Andromeda: The Chained Maiden

Andromeda looks like nothing much at first: a loose chain of medium stars hanging off a corner of the Great Square of Pegasus. Then you find the smudge near her hip, and it stops looking like nothing. That smudge is another galaxy, over two million light-years away, and on a dark night your naked eye can catch it.

Who was Andromeda, and why was she chained to a rock

In the Greek story, Andromeda was a princess whose mother, Cassiopeia, made the mistake of boasting that her daughter's beauty outshone the sea nymphs. The sea god's punishment fell on Andromeda rather than Cassiopeia: she was chained to a coastal rock, offered up as a sacrifice to the monster Cetus. Perseus arrived in time, killed the monster, and freed her. They married.

It is a small, ungenerous myth if you think about it for more than a second — Andromeda did nothing and was punished anyway — but it gave us one of the sky's oldest hero-rescue plots, and it comes as a package: Perseus, Cetus, and Cassiopeia are all up there near her, telling the same story across four constellations.

The star that is not quite the brightest star

Alpheratz, Alpha Andromedae, sits at magnitude 2.06 and marks a shared corner with the Great Square of Pegasus — it technically belongs to both patterns historically. But check the numbers before you call it the constellation's brightest star, because it isn't. Mirach, the Beta star, comes in at magnitude 2.05, a hair brighter. It is a small gap, invisible to the eye, but the alpha-is-always-brightest assumption breaks here and it is worth knowing why: Greek letter assignments were often about a star's role in the pattern, not a photometric ranking. Almach, the third bright star, rounds things out as a gold-and-blue double star, a nice target if you ever get a telescope on it.

The galaxy that is older than most of human history by the time you see it

The Andromeda Galaxy, catalogued as M31, is the nearest large spiral galaxy to our own and the most distant thing most people will ever see with the naked eye. It has two small companion galaxies riding along with it: M32, a compact elliptical, and M110, a fainter dwarf companion. All three sit close together on the sky map, though only M31 itself is an easy naked-eye catch from a dark site.

The earliest surviving written record of it is Persian, not Greek. The tenth-century astronomer al-Sufi described it in his Book of Fixed Stars, around 964 CE, calling it a little cloud. He had no way to know he was describing a separate galaxy over two million light-years away — to him it was just a fixed smudge that did not behave like a star, worth writing down.

Finding Andromeda in your own sky

Andromeda is a northern-hemisphere constellation, and its best month is November, when it rides high in the evening sky well clear of the horizon haze. Start at the Great Square of Pegasus, follow the chain of stars off its northeast corner, and you are walking Andromeda's own chain, the one that once held her to the rock.

What this means for you tonight

Open the sky map and find Andromeda for your own location and date — November evenings are when it climbs highest, but if you're not there yet, look for it rising in the northeast after dark any clear autumn night. Once you have Alpheratz and Mirach, the galaxy sits just off the chain near the second bright star; a truly dark sky is what turns it from theory into something you actually catch out of the corner of your eye. If your rising sign happens to fall in this stretch of sky, that horizon patch is worth a second look regardless of the season — it is the same real estate the chart is measuring.

For more on how this pattern got its shape, read what a constellation really is, and for why Mirach edges out Alpheratz by a hundredth of a magnitude, read how star brightness works. The rescue story continues in Perseus: The Hero, and the corner star Andromeda shares belongs to Pegasus: The Winged Horse.