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When I look at spiral galaxies they look a bit like the whirlpool of water going down a sink and I wonder if eventually they shrink to a small point (or a giant black hole?). On the other hand they may be growing and being flung apart by centrifugal forces. The sheer size and nature of galaxies means that in the few hundred years that we have been making astronomical observations, we have not come close to seeing a galaxy complete even a tiny fraction of a complete rotation, so do we really know if galaxies are growing, shrinking or stable in size?

Recently we have been informed that planets start out as very flat disk like proto-planets that eventually stabilise to near spherical oblate spheroids and I am wondering if a similar process is happening with flat disk galaxies? Are flat disks inherently unstable and tend to evolve to something more sphere like?

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Right now big galaxies like the Milky Way are growing by absorbing smaller galaxies - this is happening right now to the Sagittarius Dwarf and in the past to several other galaxies. Plus, there has been an infall of matter over time to the galaxy, but it is slowing as the universe expands.

But once a galaxy has formed, and if we ignore the above additions, it is nearly stable. Stars move through the gravitational field of all the other stars in pretty stable orbits. It is not far from how the Earth orbits the sun, except that here the orbit is around the galactic centre of mass. In principle this can go on forever.

There is a "but" here. There are some instabilities, like how the core may form an elongated bar that persists. Density waves can emerge, produce galactic arms. But over really long time there is a tendency for star velocities to get randomized due to nearby encounters. This makes the disk fuzzier, and eventually may turn it into an elliptic cloud instead (especially when the Milky Way merges with Andromeda Galaxy).

But the big problem is that close stellar encounters will from time to time give one star more than escape velocity. That star flies out of the galaxy and is lost, and the other typically gets an orbit closer to the core. Over very long time ($\sim 10^{20}$ years), this depletes the galaxy and makes it shrink into an ever denser cluster around the central black hole, which eventually eats all the remaining stars there.

So galaxies are stable on normal galactic rotation timescales (except for mergers), but unstable over very long timescales.

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    $\begingroup$ Is this "the milky way is growing" a growth in volume, or just mass? $\endgroup$ Mar 15 at 23:56
  • $\begingroup$ Re "over really long time there is a tendency for star velocities to get randomized due to nearby encounters.": Doesn't that require a very high density of stars? How long time is a "really long time"? $\endgroup$ Mar 16 at 2:36
  • $\begingroup$ @PaŭloEbermann Generally both. $\endgroup$
    – 4NT4R3S
    Mar 16 at 2:49
  • $\begingroup$ What about stars that are not exactly on the equatorial plane of the galaxy? They experience a force component towards the equatorial plane on top of the regular centre seeking force. Does this lead to a flattening (but not necessarily widening) of the galaxy disk over time? $\endgroup$
    – KDP
    Mar 16 at 7:55
  • $\begingroup$ Isn't heating the interstellar medium another mechanism to shed energy? $\endgroup$ Mar 17 at 14:27

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