# The expansion of the universe and how stars and galaxies still collide

How can it be that all galaxies and stars are moving away from each other yet they still collide? From what I have researched it as an open universe (or many open universes) yet the idea of stars and galaxies colliding boggles me.

• scientificamerican.com/article/how-can-galaxies-collide – ApproachingDarknessFish Aug 15 '16 at 22:26
• That being said, in a nutshell, the universe is expanding, but its expansion compared to the proximity between most stars is miniscule. On a Galactic scale, most galaxies are moving away from the Milky Way because their distance from us is very significant, with one major exception being Andromeda, which is very close, relatively speaking. For Galaxies that are close to one another, collision is possible only because they are so close and their proximity outweighs the Expansion. – MozerShmozer Aug 15 '16 at 22:27

• …the two types of coordinates coincide, but in the past, at a time corresponding to a redshift $z$, the relation was $d_\mathrm{phys} = d_\mathrm{com} / (1+z)$. If you view a cosmological simulation in comoving coordinates (which you almost always do), you don't see the expansion, but only see the dynamics, and then you are right that the voids expand and compress the clusters. But if you view it in physical coordinates (which measure the "true" distances), the clusters also expand. Only on such small scales at groups is gravity strong enough to prevent them from receding from each other. – pela Aug 23 '16 at 7:20