Astronomers have found evidence for the existence of very large stars in the early Universe. Weighing hundreds of times more than the mass of the Sun, such stars would have been the first to fuse hydrogen and helium into heavier elements. They left a chemical signature that the researchers have now found in an ancient, second-generation star.
Little is known about the Universe’s first stars, which would have formed out of clouds of hydrogen, helium and a tiny amount of lithium in the first few hundred million years after the Big Bang.
Simulations have long predicted that some of this first batch of stars were enormous. Such large stars only exist for a very short time before they exploded in supernovae, creating the first heavy elements from which later galaxies and stars evolved.
But no traces of their existence have previously been found. Now Professor Wako Aoki at the National Astronomical Observatory of Japan in Tokyo and his colleagues have used a technique called stellar archaeology to identify the first traces of such a star, preserved in the chemical make-up of its ancient long-lasting daughter.
The daughter star has a very low abundance of lighter elements, such as carbon, magnesium and calcium, relative to heavier elements such as iron. The most likely explanation for this signature is a type of explosion of the parent known as a “pair-instability supernova”
This type of supernova occurs when the temperature in the star’s core becomes so high that pairs of photons turn into pairs of electrons and positrons. The resulting fall in outward pressure causes the star to collapse dramatically, setting off a huge thermonuclear explosion. This would tear the whole star apart and produce the high levels of iron relative to lighter elements that Aoki’s team found.
Lower-energy supernovae, which result when normal, lower mass stars explode, create very little iron compared to lighter elements because, although the outer layer is blown away, heavier elements get sucked back into the core, forming a black hole. Most of the iron in the universe was created not in supernovae but in red giants, the phase at the end of most stars’ lives.