A star system has been found with a composition that indicates it could be 13.5 billion years old, forming not long after the Big Bang. Although stars of similar age have been observed, these two stars are in a highly unexpected place: the thin disk of the Milky Way, with an orbit quite similar to the Sun. Stars were not thought to have emerged here until around 3 billion years later, so this discovery could transform ideas of early galaxy formation.
Shortly after the Big Bang 13.8 billion years ago, the universe's only elements were hydrogen, helium and a little lithium. All heavier elements, known to astronomers as metals, were formed in the cores of these early stars and scattered through the cosmos with their deaths, so each generation of stars has a higher metal content than those before. Astronomers are keen to find survivors from the early universe, as a window into what conditions were like back then. The search for very metal-poor stars has found two dozen extreme examples such as the one found early this year thought to date to just 300 million years after the Big Bang.
Dr Andrew Casey of Monash University wasn't on such a hunt when he found 2MASS J18082002–5104378 B, however. All the very old stars we have found are in the Milky Way's halo. So a star in the disk of the galaxy, an astronomically-speaking modest 2,000 light-years from the Sun, was not where such a search would be conducted.
Instead, Casey told IFLScience his team was investigating the star 2MASS J18082002–5104378, whose motion, as recorded by another team, suggested it was in an orbital dance with a black hole or neutron star. No such object exists; the original team had made a calculation error. In the search process, however, Casey and colleagues report in the Astrophysical Journal, (preprint on ArXiv.org) they found something just as interesting.
J18082002–5104378 B is just 14 percent the mass of the Sun and by far the lowest mass star with an extreme low-metal content. Indeed, a significantly lighter object with the same content wouldn't be able to fuse hydrogen at all. J18082002–5104378 was previously identified as similarly lacking in metals, and therefore also approximately 13.5 billion years old, but has a mass three-quarters of the Sun's and similar to other very low metal stars.
The similarity between the orbits around the galactic center of J18082002–5104378 and the Sun can be seen, in contrast to other very low mass stars that just pass through the Milky Way's disk. K. Schlaufman (JHU)
Halo stars can wander into the disk, but the J18082002–5104378 pair are in a quite circular (or low eccentricity) orbit around the center of the galaxy, much like our Sun. “A star could start with low eccentricity and get bumped into an eccentric orbit,” Casey told IFLScience, but it is very hard for it to go the other way.
The discovery J18082002–5104378 is so old and yet apparently formed in the galactic disk is surprising enough; J18082002–5104378 B's mass provides an extra twist. “It was thought the stars of the early universe were massive,” Casey said to IFLScience. “Models have shown smaller stars forming nearby these large stars, and we have not been sure if they would merge, or if there was something wrong with the simulation.”
J18082002–5104378 B suggests neither is the case, that small stars could appear among the giants, and since small stars have much longer lifespans than larger ones, can still be present today. Casey added: “Ten years ago this would have been thought crazy, and it would still surprise most astronomers today.”