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When the Milky Way galaxy collides with the Andromeda galaxy, what will the mass and volume of the new galaxy be?

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The mass will be slightly less than the combine masses of the 2 galaxies since some of the stars will be hurled away.

Since the disks of the galaxies are at an angle to each other, the volume would be (roughly) the volume of the 2 galaxies as they collide. Eventually, the volume will decrease (I'm guessing to between 60 to 70%) as the stars adapt to their new environment and the central massive black holes combine.

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    $\begingroup$ The volume will be dependent on how you define the size of a galaxy. One definition is the half-light radius r_1/2, which is the radius inside which half of the luminosity is emitted. Since the merging will enhance star formation (for some time), the luminosity and hence r_1/2 will increase, so the combined volume will be larger. Another definition is the virial radius, which depends on the velocity dispersion of the stars and dark matter. This will probably also increase. It's true that as the system slowly relaxes, the size descreases somewhat, but this is on very long timescales. $\endgroup$ – pela Mar 16 '15 at 9:52

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