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Forensic evidence reveals ancient stars
Posted: 18 February 2010

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Some of the oldest, and purest, stars in the Universe have been discovered by astronomers working with the Very Large Telescope (VLT) at the European Southern Observatory, in the process plugging a gaping hole in our theories of how the Milky Way formed.

Our current ideas of galaxy formation involve smaller dwarf galaxies merging to gradually form larger galaxies. This is called hierarchical formation and means that the oldest stars in our Galaxy should have originally come from dwarf galaxies, but whenever astronomers have searched for these ancient stars in the surviving dwarfs nearby, they have always turned up a blank. This was a problem until a multinational team of European astronomers led by Dr Else Starkenburg, of the Kapteyn Astronomical Institute at the University of Groningen in the Netherlands, took a careful look at what the starlight from these ancient stars should be like.

The Fornax dwarf galaxy, where extremely low metallicity stars have been discovered. Image: ESO/DSS.

Any elements heavier than the primordial hydrogen and helium formed in the big bang are termed ‘metals’ by astronomers. They are cooked up by the nuclear fusion reactions inside the cores of stars. Younger stars, like the Sun, contain a greater abundance of ‘metals’ because there have been many previous generations of stars to create these elements. Conversely, older stars don’t have the benefit of many generations of ancestors, and consequently have lower metal abundance. The older the star, the lower its metallicity.

So astronomers have been searching for these extremely metal-poor stars in dwarf galaxies, but when Starkenburg compared the spectra of the starlight they were seeing with computer models of what the spectra should look like, they realised that the difference between normal metal poor stars and extremely metal poor stars was very subtle and could be easily missed.

“We have, in effect, found a flaw in the forensic methods used until now,” says Starkenburg. Realising what they were looking for, Starkenburg’s team used the Ultraviolet and Visual Echelle Spectrograph (UVES) on the VLT to go back and re-observe some of the nearby dwarfs. They found that previous measurements did not have the sensitivity to pick out the spectral signatures of the extremely low metallicity stars at the distance of the dwarf galaxies.

“Compared to the vague fingerprints that we had before, this would be as if we looked at the fingerprint through a microscope,” says team-member Vanessa Hill of the Observatoire de la Cote d’Azur, France. “Unfortunately just a small number of stars can be observed this way because it is very time consuming.”

The few that they did find included some record breaking stars. Three stars in particular were tremendously pristine, with relative abundances of metals between 1/3,000th and 1/10,000th of the Sun, the lowest abundances ever measured in any stars. It is not possible to calculate the exact age of the stars, according to Starkenburg, because the efficiency by which metals were produced in the early Universe is still uncertain, but these would have to belong to one of the first generations of stars in the Universe. Even so, we know they cannot be the oldest. Some other generation must have produced the small amount of metals in these stars. This first generation are called Population III stars (Population II stars consist of poor and extremely metal poor stars, while Population I stars are young, metal-rich stars like the Sun) and were born 400 million years after the big bang. Unfortunately we will never directly see a Population III star – they all exploded as supernovae just a few millions years after they were born.