Andromeda Galaxy

The Andromeda Galaxy is a large spiral galaxy about 2.5 million light-years from Earth and the nearest major galaxy to the Milky Way.

Spiral galaxy: A galaxy with a flattened disk, central bulge, and spiral arms containing stars, gas, and dust. Andromeda’s disk and arms place it in this broad galaxy class.
Andromeda–Milky Way collision: The predicted future interaction in which the Andromeda Galaxy and Milky Way will merge after approaching one another. Andromeda’s measured approach makes this major gravitational encounter likely.
Edwin Hubble: An American astronomer whose observations established that galaxies exist beyond the Milky Way. His distance measurement showed that Andromeda lies outside the Milky Way.
Andromeda Galaxy bulge: The dense, central concentration of stars in the Andromeda Galaxy. Its stellar populations and shape offer clues to the galaxy’s central history.
Milky Way: The barred spiral galaxy that contains the Solar System. It is Andromeda’s nearest major counterpart and the other galaxy in their approaching pair.
Messier 32: A compact elliptical galaxy that is a satellite of the Andromeda Galaxy. This close satellite helps reveal the Andromeda system’s structure and gravitational history.
Dynamical friction: A drag-like loss of orbital energy experienced by a massive body moving through a background of particles. It helps slow the galaxies’ relative orbital motion during their future merger.
Henrietta Swan Leavitt: An American astronomer who discovered the period–luminosity relation for Cepheid variable stars. Her relation enabled Hubble to estimate Andromeda’s distance from its Cepheids.
Andromeda Galaxy disk: The flattened rotating region of stars, gas, and dust in the Andromeda Galaxy. The disk contains much of Andromeda’s visible mass and its star-forming regions.
Triangulum Galaxy: A spiral galaxy in the Local Group, smaller than both Andromeda and the Milky Way. Its smaller size and satellite-like role contrast with Andromeda’s status as a major galaxy.

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