The most metal-poor stars are living fossils from the beginning of the universe

Our sun, like all stars, is made mostly of hydrogen and helium. They are by far the most abundant elements, formed in the early moments of the universe. But our star is also rich in other elements astronomers call metals: carbon, nitrogen, iron, gold, and more. These elements were created through astrophysical processes, such as supernovas and neutron star collisions.

The dust of long-dead stars gathered together into molecular clouds and formed new, younger stars such as the sun, stars rich in metals. But there are still stars out there that are not metal-rich. These extremely metal-poor stars, or EMPs, hold clues to the origin of stars in the cosmos.

The general model of stars is that with each successive generation, the amount of metal within a star increases. The very first stars were almost pure hydrogen and helium. They died as supernovas, and new stars formed from their remains. The largest of those stars soon died, and the cycle continued.

It’s estimated that the sun is at least a third-generation star. Because of this, the origin of its chemical composition is difficult to trace accurately. All sorts of processes can create metals. But extremely-metal-poor stars are different. Their chemical composition is so simple that we can treat them as the product of a single supernova explosion. Other processes may have contributed a bit, but mostly these stars are simple second-generation stars.

The reason this is important is that there are no first-generation stars left in the universe. Without heavier metals to increase their core density, these ancestor stars had to be hundreds of solar masses in order to trigger core fusion. They lived very short lives, and so we don’t know much about them.

With EMPs, we can study the composition of the first stars and better understand things such as their size and lifetimes. But one problem is that it’s very difficult to distinguish an “extremely poor” metal star from a “kind-of poor” metal star. You have to gather high-resolution spectra of a star to really tell the difference, and that takes time and resources. A new study has created an overview of what we know about EMPs so far as a way to encourage further research.

The findings are published on the arXiv preprint server.

One of the findings is that within our galaxy, not all EMPs are in the halo of the Milky Way. Most low-metal stars are old red dwarfs, and over time, close interactions with other stars would cause them to migrate to the outer halo of the galaxy. The fact that some EMPs remain in the disk of the galaxy suggests some interesting features of galactic dynamics. There is even evidence that a few EMPs are actually rather young. So EMPs could overturn some of our current models of stellar evolution.

Another feature is that the ratios of carbon, nitrogen, and oxygen in EMPs allow us to pinpoint the mass and age of first-generation stars. Since the ratio of elements produced in a stellar core depends on its mass, the simple composition of EMPs gives us a clear view of the first stars. With enough data, we could determine things such as how quickly they formed after the Big Bang and whether first-generation stars were common or rare.

The work also goes into more technical aspects of stellar evolution and how EMPs can help us understand the long-term evolution of galaxies. But to achieve that, we’ll need to gather a lot more data on EMPs.