Brian Wallheimer | December 27, 2016
University of Notre Dame astronomers have identified what they believe to be the second generation of stars, shedding light on the nature of the universe’s first stars.
A subclass of carbon-enhanced metal-poor (CEMP) stars, the so-called CEMP-no stars, are ancient stars that have large amounts of carbon but little of the heavy metals (such as iron) common to later-generation stars. Massive first-generation stars made up of pure hydrogen and helium produced and ejected heavier elements by stellar winds during their lifetimes or when they exploded as supernovae. Those metals — anything heavier than helium, in astronomical parlance — polluted the nearby gas clouds from which new stars formed.
Jinmi Yoon, a postdoctoral research associate in the Department of Physics; Timothy Beers, the Notre Dame Chair in Astrophysics; and Vinicius Placco, a research professor at Notre Dame, along with their collaborators, show in findings published in the Astrophysics Journal this week that the lowest metallicity stars, the most chemically primitive, include large fractions of CEMP stars. The CEMP-no stars, which are also rich in nitrogen and oxygen, are likely the stars born out of hydrogen and helium gas clouds that were polluted by the elements produced by the universe’s first stars.
“The CEMP-no stars we see today, at least many of them, were born shortly after the Big Bang, 13.5 billion years ago, out of almost completely unpolluted material,” Yoon says. “These stars, located in the halo system of our galaxy, are true second-generation stars — born out of the nucleosynthesis products of the very first stars.”
Beers says it’s unlikely that any of the universe’s first stars still exist, but much can be learned about them from detailed studies of the next generation of stars.