If you take a Universe worth of hydrogen and helium, and let it stew for about 13 billion years, you get us. We are the descendants of the primeval elements. We are the cast-off dust of the first stars, and many generations of stars after that. So our search for the first stars of the cosmos is a search for our own history. While we haven’t captured the light of those first stars, some of their direct children may be in our own galaxy..
Of course, which generation a star is in can be fuzzy. Clearly, the very first stars, forming entirely out of primordial hydrogen and helium are first-generation stars, and stars forming entirely out of the remnants of the first generations are true second-generation stars. But stars form at all different sizes, so it’s quite likely that some massive second-generation stars became supernova before some of the smaller first-generation stars.
In the Milky Way galaxy, most of the stars in the galactic plane are population I stars like the Sun. They formed much later in the history of our galaxy, and are younger with more metals. Older population II stars are generally found in the halo surrounding our galaxy, or in the old globular clusters that orbit the Milky Way. That makes sense since older stars have had more time to drift out of the galactic plane.
That’s the goal of a new study published on the *arXiv*. It looks at both observations of distant quasars and simulations of population III stars to determine the metallicity of truly second-generation stars. The authors found that while second-generation stars would be rare in the Milky Way halo, some could be lurking there. The key to identifying them is not their abundance of iron relative to helium, [Fe/He], but rather the ratios of carbon and magnesium to iron, [C/Fe] and [Mg/Fe].
So it seems the key is to look for halo stars with [C/Fe] > 2.5. We haven’t found any such stars yet, but as more sky surveys come online it is likely only a matter of time. We will still have to search the most distant galaxies to find a first-generation star, but we may soon find one of their children much closer to home.