A low-mass sun with few elements heavier than helium provides hope that the Galaxy might contain survivors from the very first generation of stars.

The very first stars to light up the universe — also known as Population III stars — are widely presumed to have been giants, hundreds of times heftier than the Sun. But a recently discovered puny star in our Galaxy might be an ancient specimen that shows how the first stellar generation could have contained some runts that still live among us today.

The star in question, estimated to be about 13.5 billion years old, contains very few elements heavier than helium, a sign that it was born during a much more pristine epoch before other stars got around to forging atoms such as carbon, oxygen, and iron and spewing them into space. That’s not terrible surprising, astronomers know of a couple dozen stars that have similar abundances of elements.

But two things make this star stand out. It’s just 14% as massive as the Sun, barely weighty enough to ignite hydrogen fusion in its core and call itself a star. And it’s the slimmer half of a binary star system, which gives a clue about how it formed.

Kevin Schlaufman (Johns Hopkins University) and colleagues published their finding in the November 10th Astrophysical Journal.

The best explanation for the star’s origin, says Schlaufman, is that it formed from gas clumping together in a disk that once swirled around the more massive of the duo, a process known as disk fragmentation. And if that can happen for stars as pristine as these two, he argues, then it should also be possible for stars with masses more similar to the Sun to form and survive in disks around some behemoth Population III stars as well.

Located about 2,000 light-years away in the southern constellation Ara, this system, designated 2MASS J18082002–5104378, was discovered in 2016. Researchers noted then its apparent low abundance of metals, astronomer’s lingo for any element that isn’t hydrogen or helium.

Schlaufman and colleagues took a closer look and discovered that the star was actually two stars orbiting each other about once every 35 days. Spectroscopy provided the orbital speeds of the stars, which in turn revealed their masses to be 0.14 and 0.76 solar masses. The data also showed that the amount of iron compared to hydrogen in the stars’ atmospheres — a common measure of a star’s metal abundance, or metallicity — is about one ten-thousandth that of the Sun.

“This is interesting because it says that very-low-mass stars can form even at these very low metallicities,” says Elisabetta Caffau (Paris Observatory, France), who was not involved with this research but discovered her own low-mass, low-metallicity star in 2011. “While there are theoretical claims that below some critical metallicity no low-mass stars can be formed, other theories claim that even for gas that is totally devoid of metals, low-mass stars can form through the fragmentation of collapsing clouds.”

Stars form out of collapsing clouds of gas. Metals aid that process by providing an efficient means of cooling the gas. The cooler the gas gets, the smaller it can scrunch up. That’s why Population III stars are presumed to have all been giants. Without metals, gas couldn’t squish itself into relatively small orbs.

“If you have efficient fragmentation, that increases the chance that you’ll make a solar-mass star,” says study co-author Schlaufman. And unlike gargantuan Population III stars, which may have lived for only a million years or so and exploded long ago, low-mass first-generation stars could have survived to today.

“That’s the reason we should keep looking for low-mass Population III stars in the Galaxy,” says Schlaufman. “We shouldn’t despair, there’s good reason to think they’re still there.”

Reference:

K. C. Schlaufman et al. "An Ultra Metal-poor Star Near the Hydrogen-burning Limit." Astrophysical Journal. November 10, 2018.

E. Caffau et al. "An Extremely Primitive Star in the Galactic Halo." Nature. September 1, 2011.