Andromeda–Milky Way collision

The predicted close encounter between the Andromeda Galaxy and the Milky Way, which may lead to their merger under mutual gravity.

Andromeda Galaxy: The nearest large spiral galaxy to the Milky Way, about 2.5 million light-years away. Its measured motion toward the Milky Way makes it the other participant in the predicted encounter.
William Herschel: An astronomer who discovered Uranus and studied nebulae and stellar systems. His early observations established Andromeda as an object whose nature and motion invited comparison with the Milky Way.
Milkomeda: A proposed name for the remnant galaxy that could form if the Milky Way and Andromeda merge. It names the possible large-scale outcome of the encounter.
Triangulum Galaxy: A spiral galaxy in the Local Group, also known as Messier 33. Its gravity may affect the Milky Way–Andromeda orbit and its own future participation is uncertain.
Milky Way: The barred spiral galaxy containing the Solar System. Its gravity and motion jointly determine the orbit of the approaching Andromeda Galaxy.
Great Debate: The 1920 debate between Harlow Shapley and Heber Curtis over the scale of the universe and the nature of spiral nebulae. The debate framed whether Andromeda was a nearby nebula or a separate galaxy like the Milky Way.
Star formation: The process by which dense regions of interstellar gas collapse to form stars. Tidal forces during a merger can compress gas and alter the galaxies’ star-forming activity.
Gaia: A European Space Agency space observatory that measures positions and motions of stars and other objects. Astrometric data from Gaia help refine the motions of Local Group galaxies and their companions.
Local Group: The galaxy group containing the Milky Way, Andromeda, and dozens of smaller galaxies. The encounter unfolds within this group’s shared gravitational environment.
Edwin Hubble: An astronomer whose observations established that galaxies exist beyond the Milky Way. Hubble’s work confirmed Andromeda as an external galaxy, making a galaxy-scale encounter a meaningful possibility.

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