Artist's impression by National Astronomical Observatory of Japan

Astronomers have found evidence for the existence of the monster stars long thought to have populated the early Universe. Weighing in at hundreds of times the mass of the Sun, such stars would have been the first to fuse primordial hydrogen and helium into heavier elements, leaving behind 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 enormous1, 2. With masses of more than 100 times that of the Sun, they would have lived and died in the cosmic blink of an eye, a few million years. As they exploded in supernovae, they created the first heavy elements from which later galaxies and stars evolved. But no traces of their existence have previously been found.

Now, using a technique called stellar archaeology, Wako Aoki at the National Astronomical Observatory of Japan in Tokyo and his colleagues have found the first hint of such a star, preserved in the chemical make-up of its ancient daughter. The chemistry of this relic — a star called SDSS J0018-0939 — suggests that it may have formed from a cloud of gas seeded with material created in the explosion of a single, very massive star. The results were published in Science on 21 August3.

“This is a much awaited discovery,” says Naoki Yoshida, an astrophysicist at the University of Tokyo who was not involved in the study. That such chemical signatures have never been found in the Universe, despite many theoretical studies predicting their existence, is a long-standing puzzle, he says. “It seems Aoki et al. have finally found an old relic that shows intriguing evidence that there really was such a monstrous star in the distant past.”

Strange mix

Astronomers look for evidence about the first stars in small, low-mass second-generation stars. These are slow burners that have now been around for about 13 billion years, and their tiny levels of heavier elements suggest that they coalesced from a gas enriched by products of just one or perhaps a few previous supernovae .

“These are like the beans in the tin can at the back of the cupboard — they sit there forever and you can open them at any time,” says Anna Frebel, an astrophysicist at the Massachusetts Institute of Technology in Cambridge, who was not part of the study. “From studying them, you can piece together the composition of the gas cloud from which the stars formed, and so what elements and how much of them came out of the very first stars.”

Until now, such studies have failed to reveal the huge stars that numerical simulations suggest should have formed in the early Universe. Aoki and colleagues discovered their candidate by studying stars originally found by the Sloan Digital Sky Survey, based at Apache Point Observatory in New Mexico. Analysing SDSS J0018-0939 using the High Dispersion Spectrograph on Japan’s Subaru Telescope on Mauna Kea, Hawaii, they found that the star has a highly unusual mix of elements.

“The chemical peculiarity is very remarkable," says Aoki. Analysis shows that the 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 a very massive star known as a pair-instability supernova, say the authors.

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, he says. Lower-energy supernovae, on the other hand, 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, says Frebel.

Continuing the hunt

Although the star's profile suggests it is more likely to have formed from the remnants of such a mega explosion than from a more conventional supernova, the fit is not exact, says Aoki. Some elements of the signature remain unexplained – something that may be resolved when models improve, he says. For now, Frebel says she would hesitate to make firm predictions on the original star's mass, or to claim this is the sole explanation.

Thomas Greif, a theoretical astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says the result is exciting, but also cautions that it does not mean theorists can claim victory. “At the moment, this is just one star, and it doesn’t mean all these stars would have exploded in this way,” he says.

Aoki's team estimates that 1 in every 500 stars that show only tiny levels of heavier elements might have been formed from the leftovers of these monsters, and the researchers are now looking at the chemical signature of more stars in a hunt for further examples. If such stars are this common, future astronomical surveys, such as the James Webb Space Telescope due for launch in 2018, or next-generation ground-based mega-telescopes, may even be able to observe the massive-supernova deaths of such stars directly, in ancient light coming from the most distant galaxies, says Yoshida. “That will ultimately prove the existence of very massive [first-generation] stars,” he adds.