The Lives and Times of Very Ancient Stars
There’s recently been a lot of hullabaloo about what may be the oldest star ever discovered. Official name HD 140283, this second-generation star is thought to be at least 13.2 billion years old. (For comparison, the universe is presently thought to be ~13.8 billion years old.) “Second generation” means exactly that- the first stars following the Big Bang coalesced and died, most of them in supernovae due to their extraordinary size, and second generation stars formed from their remains. Though the article recently published in Nature magazine did not specify stellar mass, this ancient star must be very small to have husbanded its nuclear fuel for so very long.
Many articles have been published on this topic since the discovery was announced January 10. However, I see many of the same scientific questions arising in the commentary of these articles. I’d like to take some time to address the most common of these as they raise basic questions of stellar mechanics and evolution that are required for understanding many topics in astronomy.
The article reports HD 140283 contains traces of heavier elements that identify it as a second generation star. Shouldn’t the abundance be higher after all those years of fusion?
Stellar fusion is a much more complicated process than it’s made out to be. The key point for our very old star is that it is likely to still be on the main sequence (stars still fusing hydrogen in to helium). I was unable to find sources confirming its stage of evolution, but given that it must be a red dwarf (a class of very low-mass stars) this seems probable. Not enough time has passed for the star to exhaust its hydrogen fuel, as the lower the mass the slower the star burns. The star has simply not advanced far enough to begin to fuse heavier elements. It’s also important to note that in astronomy, “heavy elements” can refer to anything heavier than hydrogen and helium, since those elements dominate the universe and are primordial, having been created in quantity during the cooling of the early universe.
Some heavy elements are only created via special processes rather than fusion reactions. All elements heavier iron were first created by supernovae. (Once they existed, it became possible to generate elements via something called the s-process, using the heavy elements as seeds, but that’s another topic.) We know HD 140283 must be a second-generation star because it contains elements that simply did not exist in the universe when the first generation of stars formed, and it has them in the right abundance for the second generation. The universe is becoming increasingly enriched in heavy elements as time goes on.
The distance to HD 140283 is estimated at 190 light years, relatively close to the Sun in astronomical terms. How is it possible the oldest star is within striking distance, or that two stars of such different age could exist in close proximity?
First of all, nobody is claiming HD 140283 is the “oldest” star, though I’ve seen this assumption made repeatedly. It’s simply the oldest we’ve detected to date (probably).
The Sun’s age is estimated at only 4.6 billion years. It coalesced well after galaxies were firmly established in our universe. HD 140283, however, first began to shine while galaxies were still forming. It’s “allegiance”, as it were, may not be as solid as our sun’s. Rather than forming within the Milky Way, it may have simply found itself in the large gravitational hole that became our galaxy. Or it could have been one of its very first stars.
Stars tend to form in “nurseries” known as giant molecular clouds, enormous nebulae of gas and dust, often staying together as a loosely bound group after formation. These groups are known as open clusters and can contain up to a few thousand stars that possess similar age and chemical composition. However, open clusters are easily disrupted by close encounters with other gravitational bodies, and over the course of 13 billion years, it’s not surprising that some of those early stars may be wandering alone. Additionally, some stars are interlopers in our galaxy, having been ejected from satellite galaxies, globular clusters orbiting the Milky Way, or even more exotic locales. Information about the orbit and velocity of HD 140283 could tell us much about its possible origins.
Finding this ancient star in our backyard isn’t so much of a stretch as a happy coincidence, and not as statistically unlikely as it first sounds.
