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Discovery of the chemically oldest star in the Milky Way

October 31, 2002

During the last 30 years researchers have tried to find stars that still carry vestiges of the very origin of the Milky Way Galaxy, when it formed from a gigantic collection of gas soon after the Big Bang. The gas of our galaxy, which was presumably composed of hydrogen and helium at the beginning, is continuously polluted by exploding stars that enrich it with heavier chemical elements. Consequently, the oldest stars should contain very little of such elements. Now, a research group of astronomers from Uppsala university together with collegues from Germany, Australia and the USA has found a giant star with less than 1/100,000 of the solar content of heavy elements, which is about 20 times more metal-poor than the previous record for this kind of star.

The discovery of the star, named HE 0107-5240 after the Hamburg/ESO Survey and its rough position in the sky (in the direction of the constellation Phoenix, at a distance of 36,000 light years) gives astronomers a unique possibility to study stellar gas with a composition close to the state it had directly after the Big Bang. But this discovery poses new questions: What is the origin of the small amount of heavy elements in stars? Most of the present theoretical calculations show that it is very difficult to form low-mass stars shortly after the Big Bang. The discovery of HE 0107-5240 also demonstrates that stars with masses slightly below that of the Sun can form from very metal-poor gas. It therefore appears that many of the theoretical models need to be improved.

The star HE 0107-5240 is about 10,000 times fainter than the faintest stars that can be seen with the naked eye. How was it possible to find it? The discovery was actually a by-product of a project with the main aim to find quasars. Pictures of the southern sky were taken with a wide-angle telescope at the European Southern Observatory (ESO) in Chile. Included in the harvest was about 8 000 spectra of very metal-poor stars. These stars are now being systematically scrutinized with larger telescopes, like ESO`s Very Large Telescope in Chile, to obtain spectra of high spectral resolution, which will allow astronomers to determine the chemical composition of these stars.This part of the project is led by Dr. Norbert Christlieb from Hamburg, presently at Uppsala Astronomical Observatory.

"This is, in a way, the closest we have come to the conditions directly after the Big Bang by studying stars," say Dr. Norbert Christlieb and Prof. Bengt Gustafsson, who in collaboration lead the chemical analysis. "But obviously a lot must have happened between the Big Bang and the formation of this star. In spite of its extreme metal-poorness it evidently carries some metals, and they were most probably formed in a still more early massive star that exploded as a supernova. Moreover, the star has an abnormally large content of carbon and nitrogen, relative to other heavy elements. Those elements may possibly have been formed by nuclear reactions with helium and hydrogen inside the star. After that, material was mixed to the stellar surface. It is also possible that a neigbouring star polluted our star by transferring mass towards the end of its life. However, we do not trace any more direct evidente that shows that this has happened."

If a star of about 0.8 solar masses and 1/100,000 of the metal content of the Sun did indeed form in the early Universe, then it should also have been possible for other low-mass stars to form. If so, they would have survived until today, giving us the chance to find them with large, systematic searches like the Hamburg/ESO Survey. Because only 1/4 of the 8000 candidates identified in that survey have been vetted as of today, it is well possible that such a star will eventually be found in these efforts.

Vetenskapsr�det (The Swedish Research Council)