Astronomers have discovered what they once believed was impossible. A star system with a composition that indicates it could be 13.5 billion years old, forming not long after the Big Bang. Although other stars have been observed to have a similar age, these two stars are located somewhere unexpected. Instead of being in the halo of the Milky Way like other old stars, they exist in the thin disk, with an orbit similar to our own Sun and only 2,000 light-years away from us.
The metal-poor stars
Shortly after the Big Bang 13.8 billion years ago, the universe only consisted of 3 elements: hydrogen, helium, and a little lithium. All heavier elements, known to astronomers as metals, were formed inside the cores of early stars and scattered through the cosmos after they died.
The search for these stars has found two dozen examples such as one found earlier this year that was thought to form just 300 million years after the Big Bang.
An old star left alone
Dr. Andrew Casey of Monash University wasn’t looking for these stars when he came across 2MASS J18082002-5104378 B. He was only investigating this star because another team had recorded that the star’s motion suggested it was in an orbital dance with a black hole or neutron star which wasn’t the case at all.
It turns out that J18082002-5104378 B is just 14% the mass of the Sun and the smallest star with an extreme low-metal content. A lighter object with the same content wouldn’t even be able to fuse hydrogen at all so this was a huge discovery.
The significance of this star
Halo stars are known to wander into the disk but J18082002-5104378 is in a circular (or low eccentricity) orbit around the center of our galaxy. Astronomers once thought that the stars of the early universe were massive but J18082002-5104378 doesn’t fit that belief.
Since small stars have much longer lifespans than larger ones, they are still being discovered today. The discovery of J18082002-5104378 proves that there could be some more small stars among the giants.