CowedbyWisdom said:
What determines the traveling speed of a star?
It goes like this:
When you've got just two bodies orbiting common centre of mass(e.g.,Earth-Moon) then the velocities of both can be exactly predicted from laws of motion and the law of gravitation. The farther the bodies apart, the lower their orbital velocities.
If the two bodies are sufficiently far away from any other gravitational influences, then treating them as isolated is a good approximation(so we don't really need to think about the Sun when calculating Moon's orbit).
But when you try to do the same calculations with three or more gravitating bodies, the equations become unsolvable apart from certain special cases(e.g., the
lagrangian points,
resonances; see:
http://en.wikipedia.org/wiki/N-body_problem). In general, though, the motion of the bodies is unpredictable in the long run, as the multiple masses tug on each other and perturb their orbits.
The bottom line is, with many bodies, you may end up with a semi-stable configuration like our solar system, or with some bodies gaining enough speed from the gravitational interactions to be completely flung out of the system. The actual result is pretty much random.
These unpredictable interactions are what determines the peculiar velocities of stars. Peculiar velocities are local velocites, relative to neighbouring stars, not directly related to the global orbital speed around the galactic centre.
If you now take the whole galaxy, with its myriads of stars distributed in somewhat regular fashion across vast distances, then their effects on some particular star you're interested in become predictable again(to an extent). You can think of the galaxy as if it were a giant disc-like body made of an insubstantial soup of particles(stars) that collectively act on a given star.
In particular, the distribution of mass in the disc causes the stars to be attracted to the centre of the galaxy - inducing orbital velocities, and to the plane of the disc - causing the orbits to "wobble" up and down as the stars traveling above the disc are pulled back in, pass throuh it, and are pulled in again.
So, taking the Alpha Centauri(a binary system) for example:
At the local level, the motion of the two stars αCenA & αCenB are determined by mutual gravitational attraction of the two binary components.
The star Proxima Centauri is just enough removed from the other two to make it difficut to say if its motion is still bound to the two other stars, or if its motion is already outside their influence, being significantly tugged in random directions by its other neighbours, including the Sun.
This is the level on which it begins to make sense to think in terms of peculiar velocities.
As you move farther away, you begin to notice that the whol Alpha Centauri system, and its neighbours, despite moving somewhat randomly amongst themselves, have a common motion around the galactic centre.
As an additional note, it is worth mentioning that the orbital motion in the galaxy does not conform to the Keplerian laws of planetary motion, in that the orbital speed remains roughly constant for all stars but those closest to the centre(while for planets it goes down with distance). A discrepancy which prompted the introduction of Dark Matter to explain it.
If black holes are not bound to stillness does that mean that black holes can travel at high velocities? So fast that they travel faster than something, say our sun?
It's best to think of black holes as just another kind of star. As far as motion goes, they behave identically in all respects. They just don't shine, and when you get very, very close to them, you get spaghettified(
http://en.wikipedia.org/wiki/Spaghettification) instead of burning up.
If the escape velocity of the milky way is about 232 km/s and an object traveling towards the "walls" of the galaxy reaches that "wall" what happens if it is not traveling that fast?
First of all, I'm pretty sure the escape velocity in the galaxy is not 232 km/s, but something closer to 500km/s(from where the Solar System is located). ~230 km/s sounds like the orbital speed of the Solar System around the galactic centre.
As for the question, I've already touched on it earlier. The star that travels above the galactic disc will eventually get pulled back in, pass through it and repeat the same thing ad infinitum, unless it gains enough peculiar velocity from local interactions to exceed the escape velocity.
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