Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: RubyO on 23/05/2019 11:35:49

Title: How do galaxies keep a stable size?
Post by: RubyO on 23/05/2019 11:35:49
Peter has this cosmic query:

How can each and every galaxy, given their multiplicity of sizes and shapes,exist in a finely-tuned dynamic equilibrium with the expansion of space such that no individual galaxy expands or contracts?

Can anyone expand on an answer?
Title: Re: How do galaxies keep a stable size?
Post by: Janus on 23/05/2019 16:40:07
Individual galaxies do not expand due to the expansion of space because they are compact enough that the mutual gravitational attraction holds them together. This effect actually extends all the way out to include groups of galaxies.  The individual galaxies of our local group are not receding from each other but orbit each other as a gravitationally bound system.  It the larger groups of galaxies that are moving apart from each other.

The individual stars in a galaxy are in orbit around the their mutual center of gravity.  Orbits are not as fragile as some people tend to think they are.  If you slow down or speed up an object in orbit, it will not necessarily "fall out" or "fly off" of orbit.  Unless you make a large change in its speed, it just settles into a new orbit.   So for example,  if you wanted to make the Moon "fall" into the Earth,  you would have to reduce its orbital speed to less than 1/5 of its present orbital speed, and to make it fly away into space, you would have to increase it by almost 41%.   
Galaxies are made of stars that fall somewhere between.  It's a bit more complicated with galaxies, but the general idea is the same. Stars in the galaxies don't have to maintain a delicate balance to remain in orbit and have a fair amount of "wriggle room". *
This is not to say that galaxies don't evolve over time.  Early in their lifetimes, things are a lot less stable.   You still had material falling in towards the center feeding the black hole their, etc.   And even later, and occasional close encounter will fling a star clear of its parent galaxy.  But the vast majority of stars in a mature galaxy are in that "comfort zone" as far as their orbit goes, and would take quite a bit to knock them completely out of it.

* As an analogy, think of a rocking chair.  If you give it a push, it will rock back and forth, but won't fall over. It takes quite a large shove to make it completely topple over.