Andromeda - The Chained Maiden

Andromeda is visible from August to January in the northern hemisphere. She lies between Pisces and Cassiopeia, with her head at the upper left corner of The Great Square of Pegasus, and her feet near Perseus. The star Sirrah marks her head, Mirach marks her hips, and Almach represents her chained foot. Andromeda is seen with her arms outstretched.

History and Mythology

This pattern of stars dates from the 2nd century AD. Andromeda was the daughter of King Cepheus and Queen Cassiopeia. Her mother offended Poseidon, the god of the sea, by boasting that she and Andromeda were more beautiful than any sea nymph. The Sea Nymphs were goddesses and disliked being compared with a mere mortal. They complained to the God of the Sea, Neptune, who sent out a monstrous sea creature, Cetus, to punish Cassiopeia.

To save his kingdom, Cepheus was told he must sacrifice Andromeda to get rid of the monster. To appease Neptune, the king chained Andromeda to a rock by the sea, to await her death from the sea monster Cetus. Soon the Whale emerged from the water and was about to swallow Andromeda, when the hero Perseus was passing by on his winged horse Pegasus. Looking down, he saw Cetus approaching the chained Andromeda. At the last moment he rescued Andromeda by turning the monster Cetus to stone.

Notable Stars

Second-magnitude Alpheratz, or Alpha Andromedae, is the brightest star in Andromeda. It is a blue-white subgiant 97 light years away, with 200 times the Sun's luminosity. Its name is now thought to mean "the horse's shoulder", showing that the star originally belonged to Pegasus, though now it is formally within the boundaries of Andromeda. As a star belonging to both constellations, Alpheratz also carries the Greek letter Delta Pegasi, though that designation is no longer commonly used.

Mirach, or Beta Andromedae, has an identical magnitude to Alpheratz, but is a cool red giant 200 light years away. Its name is a misspelling of Mizar, which means "loin".

Gamma Andromedae, also called Almach, is one of the most beautiful triple stars in the sky. The 2nd magnitude primary star is a bright orange, and the secondary appears bluish or blue-green. Their separation is presently about 10 arcseconds. The secondary is also double, though its 5th and 6th magnitude stars are only 0.5 arcseconds apart, requiring a large telescope to split. This triple system is 355 light years away.

Andromeda contains two red dwarf systems, Ross 248 and Groombridge 34, which are among the closest star systems to our own. Ross 248 is a 12th magnitude star 10.3 light years away, with only 1/9000th the luminosity of the Sun. Groombridge 34 is a red dwarf binary system 11.7 light years away, whose 8th and 10th magnitude components are separated by 40 arcseconds.

Fourth-magnitude Upsilon Andromedae is an ordinary sunlike star lying 44 light years away. It was one of the first stars discovered to have an extrasolar planet in orbit around it, and is now known to have three. All of these planets are "hot Jupiters" with masses comparable to Jupiter, but orbit very close to the star itself.

Clusters, Nebulae, and Galaxies

NGC 752 is a bright, 6th magnitude open cluster; this very large, loose collection of 60 to 70 stars is scattered across an area twice the diameter of the full Moon.

The nearest spiral galaxy to our own Milky Way is the beautiful M 31 in Andromeda. On a clear dark night you can see it with the naked eye as a fuzzy patch. When you see this galaxy, you are looking at light which left M31 over 2.2 million years ago. The Andromeda Galaxy contains almost twice as many stars as there are in the Milky Way, and it dominates our "local group" of galaxies.

M 31 is thought to be similar to our Milky Way Galaxy in structure, but about half again as large in diameter. It has two major spiral arms, but they are difficult to trace because there is so much dust in the plane of the galaxy, which is inclined to us at over 70 degrees.

M31 has two small companions, the elliptical galaxies M 32 and M 110. Another bright galaxy in Andromeda is the edge-on spiral NGC 891, which appears in small telescopes as a 10th-magnitude smudge of light.

Often overlooked because of its proximity to M 31 is the bright planetary nebula NGC 7662. This planetary nebula, nicknamed the "Blue Snowball", is a bright 9th-magnitude object easily observed in small telescopes.