Usually, if you want to gaze at one of the oldest stars in the known universe, you have to look very far away, and back in time, into the furthest reaches of the cosmos. That's because the oldest stars were the ones that formed just after the Big Bang over 13 billion years ago. The last place you'd expect to find one of these ancient stars is right here in our very own Milky Way galaxy.
So you can imagine the surprise of astronomers who recently discovered a faint glimmer in the orbit of the Milky Way's thin disc — the star-dense plane of the galaxy's rotation — that had all the markings of a very, very old star, reports Science Alert.
The best way to tell whether a star is among the oldest stars is to look at its metal content. That's because in the very early universe, there were no metals. It took the first few generations of stars to forge the first metals before they started to be incorporated into the content of later stars. In fact, the younger the star, the higher its metal content.
So when scientists find a star with low metal content, they know it must be ancient. And that's exactly what was found in this newly discovered faint star, now labeled as 2MASS J18082002–5104378 B. This star's metal content is so low that it actually registers as the lowest metallicity of any star ever discovered.
Reworking the history of the universe
"We've never discovered a star so low mass and made of so few grams of metals," said astrophysicist Andrew Casey of Monash University in Australia. "This discovery tells us that the very first stars in the Universe didn't have to all be massive stars that died long ago. These ancient stars could form from very small amounts of material, which means some of those relics from soon after the Big Bang could still exist today. That gives us a new viewpoint for star formation in the early Universe!"
In other words, for a star to last from just after the Big Bang and still have fuel to burn today, it would have to be a very small, faint star. That's because bigger, brighter stars burn out too fast. It's the universe's version of "the bigger they are, the harder they fall." The catch is that scientists never thought small stars like this were produced until much later into the universe's history.
Not only have those theories been proven wrong thanks to 2MASS J18082002–5104378 B, but it appears that we've had one of the universe's oldest stars lurking in our backyard the whole time.
The star is so faint that it's not really a surprise that it took us so long to discover it. In fact, the only reason it was spotted was because it has a brighter binary companion. Of course, this means there's a chance other ancient stars are also hovering about in our galaxy; we just have to squint our eyes, so to speak, to look for them.
"These stars are extremely rare — it's much like finding a needle among an acre of haystacks," said Casey. "But with huge amounts of data from ground-based and space-based telescopes, the future looks good: we are closer than ever to understanding how stars formed in the early Universe."