.
The team believes it could produce even more data about Population III stars by studying it more carefully. But if it could speak, imagine the tales it could say.
The research was published in the Royal Astronomical Society’s Monthly Notices.