The discovery of an ancient star formed approximately 13.6-billion years ago just after the Big Bang is telling us much about the early universe.
The star – designated SMSS J031300.36-670839.3 – lies within our Milky Way galaxy and a mere (relatively) 6,000 light years away but is the oldest known star discovered so far, we’re announcing in a paper published in Nature.
By studying the light from this star in detail we have, for the first time, seen the chemical fingerprint of the first stars to form in the universe.
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.
Starlight shines
Starlight percolates out from the hydrogen fusion reactions taking place in the star’s interior and as it passes through the outer layers, the atoms of elements present absorb light at specific wavelengths.
The spectrum of starlight has imprinted on it a unique chemical fingerprint of absorption lines that tells us what elements are present in it, and how abundant they are.
In the case of the Sun there are literally millions of absorption lines arising from a broad range of elements, from the ubiquitous hydrogen and helium to iron and rare elements such as europium.
The spectrum of the star we have discovered shows only a handful of elements – hydrogen, carbon, magnesium, and calcium. This star is made to a recipe that is remarkably different to that of our Sun, and that can tell us a lot about how and when it was formed.
Back at the Big Bang
The Big Bang gave rise to a universe filled with hydrogen, helium, and a trace of lithium. All the other elements that we see around us today – those fundamental to life – were made in stars.
As stars struggle against the inward pull of gravity they fuse hydrogen to helium, helium to carbon and oxygen. If massive enough, a star continues fusion on to iron.
At the end of a massive star’s life these products are then recycled back into the surrounding gas through a supernova explosion.
Any old iron?
The iron level of the universe increases with time as successive generations of stars form and die. We can use the iron abundance of a star as a qualitative “clock” telling us when the star was formed.
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 star. This indicates that our star is the most ancient yet found.
Stars are like time capsules, they lock away a sample of gas from which they form. In the case of the star we have discovered, this has enabled us to study in detail a sample of gas from approximately 13.6 billion years ago.
This is so long ago that the star predates the formation of the Milky Way. It likely formed in a small cloud of gas and eventually many of such clouds fell together under gravity to form the grand spiral galaxy we call home.
This star has born silent witness to 99% of the life of the universe - it has spun impervious, slowly converting hydrogen into helium as demanded by gravity.
What are older stars made of?
The chemistry of the gas shows that our star formed in the wake of a primordial star around 60 times the mass of the sun that died in a supernova explosion. This supernova explosion was radically different to those supernova that occur today.
The explosion was of surprisingly low energy, such that although the star disintegrated, almost all of the heavier elements, such as iron, that form near the core of the star, were swallowed by a black hole formed at the heart of the explosion.
The shockwave from the dead primordial star spread its outer layers, enriched with carbon and magnesium formed over its lifetime, into the surrounding gas. Some of this gas subsequently condensed leading to the star we have discovered.
In this way, our star is a member of the second generation of stars in the universe and is unique in that it unambiguously incorporates material from the first stellar generation.
The early stars
The first generation of stars to light up the universe are understood to be fundamentally different from the generations that followed. Formed from the pristine hydrogen and helium of the Big Bang, the first stars were massive, hundreds of times the mass of the sun.
Without iron and molecules of carbon and oxygen, condensing stars cooled very slowly and small stars could not form. A first generation of mammoth stars lived fast and died after only a few million years (compare this to the nine-billion year lifetime of the sun).
For this reason we don’t expect to find any member of the first star generation today. But we can use the forensic evidence left in the wake of their explosive deaths – as encapsulated in the second-generation stars such as the one we have found – to describe what they were like.
Finding the oldest stars is very much a needle in a haystack search. Our star was one of 60-million stars in our search.
To cleanly separate the oldest stars from the vast bulk of pedestrian stars is made possible by the SkyMapper telescope operated by the Australian National University from our dark-sky site at Siding Spring Observatory, near Coonabarabran, NSW.
The optical filters used in SkyMapper enable us to find stars with low iron from their colour. With a large digital camera such as SkyMapper’s we can screen 100,000 stars per hour. Using SkyMapper we continue to map the southern sky every clear night.
Our best candidates are then examined in great detail using one of the twin 6.5m Magellan telescopes in Chile.
A dozen left?
We expect that there may be only as few as a dozen other ancient stars to be found. Bringing these stars to light will allow us to characterise the population of the first stars and obtain insight into an era of cosmic evolution hidden from modern telescopes: the switching on of the first stars.
This was a turning point in the history of the universe. They mark the transition from warm, dark gas to one capable of generating material for rocky planets and life.