Astronomers led by Dr Stefan Keller of the Australian National University say they have discovered the oldest star ever seen in the Universe.
The star has been named SMSS J031300.36-670839.3 (SM0313 for short). SMSS stands for SkyMapper Southern Sky Survey, the 031300 refers to the right ascension of the star on the sky and the 670839.3 refers to the declination.
SM0313 has visual magnitude of 14.7 and is located in the southern constellation Hydrus, about 6,000 light years away from Earth. It formed within the 2nd generation of stars in the Universe, soon after the very first stars exploded.
To find the earliest generations of stars, astronomers look for small abundances of the first heavy elements created, such as iron. Stars with very low chemical abundances may have formed shortly after the Big Bang 13.7 billion years ago.
“First generation stars are predominantly tens or hundreds of times more massive than the Sun. They live fast, die young and have not survived to the present day. The second generation star we have discovered is, on the other hand, a little smaller than the Sun providing it with an enormous lifespan of over 13 billion years,” explained Dr Keller, who is the first author of a paper published in the journal Nature (arXiv.org).
To find such a stellar candidate, Dr Keller and his colleagues went through the spectral data of 60 million stars surveyed with the SkyMapper telescope. They weeded through the data, discarding any stars with spectra similar to the Sun – a modern analogue with relatively large chemical abundances, and identified a handful of stars containing very low chemical signatures.
The astronomers then got a closer look at these stars using the 6.5m Magellan telescope in Chile to obtain high-resolution spectral data. From these data, they analyzed absorption lines of each star (every chemical element gives off a characteristic absorption line, or wavelength of light; the fainter this line, the less of the chemical is present).
In the case of SM0313, the team calculated that the star’s iron content is at least 10 million times less than the iron found in the Sun – which is the lowest iron abundance ever detected in a star. The star, they concluded, must be a true second-generation star.
“Truthfully, we don’t actually know how old SM0313 is. This is because, sadly, we can’t determine a specific age of these kinds of objects. However, the chemical composition of SM0313 tells us that it is a second-generation star in the Universe which naturally makes this star nearly as old as the Universe itself,” said co-author Dr Anna Frebel from Massachusetts Institute of Technology and Kavli Institute for Astrophysics and Space Research.
The astronomers also measured the abundance of carbon in SM0313, and found that this element was in much higher supply – more than 1,000 times greater than iron.
“The discrepancy is illuminating: according to computational models, stellar formation occurs from the inside out. Chemical elements that are fused in a star’s core are pushed further out to its perimeter, making way for new elements to form. The outer layers of the very first generation of stars were likely composed of the first heavy elements, leaving heavier elements like iron in their cores. According to theory, when these very first stars exploded as supernovae, they spewed all their chemical elements into space,” Dr Frebel explained.
“But the new second-generation star may change scientists’ understanding of just how active the very first generation of stars was.”
Because SM0313 has both very low iron and relatively high carbon content, the team envisions an alternative scenario: that this star arose from a low-energy, first-generation star whose explosion expelled the contents of its outer layers, but was not strong enough to release chemicals such as iron from its inner layers.
The resulting gas cloud – high in carbon, and low in iron – eventually coalesced to form SM0313.
“This is the first time that we’ve been able to unambiguously say that we’ve found the chemical fingerprint of a first star,” Dr Keller said.
“This is one of the first steps in understanding what those first stars were like. What this star has enabled us to do is record the fingerprint of those first stars.”
The composition of SM0313 shows it formed in the wake of a first generation star, which had a mass 60 times that of our Sun.
“To make a star like our Sun, you take the basic ingredients of hydrogen and helium from the Big Bang and add an enormous amount of iron – the equivalent of about 1,000 times the Earth’s mass,” Dr Keller said.
He added: “it was previously thought that primordial stars died in extremely violent explosions which polluted huge volumes of space with iron. But the ancient star shows signs of pollution with lighter elements such as carbon and magnesium, and no sign of pollution with iron.”
“This indicates the primordial star’s supernova explosion was of surprisingly low energy. Although sufficient to disintegrate the primordial star, almost all of the heavy elements such as iron, were consumed by a black hole that formed at the heart of the explosion.”
The discovery of SM0313 gives astronomers a clearer idea of what the Universe was like in its infancy.
S. C. Keller et al. A single low-energy, iron-poor supernova as the source of metals in the star SMSS J031300.36−670839.3. Nature, published online February 09, 2014; doi: 10.1038/nature12990