An Impossible Star

Astronomers observe a 13-billion-year-old star with an unusual chemical composition

09-Sep-2011 - Germany

A team of European astronomers led by a scientist of Heidelberg University has discovered a star that according to standard astronomical thinking should not be able to exist: it is made up almost entirely of hydrogen and helium with only minute traces of other elements. In terms of the generally accepted theory of star formation, this unusual chemical composition puts the star dating from the early stages of the Universe into a “forbidden zone”. “For all we know, it should never have come into being in the first place,” says Dr. Elisabetta Caffau of Heidelberg University’s Centre for Astronomy (ZAH). The findings of the investigation, which was carried out with the Very Large Telescope of the European Southern Observatory (ESO), were published in “Nature” on 1 September 2011.

The extremely faint star in the Leo constellation goes by the cumbersome designation “SDSS J102915+172927”. It was catalogued in the course of the Sloan Digital Sky Survey (SDSS), an international project scanning certain sectors of the sky with spectral lines. The figures in the designation refer to its position in the sky. The star has slightly less mass than the Sun and is probably over 13 billion years old. According to the observations of the European research team, SDSS J102915+172927 has the lowest proportion of chemical elements heavier than helium of any star investigated so far.

The properties of the star were investigated with the two spectrographs X-Shooter and UVES of the ESO’s Very Large Telescope (VLT) in Chile. With these devices, the light of celestial bodies can be split into its colour components. Spectrum analysis, developed by Gustav Kirchhoff and Robert Bunsen in the mid-19th century in Heidelberg, is used to determine the relative proportion of chemical elements in a star’s atmosphere. This is how the astronomers established that the metal content of SDSS J102915+172927 is 20,000 times lower than that of the Sun. In their first measurements the astronomers spotted just one chemical element heavier than helium: calcium. It was only through additional observation that the scientists from France, Germany and Italy were able to identify other metals.

“The generally accepted theory states that because of their low mass and the extremely small proportion of heavy elements, stars like this one should not be able to exist at all,” says Dr. Caffau, who conducts research at the Königstuhl State Observatory of Heidelberg University’s Centre for Astronomy. “Received thinking suggests that in this case the gas and dust clouds giving rise to such a star could not have condensed sufficiently for the purpose. This is the first time a star has been discovered in the ‘forbidden zone’ of star formation and it was a big surprise for us. Now astrophysicists will have to rethink some of their models for the formation of stars.” Dr. Caffau is the lead author of the study published in “Nature”.

Cosmologists believe that, along with traces of lithium, the lightest elements hydrogen and helium came into existence shortly after the Big Bang. Almost all heavier elements were formed a great deal later, either through fusion processes in the interior of stars or in the course of supernova explosions at the end of a star’s life. After the explosion the metal-rich material mixes with the interstellar medium, i.e. the matter in the space between the stars. The next generation of stars is formed from this metal-enriched material. The metal content of the newly formed stars is higher than in the previous generation. “This means that the proportion of metals also tells us how old a star is, or rather how many generations of stars the material it consists of has been through,” says Dr. Caffau. “The fact that SDSS J102915+172927 is extremely metal-poor means that this star must stem from the early days of the Universe. In fact, it may be one of the oldest stars ever found.”

Another surprise is the paucity of lithium in SDSS J102915+172927, as according to Dr. Caffau, a star of this age should have more or less the same chemical composition as the Universe just after the Big Bang. But the star’s lithium content is fifty times smaller than cosmological calculations on element formation would lead us to expect. For the European astronomer team, it is a mystery how the lithium that must have formed at the beginning of the Universe was destroyed in this particular star. But the scientists are convinced that this strange star is not alone. “We have a whole series of candidates whose metal content may be just as low as that of SDSS J102915+172927, if not lower,” says Dr. Caffau. “We are planning to observe them with the VLT to see if this is the case.” For the astronomers, it is the next step towards exploring the very first generation of stars.

Original publication

E. Caffau, P. Bonifacio, P. François, L. Sbordone, L. Monaco, M. Spite, F. Spite, H.-G. Ludwig, R. Cayrel, S. Zaggia, F. Hammer, S. Randich, P. Molaro, V. Hill: An extremely primitive halo star in the Galactic halo, Nature  (1 September 2011)

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