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  #51  
Old 04-26-2012, 11:55 AM
Chronos Chronos is offline
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There are a few stars that orbit in wild and crazy directions, though. Barnard's Star, the next closest star after the alpha Centauri system, is a halo star that just happens to be passing through our neighborhood right now, making it the galactic equivalent of a comet.
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  #52  
Old 04-26-2012, 12:04 PM
Anne Neville Anne Neville is offline
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Every galaxy has a halo component, which includes stars and globular clusters orbiting the center of mass of the galaxy in a spherical distribution (ie, not confined to the disk).

In our solar system, there are some moons that orbit their planets retrograde (the other way around from how the planets orbit the Sun). If you were looking at the solar system in such a way that the planets seemed to be going around the Sun counterclockwise, these moons would be going around their planets clockwise. We think these moons were captured into orbit around their planets- they weren't formed in orbit around the planets, the way the Earth's moon and most other large moons in the solar system were. Galaxies capture stars and smaller galaxies, too, so there's no reason to think there wouldn't be stars orbiting our galaxy the "wrong way".
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  #53  
Old 04-26-2012, 01:54 PM
Capt Kirk Capt Kirk is offline
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Quote:
Originally Posted by JBDivmstr View Post

And on a different note... Whose insurance is going to have to pay for damages?
Obviously Andromeda Galactic Insurance as we obviously have the right of way, look at the crazy angle they are speeding at us from!
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Old 04-26-2012, 02:41 PM
Polycarp Polycarp is offline
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Quote:
Originally Posted by JBDivmstr View Post
Whose insurance is going to have to pay for damages?
Nobody's. They've got several billion years to find a reason for rescission!

Last edited by Polycarp; 04-26-2012 at 02:42 PM..
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  #55  
Old 04-27-2012, 03:28 AM
dtilque dtilque is offline
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Quote:
Originally Posted by Jragon View Post
So we have

Luna->Earth->Sol->Solar System's CM->?...->Galaxy CM->Local Group CM->...

How many non-trivial "?" are in there? Is there any formal hierarchy like "group of solar systems"->"cluster of solar system groups"->"group of clusters" etc in between? The terms themselves I made up, but I hope they kind of illustrate what I'm getting at.
None. The Sun orbits the center of mass of the Galaxy. Note however that it's not a simple elliptical orbit, but then you can't appoximate the Galaxy as a point mass, which is a prerequisite for an elliptical orbit.. The Sun kind of bobs up and down "above" and "below" the plane of the galactic disk. If you plotted the path with respect to that plane, it would look like a sine wave with a period of (I think) about 80 million years. Much less than the orbital period, anyway. You can think of this bobbing motion as something vaguely like an orbit about the galactic disk.

While there are star clusters, the Sun is not in one. In general, star clusters are either extremely old (globular clusters, which pre-date the galactic disk) or very young (open or galactic clusters). In general, stars form in clusters, but those evaporate as interactions between the various members eject stars. The Sun left its natal cluster a very long time ago.

Globular clusters evaporate too, but they started with many more stars than open clusters do, so they haven't completely evaporated yet. The stars that are left are orbiting very close to each other compared to stars in the galactic disk. And sometimes they do collide. When they do they form blue stragglers.



JBDivmstr Sorry for any confusion I caused. I suppose I should have said "almost certainly won't collide" or something like that.
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