The Andromeda Galaxy

A giant spiral next door

Andromeda is the nearest large galaxy to the Milky Way and the most distant object most people can see with the naked eye. Under a dark sky it appears as a faint, elongated smudge larger than the full Moon.

It is a grand spiral galaxy: a bright central bulge wrapped in well-defined arms traced by young, hot blue stars and glowing pink nebulae. A faint halo of old stars extends far beyond the bright disc.

The bright nucleus

At its heart, Andromeda has an unusually dense, double structure of stars. Long thought to be a single supermassive black hole, the nucleus appears instead to hold a compact cluster of old stars around a central black hole of roughly 100 million solar masses.

This bright, compact core is one of the most studied regions in the galaxy, visible in images as the brilliant centre of the spiral.

Its companion galaxies

Like the Milky Way, Andromeda has its own retinue of satellites. The two brightest are M32, a compact elliptical galaxy close to the disc, and M110, a dwarf elliptical a little farther out. More than a dozen fainter dwarf galaxies have been found around it.

Together, Andromeda, the Milky Way, and their satellites make up the largest members of our Local Group of galaxies.

A hidden halo

Surrounding Andromeda's bright disc is an enormous halo of gas and old stars, detected in ultraviolet and X-ray light. The halo is so large that it may already overlap the halo of our own Milky Way, even though the two galaxies are still 2.5 million light-years apart.

A future collision

Andromeda is moving toward the Milky Way at about 110 kilometres per second. In roughly four to five billion years, the two galaxies are expected to make their first close pass and become gravitationally bound, before merging over several billion more years.

Despite the dramatic name, such a collision is gentle for the stars themselves: they are so far apart that individual stars almost never strike one another. Gravity and gas reshape the galaxies, not impacts.