Young stars should, the theory goes, be found close to the place of their birth, in star forming regions – giant dark clouds of dense gas (dense compared to the vacuum of space, anyway). The closest of those are 400-500 light years away, and all lie in a particular band on the sky called “Gould’s Belt”. Stars are supposed to form when a gas cloud gets compressed (either a supernova shockwave, gravitational effects from a passing galaxy, or the expanding bubble produced by the winds of many hot stars), and they’re born in huge batches.
A little over 30 years ago, two astronomers discovered something strange that shouldn’t have existed.
Slavek Rucinski & Joachim Krautter* were getting observations of TW Hydra, a star that had previously been identified as probably young-ish, because it had signs of an active stellar photosphere (lots of starspots, flares, etc). What they found was that the light from TW Hydra wasn’t just indicating lots of flares and starspots, it was incredibly variable and chaotic (they use the term “highly erratic”), with enormous changes in the amount of light coming from the star… it wasn’t just young-ish, it was a young star. In human terms, it was a toddler throwing a tantrum. TW Hydra was a genuine example of a type of star called a T Tauri star – a class of variable star whose namesake, T Tauri itself, is a young star in the Taurus-Auriga star forming region, about 500 light years away.
TW Hydra, however, seemed to be isolated. It wasn’t near any dark clouds, and (as it turns out) it’s more like 180 light years away than 500. That was a problem. Sure, there are the Hyades and the Pleiades that are nearby, but they’re not all that young, and certainly not young enough to be a T Tauri star. Anything nearby isn’t terribly young, and anything terribly young isn’t nearby. So what was TW Hydra?
Six years later, in 1989, another group of people found two stars that seemed to share the motion of TW Hydra; in 1992, two more were found. By 1997, it was clear that there was an entire group of stars moving through space in the same direction as TW Hydra. It was a tiny loose little group of stars about 12 million years old – that’s still young when you consider most stars live for BILLIONS of years. And there was no way any of them could be near any of the star forming regions you’d normally find stars that age.
So what happened? What the TW Hydra association represents is the smallest scale of star formation. We’re used to stars being formed in huge batches, like the Orion Nebula, or Taurus-Auriga, or any number of other famous and beautiful regions. But TW Hydra and its friends (and others like it found since) represent something smaller: Rather than being born in huge batches, a tiny wisp of gas has been compressed just enough to set off the cascading gravitational collapse that forms stars, and a tiny batch has been made. Where the Pleiades has at least a thousand stars all held together (for now) by gravity, TW Hydra never was. If normal star formation is a firework going off, TW Hydra was a firecracker whose fizzling produced somewhere around 50 stars (and probably a few more as yet unnoticed) that are steadily dispersing into space. What little gas there was, is gone.
Since roughly the year 2000, we’ve steadily learned that TW Hydra is not alone. As you’d expect, if it can happen once, it can happen again. There are now somewhere around 15 groups of nearby young stars (the number is still under debate*) with ages between 5 million years old and 500 million years old (also under debate*).
This animation I’ve made shows where the stars are relative to the Sun. The circles are at 25 parsecs (81 light years) and 100 parsecs (326 light years), and at the center is the Sun.
And yes, the Sun is actually in the middle of some of these groups. Ursa Major (500 million years old), beta Pic (25 million years old), and AB Dor (125 million years old) all overlap with each other and surround the Sun.
*Both Rucinski and Krautter are still around and doing science. That’s one of the most fun things about this field of research: it’s so new.