Andromeda

Astronomy Class

Andromeda in mythology was a daughter of Kifeas, king of Kifines and Cassiope, grand children of Aiolos god of the winds.

Andromeda considered herself more beautiful than goddess Venus and Hera so, Poseidon, in order to punish her arrogance, sent floods and a sea monster to ruin the land of Kifeas.

The Oracle said that Kifeas should sacrifice his daughter Andromeda to stop the tragedy. Kifeas against his will obeyed and tied his daughter on a rock near the sea. At that time Perseys was passing with his ship near that coast returning from Medusa's lair. When he listened to her story he felt pity for her and took her along.

However Andromeda was so beautiful that Perseys fell in love with her. He married her and she gave him six sons and two daughters.

Observing Galaxy M31 one can easily tell why it is called Andromeda. It is one of the most spectacular and beautiful sights on the night sky.

Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies together with M32 M110 two bright elliptical dwarf galaxies, M33 and others. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects.

Visible to the naked eye under conditions, this object was known to the Persian astronomer Abd-al-Rahman Al-Sufi as the little cloud, who described it in 964 AD in his Book of Fixed Stars: It must have been observed by and commonly known to Persian astronomers at Isfahan as early as 905 AD, or earlier. R.H. Allen (1899/1963) reports that it was also appeared on a Dutch star map of 1500. Charles Messier, who cataloged it on August 3, 1764, was obviously unaware of these early reports, and ascribed its discovery to Simon Marius, who was the first to give a telescopic description in 1612, but (according to R.H. Allen) didn't claim its discovery. Unaware of both Al Sufi's and Marius' discovery, Giovanni Batista Hodierna independently rediscovered this object before 1654. Edmond Halley, however, in his 1716 treat of "Nebulae", accounts the discovery of this "nebula" to the French astronomer Bullialdus, who observed it in 1661; but Bullialdus mentions that it had been seen 150 years earlier (in the early 1500s) by some anonymous astronomer (R.H. Allen, 1899/1963).

It was Longley believed that the "Great Andromeda Nebula" was one of the nearest nebulae. William Herschel believed, wrongly of course, that its distance would "not exceed 2000 times the distance of Sirius" (17,000 light years); nevertheless, he viewed it at the nearest "island universe" like our Milky Way which he assumed to be a disk of 850 times the distance of Sirius in diameter, and of a thickness of 155 times that distance.

It was William Huggins, the pioneer of spectroscopy, who noted in 1864 the difference between gaseous nebula with their line spectra and those "nebulae" with star-like, continuous spectra, which we now know as galaxies, and found a continuous spectrum for M31.

In 1887, Isaac Roberts obtained the first photographs of the Andromeda "Nebula," which showed the basic features of its spiral structure for the first time.

In 1912, V.M. Slipher of Lowell Observatory measured the radial velocity of the Andromeda "nebula" and found it the highest velocity ever measured, about 300 km/sec in approach. This already pointed to the extra-galactic nature of this object. According to Burnham, a better value is about 266 km/sec, but R. Brent Tully gives 298 km/sec, and NED has again 300 +/- 4 km/s as the modern value. Note that all the previous values describe the motion with respect to our Solar System. The latter value can be obtained by correcting for the motion of our Solar System around that center. The modern values for Galactic rotation and heliocentric radial velocity yield that the Andromeda Galaxy and the Milky Way are approaching each other at about 100 km/sec.

In 1923, Edwin Hubble found the first Cepheid variable in the Andromeda galaxy and thus established the intergalactic distance and the true nature of M31 as a galaxy. Because he was not aware of the two Cepheid classes, his distance was incorrect by a factor of more than two, though. This error was not discovered until 1953, when the 200-inch Palomar telescope was completed and had started observing. Hubble published his epochal study of the Andromeda "nebula" as an extragalactic stellar system (galaxy) in 1929 (Hubble 1929).

At modern times, the Andromeda galaxy is certainly the most studied galaxy. It is of particular interest because it allows studies of all the features of a galaxy from outside which we also find in Milky Way, but cannot observe as the greatest part of our Galaxy is hidden by interstellar dust. Thus there are continuous studies of the spiral structure, globular and open clusters, interstellar matter, planetary nebulae, supernova remnants, galactic nucleus, companion galaxies, and more.

Some of the features mentioned above are also of interest for the amateur: Even Charles Messier found its two brightest companions, M32 and M110 which are visible in binoculars and conspicuous in small telescopes, and created a drawing of all three. These two relatively bright and relatively close companions are visible in many photos of M31, including the one in this page. They are only the brightest of a "swarm" of smaller companions which surround the Andromeda Galaxy, and form a subgroup of the Local Group. At the time of this writing (September 2003), at least 11 of them are known: Besides M32 and M110 these are NGC 185, which was discovered by William Herschel, and NGC 147 (discovered by d'Arrest) as well as the very faint dwarf systems And I, And II, And III, possibly And IV (which may however be a cluster or a remote background galaxy), And V, And VI (also called the Pegasus dwarf), And VII (also Cassiopeia dwarf), and And VIII. It is still not clear if M33, the smaller spiral galaxy in Triangulum, and its probable companion LGS 3 belong to this subgroup, or the more remote Local Group member IC 1613, or one of the possible member candidates UGCA 86 or UGCA 92.

The Andromeda Galaxy is in notable