The predicted collision and merger between our Milky Way galaxy and the neighboring Andromeda galaxy 3.5 billion years from now will result in an enormous new elliptical galaxy, but not for another 6 billion years, researchers say.
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How colliding galaxies would look in our night sky
The event “will indeed be unprecedented” in the history of the two galaxies. Andromeda, also known by its astronomical designation M31, is a Milky Way-size galaxy some 2.5 million light-years away. On a clear night under dark skies, its nucleus is visible to the naked eye as a small, glowing fuzz ball.
Although it looks as if Andromeda is setting still, the two galaxies are actually careening toward each other at about 250,000 miles an hour.(This is about 25 times faster than the speed of a bullet.)
The closing speed increases with time because each galaxy's gravitational tug on the other grows stronger as they two draw closer. By the time of the first encounter about 4 billion years from now, the two will have reached a combined collision speed of roughly 1.25 million miles an hour. This means that an already small sideways motion, which doesn't change with time, becomes increasingly insignificant compared with the rising collision speed.
Upon impact the stellar populations of both galaxies are jostled and the Milky Way loses its flattened pancake shape with most of the stars on nearly circular orbits. The galaxies’ cores merge, and the stars settle into randomised orbits to create an elliptical-shaped galaxy.This will be a major makeover for our galaxy as we know it; the sun will be flung into a new region of the galaxy, but the Earth – our Earth – and solar system will not be destroyed. With light years between neighboring star systems galaxies are mostly empty space and it is very unlikely that any two stars will collide.
Immense gravity interaction will be the main cause in these changes, not impacts between solid matter.