if there is a super massive black hole in the middle of our milky way?

Question

if there is a super massive black hole in the middle of our milky way, "galaxy"
why are all the stars floating away from it? like all the stars in the milky way over time spread out more, right? then if there was a black hole in the center of the milky way "galaxy", why are they all floating away?

Answer 1

No, the stars are not floating away from the galactic center (which harbors a super massive black hole), they are in orbit around it.



What is spreading out are OTHER galaxies, with their own central black hole and their own stars orbiting.



A black hole behaves just like any other object of equal mass does. At a distance, you just orbit it. The difference is that one can get really close to a black hole because it is so compact, that is when things get bad.

Answer 2

There are a number of good partial answers here, and a few good total ones, as well as a few misleading statements.



First, let's dispense with the expansion of the universe as having any effect on the galaxy. Do a little math, and you can see this. The current best estimate of the Hubble expansion parameter, H0, is in the neighborhood of 72 km/s/Mpc (kilometers per second per megaparsec). Across the entire galaxy, a distance, D, of about 100 k lt-y, or 30 kpc = 0.03 Mpc, the total effect, as a velocity difference, of that expansion rate, amounts to



∆v = H0*D = 2.2 km/s



The orbital velocity of stars about the galactic center, once you get away from the central bulge, is a fairly uniform 210 - 240 km/s, or more than 100 times the Hubble expansion rate. In short, the motions of stars within any galaxy due to mutual gravitation, completely dominate any effect due to universal expansion. This is still true on the scale of galactic clusters. In fact, the Andromeda Galaxy, M31, is actually *approaching* our Milky Way (they and we are the two largest galaxies in our Local Cluster).



Next, Vincent G's statement that at any decent distance from a black hole (BH), its gravitating effects are just those of any compact object of the same mass, is totally correct. It's only when you get very close to the event horizon, or, for a rotating BH, the static limit, that things get crazy. Matter that gets that close, tends to wind up *joining* the BH (RIP).



And, yes, it is widely thought that there is a supermassive BH at the center of the galaxy, and no, the stars are not all floating away from it, nor are they "spreading out over time." They're following their regular orbits around the center. I know that a lot of popular portrayals of BH's make them out to be some sort of giant super-vac, sucking in everything in creation, but that isn't how they work, even in theory (and BH's really are in the theory stage at present, because by their very nature, if they exist, they would be nearly impossible to detect directly -- we have to rely on lots of other kinds of evidence for them, and it is quite substantial -- many things that we see, if not caused by BH's, would have to have even more exotic explanations).

Answer 3

They aren't.



The stars in the Milky Way galaxy are not "floating away" or "spreading out". The stars, including our Sun, orbit the center of the galaxy over a very long period.

Answer 4

There is a massive black hole. It's about 4 million solar masses. That's a small fraction of the total mass in the inner galaxy, much less than 1%.



At 4 million solar radii, the Schwarzschild radius is only about the size of the orbit of Mercury, and so the likelihood of a star colliding with it is small, even though the density of stars in the galactic center is small.



Since angular momentum and energy are conserved, and since stars are well separated, all the stars that are going to fall into the black hole have already fallen in. The rest are in orbits that don't intersect it.

Answer 5

It's indeed presumed that there is a massive black hole in the center of the Milky Way. Our galaxy is spinning and the pinwheel pattern we observe aided by the doplar effect help gives us that estimation.



We have a pinwheel shaped galaxy because there's dark matter surrounding it, otherwise the pattern would look like bicycle spokes.



Our galaxy is increasing in size due to expansion of the universe (theory) and not just centrifical force as a result of the gravitational spin.

Answer 6

Gravity is what keeps the stars clustered together

there is more and more evidence for a very large B-H in the center. we can only see stars not B-H



the Galaxies are moving apart due to the "expansion of (empty) space ( we think"

Answer 7

definite scientists have shown that there is a large great black hollow interior the path of our Milky way. Stars that are smaller use much less nuclear reactions to emit easy and likewise are less warm than larger stars. subsequently smaller stars stay longer than larger stars as they use greater power for nuclear reactions. thank you

Answer 8

Expansion of the universe due to dark energy. Here's a link to read on the subject

http://www.cbsnews.com/stories/2011/03/15/scitech/main20043535.shtml