Astronomers may have discovered the signature of one of the Universe’s earliest stars.
Theoretical models predict that some of the very first stars were hundreds of times larger than the Sun.
But no clear evidence of this had been found so far.
Scientists writing in the journal Science have discovered the traces of an early massive stellar object in the exceptional chemical composition of a star in our galaxy.
“This is quite a unique star, with a very peculiar chemical pattern that has never been found previously,” said lead author Dr Wako Aoki, from the National Observatory of Japan.
In the first several hundred million years after the Big Bang, the Universe was composed only of hydrogen and helium. It had no structure, no stars, nor any black holes.
“It was a very featureless, boring Universe. Then the first stars formed and fundamentally transformed it,” said Dr Volker Bromm from the University of Texas, Austin.
Dr Bromm, who was not part of the study, has worked for many years on numerical simulations of early star and galaxy formation.
“How this happened, this transformation of the early Universe into a state of ever increasing complexity, depended very much on the mass of the first stars.”
Nuclear fusion occurring in the core of these first generation stars produced the heavier elements that we know today, like carbon, oxygen, magnesium and iron.
The stars eventually died, creating explosions – supernovas – that dispersed heavy elements throughout space.
Then the second generation of stars grew out of the gas that already contained some elements other than hydrogen and helium.
The star found by Dr Aoki’s team, named SDSS J0018-0939, is one of hundreds identified by the Sloan Digital Sky Survey with 1,000 times less iron than that found in our Sun.
Astronomers call them low-metallicity stars.
At the moment there is no way to directly observe the very first stars, so scientists rely on indirect probes. They investigate the chemical composition of ancient, extremely metal-poor stars to infer the properties of the first supernova explosions – “stellar archaeology”.
Dr Bromm told BBC News: “You have a massive star which eventually runs out of nuclear fuel and then gravity takes over, creating in the centre either a neutron star or a black hole.”
“Most of the matter will go into the black hole, which means that most of the heavy metals will also end up in the black hole.”
But theory predicts that beyond 100 solar masses, stars would suffer a completely different type of explosion called a Pair-Instability supernova in which no physical remnant is left.
These are very unusual, hyper-violent blasts. “It’s a huge thermonuclear explosion; All the fuel is burnt at once and the star rips itself apart.”
In this case, all the elements produced by the star are ejected, creating a surrounding region that is rich in metals.
Dr Aoki added: “We studied stars with extremely low metallicity, but the iron abundance of this star is not as low.” This was the first indication that the life of the progenitor star must have ended in a Pair-Instability supernova, which can only happen in massive stars.
In our present Universe, stars that are 100 times more massive than the Sun are extremely rare. The presence of more metals limits the size of a star.
As supernovas populated the cosmos with metals, the birth of huge stars became more infrequent.
Looking into the past Many more observations are planned for the coming years, in a search for other primitive stars.
Huge stars have a short life of “only” three million years, which is a thousand times shorter than what is expected for the Sun. Therefore, none of the earliest large stars will have survived to the present day.
Astronomers can look into the Universe’s past by pointing telescopes at the farthest objects, since light emitted by them takes billions of years to reach Earth.
