The Andromeda Galaxy (M31)
The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda Nebula in older texts .
Andromeda is the nearest spiral galaxy to the Milky Way, but not the closest galaxy overall. It gets its name from the area of the sky in which it appears, the Andromeda constellation, which was named after the mythological princess Andromeda. Andromeda is the largest galaxy of the Local Group, which consists of the Andromeda Galaxy, the Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, Andromeda may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping.
A spiral galaxy like the Milky Way, Andromeda contains a concentrated bulge of matter in the middle, surrounded by a disk of gas, dust, and stars 260,000 light-years long, more than 2.5 times as long as the Milky Way.
The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars, more than the number of stars in our own galaxy, which is estimated to be c. 200-400 billion. While the 2006 estimates put the mass of the Milky Way to be 80% of the mass of Andromeda, which is estimated to be 7.1 X 1011 solar masses, a 2009 study concluded that Andromeda and the Milky Way are about equal in mass.
There are approximately 460 globular clusters associated with the Andromeda galaxy. The most massive of these clusters, identified as Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the local group of galaxies.
It contains several million stars, and is about twice as luminous as Omega Centauri, the brightest known globular cluster in the Milky Way. Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 to be the remnant core of a dwarf galaxy that was consumed by M31 in the distant past. The globular with the greatest apparent brightness is G76 which is located in the south-west arm’s eastern half.
In 2005, astronomers discovered a completely new type of star cluster in M31. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that they are much larger – several hundred light-years across – and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters.
The spiral arms of the Andromeda Galaxy are being distorted by gravitational interactions with two companion galaxies, M32 and M110.
The Andromeda Galaxy has at least two spiral arms, plus a ring of dust that may have come from the smaller galaxy M32. Astronomers think that it may have interacted more closely with Andromeda several hundred million years ago.
The Andromeda Galaxy (M31) is the closest large galaxy to the Milky Way and is one of only ten galaxies that can be seen unaided from the Earth. In approximately 4.5 billion years the Andromeda Galaxy and the Milky Way are expected to collide. Andromeda is accompanied by at least 10 satellite galaxies the most notable of which is the Triangulum Galaxy.
The distance to the Andromeda Galaxy is immense: some 2,300,000 light years, but nonetheless its vast size and luminosity mean that it is still visible to the naked eye (in fact, it is the most distant object that can been seen without a telescope). Even so, much of the structure in its spiral arms is too faint to be seen, so that it appears smaller than it actually is: if we could see the entire galaxy, it would occupy an area of the sky nearly six times the size of the Moon’s disc.
Andromeda’s proximity will be deadly to our galaxy. The two galaxies are rushing closer to one another at about 70 miles per second (112 kilometers per second). Astronomers estimate that it will collide with the Milky Way in about 5 billion years. By that time, the sun will have swollen into a red giant and swallowed up the terrestrial planets, so Earth will have other things to worry about.
Still, the fresh influx of dust should boost star formation in the new Milkomeda galaxy, and the Earthless sun may well leave the Milky Way for good. After a messy phase, where arms project crazily from the combined pair, the two should settle into a smooth elliptical galaxy.
Galaxy collisions are a normal part of the universe’s evolution. In fact, both Andromeda and the Milky Way bear signs of having already crashed into other galaxies. Andromeda boasts a large ring of dust in its center, giving it an interesting shape. Astronomers believe this dust may have formed when it swallowed an existing galaxy.
In 964, the Persian astronomer Abd al-Rahman al-Sufi described the galaxy as a “small cloud” in his Book of Fixed Stars, the first known report of our nearest neighbor. When Charles Messier labeled it M31 in 1764, he incorrectly credited the discovery of what was then called a nebula to the German astronomer, Simon Marius, who provided the first telescopic observation of the object.
In the 1920s, the distant galaxy became part of the Great Debate between American astronomers Harlow Shapley and Heber Curtis. At the time, astronomers believed the Milky Way completely composed the whole of universe, and the strange patches known as nebulae lay inside of them. Curtis had spotted various nova in Andromeda, and argued instead that it was a separate galaxy.
The discussion wasn’t concluded until 1925, when Edwin Hubble identified a special kind of star known as a Cepheid variable – a star whose characteristics allow for precise measurements of distance – within Andromeda. Because Shapley had previously determined that the Milky Way was only 100,000 light-years across, Hubble’s calculations revealed that the fuzzy patch was too far away to lay within the Milky Way.
Andromeda in Mythology
Andromeda is a princess from Greek mythology who, as divine punishment for her mother’s bragging, (the Boast of Cassiopeia) was chained to a rock as a sacrifice to a sea monster. She was saved from death by Perseus, her future husband. Her name is the Latinized form of the Greek. The subject has been popular in art since classical times. In the Christian period the subject was converted into the legend of St George and the Dragon, but from the Renaissance interest revived in the original story, typically as derived from Ovid’s account.
In Greek mythology, Andromeda was the daughter of Cepheus and Cassiopeia, king and queen of the kingdom Ethiopia.
Her mother Cassiopeia boasted that she was more beautiful than the Nereids, the nymph-daughters of the sea god Nereus and often seen accompanying Poseidon. To punish the Queen for her arrogance, Poseidon, brother to Zeus and god of the sea, sent a sea monster (Cetus) to ravage the coast of Ethiopia including the kingdom of the vain Queen. The desperate King consulted the Oracle of Apollo, who announced that no respite would be found until the king sacrificed his virgin daughter Andromeda to the monster. She was chained naked to a rock on the coast.
Perseus was returning from having slain the Gorgon Medusa, he found Andromeda and slew Cetus by approaching invisible with Hades’s helm and slaying him. He set her free, and married her in spite of Andromeda having been previously promised to her uncle Phineus. At the wedding a quarrel took place between the rivals, and Phineus was turned to stone by the sight of the Gorgon’s head .
Andromeda followed her husband to Tiryns in Argos, and together they became the ancestors of the family of the Perseidae through the line of their son Perses. Perseus and Andromeda had seven sons: Perseides, Perses, Alcaeus, Heleus, Mestor, Sthenelus, and Electryon, and one daughter, Gorgophone. Their descendants ruled Mycenae from Electryon down to Eurystheus, after whom Atreus attained the kingdom, and would also include the great hero Heracles. According to this mythology, Perseus is the ancestor of the Persians.
After her death, Andromeda was placed by Athena amongst the constellations in the northern sky, near Perseus and Cassiopeia. The constellation had been named after her.