In astronomy, “metals” are all elements heavier that hygrogen (H) and helium (He). The metal content of a star is determined by its metallicity, Z, which is the mass fraction of elements other than H and He. The very first stars in the Universe should only contain H, He and traces of lithium (Li), as these were the only elements synthesised in the Big Bang – all other elements are created by stars. The first stars in the universe are through to have been very massive and very hot, and would have quickly ended their lives in a supernova explosion. If these stars were very hot, this could wipe out their Li which is destroyed at temperatures greater than about 2 million degrees. The resulting supernova explosion from the very first stars would have polluted the surrounding interstellar medium with metals, which are very important for the creation of the second generation of stars. Metals act as a very effective cooling agent, and for gas clouds to collapse to form low-mass stars, they need to cool enough so that gravity can overcome the gas pressure that holds the cloud up. If it is true that the first stars were all massive, then none would have survived today, since massive stars live fast and die young. Low-mass stars, on the other hand, live for an extremely long time and stars with masses less than 0.8 solar masses can be over 13 billion years old and detectable today. So astronomers have been hunting for low-mass, low-metallicty stars which will provide clues about the early universe.
Using the Very Large Telescope in Chile, a team lead by Elisabetta Caffau have discovered an extremely metal-poor star in the halo of our galaxy. The star, SDSS J102915+172927, has a metallicity of Z < 6.9 x 10-7, which is at least five order of magnitude lower than our Sun. The star joins a small group of very metal-poor stars, which all have a very low iron content, but a surprisingly large amounts of carbon, nitrogen and oxygen relative to their iron abundances. SDSS J102915+172927, however, does not carbon, nitrogen or oxygen. In fact, the only metal yet detected in the star is calcium. Surprisingly, no lithium has been detected, which suggests that the star must have experience temperatures above 2 million degrees (though the current effective temperature of this low-metallicity dwarf is expected to be a few thousand degrees).
For mode details, see
- The stars that should not exist, ESO press release
- A hint of normality at last?, Nature News & Views by John Norris
- An extremely primitive star in the Galactic halo, Caffau et al. (2011, Nature, 477, p67) [Requires Swinburne login]