Andromeda stands in the northern sky eternally chained to her rock. She is one of five constellations that Ptolemy described in the second century, which are all part of the same ancient myth. The others are: Cepheus, Cassiopeia, Perseus and Cetus.
Andromeda was the beautiful daughter of King Cepheus and Queen Cassiopeia of Ethiopia. Her boastful mother bragged that Andromeda - and in some versions, herself as well - was more beautiful than the sea nymphs. The nymphs complained to the sea god Poseidon. The classical gods did not tolerate mortals getting above themselves, so as punishment, he sent the sea monster Cetus to ravish the kingdom.
Cepheus consulted an oracle which said that to save the kingdom, they would have to sacrifice Andromeda to the monster. She was chained to a rock to await her fate, but, happily, 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 so he did.
As we know the story now, Andromeda's role seems to be looking beautiful and helpless, and getting rescued in the nick of time. But there was probably more to it originally. After they married, she and Perseus went on to have a large family and were the mythical ancestors of the Persians. In addition, long before the story of Andromeda, some of the stars in her 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. This diagram relates the main stars to the girl.
In this Earthsky diagram you can see that the figure of Andromeda forms a sideways V-shape off the Great Square of Pegasus. Notice that one of Andromeda's stars is one corner of the square of Pegasus. The star, Alpha Andromedae (also known as Alpheratz or Sirrah) used to be shared with Pegasus, but every star is now uniquely placed in one constellation.
Seen from Earth, Beta Andromedae (also known as Mirach) and Alpheratz are equally bright. In actuality, Mirach is a red giant that is much more luminous than Alpheratz, but it's twice as far away.
Gamma Andromedae (traditionally called Almach) isn't labeled on the diagram, but it's the star just along from Mirach. It looks slightly fainter than Alpheratz and Mirach, but that is deceptive. It's the most luminous star in Andromeda, more than one magnitude brighter than Mirach, but about 75% farther away.
Almach has been a well-known double star since the eighteenth century. It's not only easily separated in a telescope, but the contrasting colors of the stars make for a beautiful sight. The primary star appears as a bright golden yellow and the fainter secondary as a deep blue. However, things are not what they seem, because the secondary turns out to be a triple star system.
The closest star to us in Andromeda is Ross 248, a dim red dwarf just nine light years away. Despite its nearness, it's too dim to be seen 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 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 is in the star's habitable zone, but all four planets appear to be gas giants like Jupiter, but bigger.
Deep sky objects
Andromeda has one prominent planetary nebula: NGC 7662, popularly known as the Blue Snowball Nebula. This image, taken by Chris Vedeler at the Steward Observatory, shows why. 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 looked like a planet's disk in 18th century telescopes.
William Herschel discovered the edge-on spiral galaxy NGC 891. It's about the same size as the Milky Way - around 100,000 light years across - and is 30 million light years away.
A spiral much closer to us is the most famous deep-sky object in Andromeda, M31, the Andromeda Galaxy, here imaged by by the GALEX space telescope. At 2.5 million light years away, it's the most distant object visible to the unaided eye. It's twice the size of the Milky Way, but has only half the mass. 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.
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