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Unruly Milky Way
empaler writes "Space.com is running a story about the movement history of the local group near our solar system. The belief until now has been that after an initial period of chaos in our galaxy, it had since 'been rather calm'. 'But this turns out not to be true. Stars have been perturbed all the time throughout the Milky Way history.'"
Stars are so small and so far apart that they almost never come close to hitting. Main sequence stars are roughly separated by one parsec (3.26 light-years). You can think of it like having one golf ball ten miles away from you and hitting another and asking if they are likely to collide. Galaxies do collide often and interact because they are close together compared to their width.
120 chars of filth!
The can figure out where things are going about as well as they can figure out where things came from. It depends critically on our knowledge of where everything else is that the stars can gravitationally interact with. It can be highly chaotic when stars pass at all close to one another. (Using the appropriate definition of "close," which does not include any likelyhood of collision.) If there's a dense gas cloud we're not aware of in the wrong spot, that would invalidate much of this kind of simulation forward or backward.
Local stars are all moving slowly enough (relative to us) that there are no surprises likely. The only kind of surprises we could get would be a brown dwarf or the like, something not visible until it's close. In terms of human lifetimes, this isn't changing very rapidly at all.
Remember, the animation from the article covers 250 million years! Two such rotations ago, there was no life on land and fish were only just beginning to appear. One rotation ago -- the beginning of the simulation -- was the time of the Permian mass extinction. That was a freaking long time ago! That predates roaches! And mammals.
If we're lucky, we'll still be around in another 250 million years.
Stars are so small and so far apart that they almost never come close to hitting. Main sequence stars are roughly separated by one parsec (3.26 light-years). You can think of it like having one golf ball ten miles away from you and hitting another and asking if they are likely to collide. Galaxies do collide often and interact because they are close together compared to their width.
Very true! Two things though:
Plotting ahead is useful because even though stars don't collide, they do come close enough to each other to disrupt planetary systems. Even just preturbing the Oort cloud a bit would cause destructive impacts on earth that could wipe out all life.
Second of all, galaxies "collide" however it's kinda like two clouds of smoke "colliding" because there's really not much actual physical contact (although black holes might eat quite a bit during this period).
Good to point this out though! Galactic dynamics is quite fascinating, and much more complex than you'd expect!
Cheers,
Justin
Rather than just asserting that blindly, why don't we work out the numbers?
The only place in the Milky Way that stars collide are inside globular clusters, where they form "blue stragglers". There are about 3000 known blue stragglers in the Milky Way (Piotto et al. 2004). The rate is probably higher now than in the past (it should increase with time as more clusters undergo core collapse), but averaging over the 13.7 billion year history, that's an average rate of one every 4.5 million years
Now, whether you consider that "quite frequently" or not depends on your perspective. ;-)
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It's possible to trace the orbits for the stars they have forward about as well as they can trace them backwards.
The problem, though, is that getting good enough kinematical information to do this is hard for stars that aren't near the Sun. You notice that all of the stars end up right near the Sun - that's not a coincidence! Those are the ones whose orbits we can get a handle on precisely because right now they're close enough that it's easy to get their kinematics.
You can imagine that if you waited 220 million years (one orbit) and did the same exercise, you'd find the same thing - the stars came from a broader range of radii "220 million years ago" (i.e. now). So right now, most of them are too far away for us to get good proper motions and/or good distances, and therefore good spatial velocities.
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