Pristine-ish

NASA, ESA, CSA, STScI

The Big Bang essentially created two elements: hydrogen and helium. It also produced tiny traces of lithium and a few other light isotopes, but in the beginning there was hydrogen and helium. All the other, heavier elements formed later, either in the cores of stars, through stellar collisions, or other astrophysical processes. Even now hydrogen and helium make up so much of the material world that astronomers refer to all other elements as metals. Dust in the wind, you might say.

One consequence of this is that you can get a pretty good idea of a star’s age by the amount of metals seen in its spectrum. The very first stars, the progenitors of all others, would just contain hydrogen and helium. The gas dust cast off by their demise would contain some metals, and so would the second generation of stars. With each generation a bit more metal is added to the mix, so the higher the metallicity of a star, the younger it generally is. Our Sun is only five billion years old, for example, and it has a high metallicity.

The first stars have likely long-vanished from the cosmos. With only hydrogen and helium to work with, they would have needed hundreds of solar masses to trigger nuclear fusion in their core, and they would have become supernovae in a cosmic blink. To study these grandmother stars, astronomers have mostly looked for galaxies at the farthest edge of the observable universe. Galaxies with unusually low metallicity. But another approach is to look for ancient stars in our galactic neighborhood. The idea is that the first stars may have given rise to some second-generation stars with low masses. If these stars were smaller than the Sun, they would live long enough to still be around today. Recently astronomers have found just such a star.¹

Ji, et al

The star is known as SDSS J0715-7334. It is a red giant star in the halo of the Large Magellanic Cloud. The metallicity of this star is so low that even the most distant, most primordial galaxies we’ve observed have ten times the metallicity of this star. SDSS J0715-7334 is the closest thing we’ve found to a pristine, metal-free star. Its metallic abundances tell us a few interesting things about early star formation.

To begin with, by looking at the specific abundances of elements such as carbon, magnesium, and iron relative to hydrogen, we can get a handle on the size of its parent star. If SDSS J0715-7334 is a second-generation star, then it formed within the supernova remnant of a 30 solar-mass star, which is surprisingly small. Another interesting aspect of the star is that its abundance of carbon is exceptionally low. This is surprising because large stars are efficient producers of carbon, nitrogen, and oxygen due to the helium burning CNO fusion cycle. The lack of carbon suggests that there was plenty of cooled dust in the star-forming region, which is necessary for small early stars to form. Finally, the motion of SDSS J0715-7334 within the Large Magellanic Cloud suggests that it formed within the small galaxy’s halo and is not simply a passing visitor. This means that we are likely to find more of these stars in our galactic neighborhood, which means we will be able to compare observations of distant galaxies to those of local pristine stars.