The Oldest Stars are the Poorest


By Knicole Colon

A recent letter published in Nature by Keller et al.  presented a star that is composed of an extremely small amount of metals, making it possibly one of the oldest stars in the universe.  Such stars as this one (known technically as SMSSJ031300.36-670839.3, but let’s call it the “senior citizen” for simplicity) are called “metal-poor,” where the term “metal” refers to any element that is heavier than helium (at least in astronomy).  This discovery is groundbreaking because only a handful of extremely metal-poor stars were previously known, and the senior citizen is significantly more metal-poor than those other stars.

The authors first identified this star as having a particularly low amount of metals based on measurements of its apparent brightness taken at different wavelengths of light.  These measurements tell you the color of the star, and there are empirical relations that can be used to extract a star’s properties based on its color.  For a more robust determination of a star’s properties, astronomers turn to spectroscopy.  With spectroscopy, it is possible to identify absorption lines caused by different elements present in a star.  Keller et al. acquired spectra from two different telescopes and found that the senior citizen’s spectrum had very few absorption features that would be caused by heavy metals.  Furthermore, the authors compared spectra of the senior citizen to spectra of two other extremely metal-poor stars that also have similar temperatures and surface gravities as the senior citizen.  They found that where some spectral features due to elements like nickel and iron exist in the other two metal-poor stars, in the senior citizen’s spectra you simply see nothing.  Stars tend to have very complicated spectra because they are composed of many different elements, so to find a star with very few spectral features is remarkable.

If we backtrack for a minute, we have to ask how such metal-poor stars are formed in the first place, and why that means they are old.  It is believed that the very first generation of stars evolved, generated metals as heavy as iron in their cores, then died and spread their metal-rich ashes throughout the universe.  However, assuming that metal-poor stars are formed from gas that is not metal-rich, that gas must not have been enriched by that first generation of stars.  That suggests that metal-poor stars are quite old – not necessarily first generation stars themselves, but old enough that perhaps they formed in areas where the metal-rich leftovers from the first stars to die had not yet reached.

Keller et al. offer a new theory as to why this star is so metal-poor.  The authors theorize that in the early universe, a single low-energy supernova (i.e. the explosive death of one of the first stars ever formed) enriched the gas from which the metal-poor stars formed.  Their theory also includes the formation of a black hole after the supernova event, which served to trap the heaviest metals that had formed in the core of the star.  Correspondingly, lighter elements like carbon would have been successfully ejected into space by the supernova explosion and would then have enriched the gas from which new stars would form.  This theory is surprising because, as mentioned above, it has been the general belief that the first stars that ever formed in the universe were extremely massive and would go out with a very-high-energy bang, enriching the gas around them with lots of metals (regardless of whether a black hole was formed or not).  Therefore, while the discovery of such a metal-poor star is certainly one for the record books, the exact origin of such a star is still a mystery.