Andromeda Galaxy

Under a dark sky it appears near the constellation Andromeda as a faint, elliptical smudge of light. In a photograph a huge, slightly tilted spiral disk emerges — a majestic sight in which a bright yellow core is wrapped in layer upon layer of blue spiral arms and dark dust lanes. Two small satellite galaxies can be seen alongside it.

The Andromeda Galaxy (M31) is the largest galaxy in the "Local Group" to which the Milky Way belongs — more than twice our galaxy's diameter, with a stellar population approaching a trillion. At about 2.5 million light-years away it is the most distant object visible to the naked eye, and the light reaching us now set out 2.5 million years ago. Remarkably, this galaxy is approaching us at about 110 km/s, and in roughly 4.5 billion years it is predicted to collide and merge with the Milky Way to form a single giant elliptical galaxy.