How do they determine the bounderies between galaxies?
How do the Astronomers determine where one Galaxy begins, and another ends? Are there some sort of “Boundry Lines” between galaxies?
Galaxies have no real boundaries. Probably the best way to determine if an object is in one galaxy or another nearby galaxy is to decide which galaxy has a stronger gravitational pull on that object.
With the advent of the concept of Dark Matter, this question becomes even more confused, since two nearby galaxies may have intertwined Dark Matter halos, blurring any possible boundaries even further.
Planets, Stars, Nebulae, Galaxies – Universe Size Comparison 2009 [HD]
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