Andromeda is in the northern sky eternally chained to her rock. She is one of six constellations described by Ptolemy in the second century that are all part of the same ancient myth. The others are: Perseus, Cassiopeia, Cepheus, Cetus and Pegasus.
Andromeda was the beautiful daughter of King Cepheus and Queen Cassiopeia of ancient Ethiopia. Her boastful mother bragged that she – and in some versions, Andromeda – was more beautiful than the sea nymphs. The nymphs were outraged by this arrogance, and Amphitrite, who was one of the nymphs, took the complaints to her husband Poseidon. The classical gods did not tolerate mortals getting above themselves, so the sea god sent the sea monster Cetus to ravish the Ethiopian kingdom.
An oracle told the king and queen that to save the land, they had to sacrifice Andromeda to the monster. But when she was chained to a rock to await her fate, the hero Perseus saw her on his way home from killing Medusa the Gorgon. The king and queen agreed that if he killed the monster he could marry their daughter, and he did.
Perseus and Andromeda went on to have a large family, and they were the mythical ancestors of the Persians. In the story we know now, Andromeda's role seems to be looking beautiful and helpless, and getting rescued. However there was probably more to it originally. Long before the Greek myths existed, some of the stars in Andromeda's constellation were part of a Babylonian constellation representing a goddess.
The modern constellation Andromeda is a defined region of sky that includes the classical star pattern representing the chained maiden. Here is an early 19th century depiction of Princess Andromeda.
In this Earthsky diagram of Andromeda you can see that the figure of Andromeda forms a sideways V-shape off the Great Square of Pegasus. Oddly, one of Andromeda's stars is one corner of the square of Pegasus. The star is Alpheratz (Alpha Andromedae). It used to be shared with Pegasus, but every star is now assigned to only one constellation. Seen from Earth, Mirach (Beta Andromedae) and Alpheratz are equally bright. But in fact, Mirach is a red giant and much more luminous than Alpheratz, but it's twice as far away from us.
Brighter still is Almach (Gamma Andromedae). It isn't named on the diagram, but it's the star shown just to the left of Mirach. Despite its faint appearance it's the most luminous star in Andromeda. It's much brighter than Mirach, but about 75% farther away. Almach has been known as a double star since the eighteenth century, and it's popular with amateur astronomers because it's a lovely sight in the telescope. The primary star is a bright golden yellow and the secondary a deep blue. Since the secondary star has turned out to be a triple system, Mirach is, in fact, a quadruple star.
Ross 248 is a red dwarf just nine light years away. It's the Andromeda star closest to us, yet it's too dim to see without a telescope. It's moving at high speed in our direction, and in about 33,000 years it will be closer to the Solar System than Proxima Centauri.
Upsilon Andromedae (Titawin) is a binary star about 44 light years away. The primary star is similar to the Sun, though somewhat younger and brighter. There are at least four planets orbiting it. It was the first multiple-planet system discovered orbiting a sunlike star and also the first discovered in a multiple-star system. The planet Upsilon Andromedae d (Majriti) is in the star's habitable zone, but the four planets appear to be gas giants bigger than Jupiter.
Deep sky objects
Andromeda has one prominent planetary nebula: NGC 7662, popularly known as the Blue Snowball Nebula. A planetary nebula is created when a dying sunlike star swells into a red giant and starts losing its outer layers. Some of them – like this one – are quite rounded, and in eighteenth-century telescopes looked like a planet's disk.
Our nearest large neighboring galaxy is M31, the Andromeda Galaxy. At 2.5 million light years away, it's the most distant object visible to the unaided eye. We don't know who was the first to notice it, but the first record we have is the Arab astronomer al Sufi's Book of the Fixed Stars in 964.
In the future, M31 will be much closer to us, as it's on a collision course with the Milky Way. The two galaxies will eventually merge. The good news is that individual stars are so far apart they won't crash into each other, but the Sun and Solar System could well move into a new neighborhood. The excellent news is that this isn't going to happen for several billion years so we needn't lose sleep over it.