Object Analysis

The Andromeda Galaxy (Messier 31) is a barred spiral galaxy approximately 2.5 million light-years from Earth and the nearest major galaxy to the Milky Way. It is the largest member of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 44 other smaller dwarf galaxies. Despite earlier findings that suggested the Milky Way contains more dark matter and could be the most massive in the grouping, 2006 observations by the Spitzer Space Telescope revealed that Andromeda contains one trillion stars: at least twice the number of stars as the Milky Way.

Visually, Andromeda appears as a smudge of light to the naked eye in moderate skies, making it one of the furthest objects visible without equipment. Through a telescope, its bright core and dust lanes become apparent. It is approaching the Milky Way at about 110 kilometers per second, and the two galaxies are expected to collide and merge in about 4.5 billion years to form a giant elliptical galaxy.

The galaxy has a crowded double nucleus, a massive star cluster at its very heart, and a supermassive black hole hidden within. It is surrounded by a massive halo of hot gas that is estimated to contain half the mass of the stars in the galaxy itself. Andromeda has played a crucial role in our understanding of the scale of the universe; it was observing variables in Andromeda that proved "spiral nebulae" were actually separate island universes outside our own.