What fascinates me most about the nearby stars is how little we know about them. We’ve been going so far out on this branch of inquiry that leads to galaxies, halos, and the very edge of the universe itself that it’s easy to forget we don’t even know where all the nearest stars are. Or, even, what they are.
I used to use the analogy that it’s like not knowing how many states there are in the United States (well?) but it’s been pointed out that it’s a very flawed analogy. I still like it though.
At any rate, the fact that we know so little about the nearest stars lead me to the fascinating subject of nearby young stars. By rights, they shouldn’t exist… we are in an inter-arm region of the Galaxy (assuming, of course, that the Galaxy even HAS arms… they apparently aren’t well-defined) where there are very few star forming regions… there’s Taurus, there’s Orion, and there’s the Scorpius-Centaurus complex, which is this big snake-like band stretching across a large portion of the sky containing regions that have formed stars, will form stars, and might be forming stars now. But all of those are over a hundred parsecs away, and they’re not very big, especially when compared to the star-forming complexes in Carina, or the Arches cluster. The nearest O-type star is Zeta Ophiuchus, 140 parsecs away (that’s 400+ light years). We are not exactly surrounded by massive stars that don’t live long, that are the hallmarks of star forming regions.
And yet there are young stars here. Young stars like the Pleiades cluster, which is easily visible in the night sky, or the somewhat older Hyades cluster (also easily visible, but less distinctive), or the Ursa Major moving group discovered in the late 1800s.
That last one is worth some discussion, because it’s an exemplar of the unusual groups nearby- Ursa Major was noticed because the bright stars in the Big Dipper and its environs were found to all be moving in the same direction – a moving group. We now believe they, and an associated cloud of other objects spread over the entire sky, were all formed from a single burst of star formation at some point in the past – between 250 and 600 million years ago – in the Scorpius-Centaurus star forming region, and have since drifted into the Sun’s path. Because there are so few of them, they are quickly dispersing into the galactic background. This is the sad state of the Sun, where we’ve thus far been unsuccessful at finding any solar siblings. Studies like the Geneva-Copenhagen survey suggest that 4.5 billion years can scatter a group of stars all across the Galaxy, so it’s not all that surprising.
But Ursa Major (which may or may not include Sirius, on the other side of the sky from the stars in the constellation itself) is young enough that we can still find its other members. There are still others: what were bizarre isolated T Tauri (young and variable) stars are now whole clouds mixed into the general mishmash of nearby objects. People have found something like 20 of them, and not all of them are real or well-supported by evidence. These are things hiding in plain sight, right where we can (and have) been looking, like a vein of silver in a rock.
My paper, which comes out on the ArXiV preprint server tomorrow, is my second published foray into the chaotic and fascinating world of young stars. My first paper involved the discovery that a previously known star was probably 40 million years old, and only 27 light years away. This time, I’ve got two star systems that don’t really match any known moving group (but are definitely young), and one object that I claim is a member of the TW Hydra moving group, even though it’s really far from the rest of the group. And yes, given that it fails most of the tests for TW Hydra group membership, it may be somewhat unfair of me to claim that it IS a member… but I can’t think of anything else it could be: the star is near the known TW Hydra stars, is clearly at least as young as they are, and… well, if I’m forced to choose between “We just don’t know the TW Hydra moving group very well” and “I’ve found a completely new moving group” I’m going to assume we just don’t know TW Hydra very well. Someone else might make the opposite call.
The other two unusual stars have their own quirks. One is a fairly perfect match for a 30 million year old star system in the Carina moving group (never mind that several researchers claim, with evidence, that Carina doesn’t exist) but is close to us in the northern hemisphere, some 70 parsecs away from other known members. Is this a new group too, or are the existing groups simply not big enough? Does this mean there are other objects we should know about? The other is in the southern hemisphere where a well-behaved member of Tucana-Horologium should be, but it’s too bright. Way too bright. So bright that it appears to be a contracting star younger than the beta Pictoris moving group, not older like Tucana-Horologium is supposed to be. What is it? I don’t know, but for that one I AM tending toward “new group”.
And these discoveries were all made among nearby stars… very nearby stars, the kind you’d think we should definitely know everything about. But we clearly don’t. There are wild and wacky things everywhere, even in our own back yard. We don’t understand them yet, but that also leaves me a chance to be the guy who answers those questions. It’s a lot of fun.