We can’t see the first stars yet, but we can see their direct descendants

For those who take a universe value of hydrogen and helium, and let it stew for about 13 billion years, you get us. We’re the descendants of the primeval parts. We’re the cast-off dust of the primary stars, and plenty of generations of stars after that. So our seek for the primary stars of the cosmos is a seek for our personal historical past. Whereas we’ve not captured the sunshine of these first stars, a few of their direct kids could also be in our personal galaxy.

The primary stars have been huge. With none heavier parts to weigh them down, they wanted to be about 300 instances that of our sun in an effort to set off nuclear fusion of their core. Due to their measurement, they went by means of their fusion cycles fairly shortly and lived very brief lives.

However the supernova explosions signaling their deaths scattered heavier parts akin to carbon and iron from which new stars fashioned. Massive second-generation stars additionally died as supernovae and scattered much more heavy elements. Consequently, every technology of stars contained an increasing number of of those parts. In astronomy lingo, we are saying every technology has a better metallicity.

After all, which technology a star is in might be fuzzy. Clearly, the very first stars, forming fully out of primordial hydrogen and helium are first-generation stars, and stars forming fully out of the remnants of the primary generations are true second-generation stars. However stars kind in any respect completely different sizes, so it is fairly seemingly that some huge second-generation stars grew to become supernova earlier than a few of the smaller first-generation stars.

Many early stars may have fashioned from principally first-generation materials with a contact of second-generation dust, whereas others fashioned principally from second-generation stars with a sprinkling of first-generation heritage. Stars like our sun are seemingly a mixture of materials from a number of generations.

For contemporary stars, fairly than attempting to find out their technology, we categorize them into populations primarily based on their metallicity. A star’s metallicity is taken because the ratio of iron to helium [Fe/He] on a logarithmic scale. Inhabitants I stars have an [Fe/He] of not less than -1, that means they’ve 10% of the sun’s iron ratio or extra. Inhabitants II stars have an [Fe/He] of lower than -1. The third class, Inhabitants III, is reserved for true first-generation stars.

Within the Milky Way galaxy, many of the stars within the galactic aircraft are inhabitants I stars just like the sun. They fashioned a lot later within the historical past of our galaxy, and are youthful with extra metals. Older inhabitants II stars are typically discovered within the halo surrounding our galaxy, or within the outdated globular clusters that orbit the Milky Way. That is smart since older stars have had extra time to float out of the galactic plane. Given the evolution of our galaxy, it is fairly seemingly that a few of the inhabitants II stars in our halo are actually second-generation stars. However how can we distinguish them from different outdated stars?

That is the purpose of a brand new research printed on the arXiv preprint server. It seems to be at each observations of distant quasars and simulations of inhabitants III stars to find out the metallicity of actually second-generation stars. The authors discovered that whereas second-generation stars can be uncommon within the Milky Way halo, some might be lurking there. The important thing to figuring out them just isn’t their abundance of iron relative to helium, [Fe/He], however fairly the ratios of carbon and magnesium to iron, [C/Fe] and [Mg/Fe].

Carbon is fashioned in stars as a part of the CNO cycle, which is the second-level fusion cycle after hydrogen burning. Magnesium is a product of a 3-stage fusion of carbon with helium. Many first-generation stars exploded as high-powered supernovae, however some exploded with decrease vitality. These low-energy supernovae would forged off parts akin to carbon and magnesium, however not a lot iron. So, stars with an exceptionally excessive [C/Fe] ratio seemingly fashioned from the remnant materials of a single first-generation star. The decrease the [C/Fe] ratio, the extra seemingly a inhabitants II star fashioned from first and second-generation stars.

So it appears the secret is to search for halo stars with [C/Fe] > 2.5. We’ve not discovered any such stars but, however as extra sky surveys come on-line it’s seemingly solely a matter of time. We’ll nonetheless have to look essentially the most distant galaxies to discover a first-generation star, however we might quickly discover one in all their kids a lot nearer to house.