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In watching this Hubble video on Omega Centari it got me thinking about how dense a galaxy can get. Do we know how dense a galaxy can get?

Are there any good resources on reading more about very dense galaxies?

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  • $\begingroup$ How can you define a boundary for the galaxy, to get its dimensions? $\endgroup$ Aug 30 '16 at 18:44
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    $\begingroup$ And just what do you mean by density? Do you mean the number of stars per cubic parsec? The amount of mass per unit volume? Does that include dust? Dark Matter? Dark Energy? $\endgroup$
    – zephyr
    Aug 30 '16 at 19:58
  • $\begingroup$ To extend zephyr's comment: Do you mean the average density of the galaxy, or the maximum density somewhere within the galaxy? (The center of the Milky Way is much denser than the average density of the Milky Way.) $\endgroup$ Sep 3 '16 at 11:12
  • $\begingroup$ I know this is probably a dumb question, but what I'm interested in is how crowded could a field of stars get near the center of a galaxy and still be somewhat stable. So how many stars in a cubic light year might be a more intuitive way for me to understand it. $\endgroup$ Sep 3 '16 at 13:34
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As a general rule, galaxies aren't very dense at all, though precise numbers are impossible, rough estimates can be made.

Mass of the Milky-way: with it's dark matter halo, about 6 x 10^42 kg

Volume of the Milky-way (per Wikipedia) about 3.3 × 10^61 cubic meters, or, 3.3 x 10^52 cubic km.

That gives us a rough density of the milky way of about 1 kg for every 5 billion cubic km. That's obviously not very dense. There are probably denser galaxies out there and larger galaxies, but the overall density is going to be very low, even for the densest galaxies. Space is, generally speaking, mostly empty.

Even in regions that are so called "densely packed with stars", if you measure the density in the traditional way, mass to volume, you end up with very low density, 1 kg per thousands if not millions of cubic km. On earth, that density of air would essentially be a vacuum.

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