The Andromeda Galaxy

Along with its cool, lengthening nights, autumn's arrival in the Northern Hemisphere brings some of the year's most enjoyable observing. Shortly after darkness falls in late September, the Andromeda Galaxy is ideally positioned high in the east for Northern Hemisphere observers, near the W-shaped constellation Cassiopeia. If you are new to hunting deep-sky targets, the Andromeda Galaxy provides an enticing introduction. This enormous spiral galaxy is called Messier 31, or M31 — the 31st object in 18th-century French astronomer Charles Messier's catalog of nonstellar objects. The location of M31 within the constellation of Andromeda is marked on our interactive sky chart; click on the "change" button to alter either the date and time or viewing location displayed by the chart.

M31 is generally considered the most distant object visible to the unaided eye on a dark night. It is about 2.5 million light-years away — distant enough to reduce the combined luminosity of its estimated 400 billion suns to a faint glow. City lights wipe it out for naked-eye viewers, but you can still detect its elongated central portion with binoculars. Under a truly dark sky the binocular view of M31 is magnificent, extending much of the way across the field of view. Backyard-type telescopes can typically encompass only a portion of its glowing expanse in a single view, and rarely do they show any detail. The view of M31 through larger telescopes reveals dust lanes in the galaxy's spiral arms.

Once you become a deep-sky aficionado, you will spend the rest of your observing career hunting out galaxies much fainter and smaller than this one. If Andromeda's subtle beauty fails to impress at first, consider this: if our own Milky Way galaxy were as distant, it would prove to be an even greater observing challenge!