M31, the famous Andromeda Galaxy, is the largest member of the Local Group that also includes the Milky Way, the Triangulum Galaxy (M33) and about 50 other smaller galaxies. With an apparent mag. of +3.4, it's one of the brightest Messier objects and easily visible to the naked eye even from areas with a certain amount of light pollution. M31 is usually regarded as the most distant object that can be easily seen without optical aid.
The Andromeda Galaxy has been known for a long time. It was first recorded over 1000 years ago by Isfahan based Persian astronomer Abd-al-Rahman Al-Sufi. In 964 AD, he described it as the little cloud in his Book of Fixed Stars. This object was almost certainly known - for a number of years before this date - to other Persian astronomers. The first person to telescopically observe and describe M31, was German astronomer Simon Marius on December 15, 1612. Unaware of both Al Sufi's and Marius' discovery, Giovanni Batista Hodierna independently rediscovered the object sometime before 1654. Then on August 3, 1764, Charles Messier added the great spiral to his catalogue. Incidentally, Messier incorrectly credited Marius as the discoverer, apparently unaware of the earlier work of Al-Sufi.
For a long time, the true nature of M31 was unknown. It was simply regarded as a large nebula located within our galaxy. Up until the 20th century, it was referred to as either the Andromeda Nebula or the Great Andromeda Nebula. In the 18th century, William Herschel incorrectly believed M31 was one of the nearest nebulae, located at a distance of not more than 17,000 light-years. However, he was correct in viewing the galaxy as an island universe like the Milky Way, although at such a small distance it would be much smaller in size than our own galaxy.
In 1887, Isaac Roberts from his private observatory in Sussex, England captured the first photographs of M31. His pictures showed for the first time the basic features of its spiral structure. Clues about the extragalactic nature of the Andromeda galaxy were obtained in 1912 by Vesto Slipher of the Lowell Observatory in Flagstaff, Arizona. He determined its velocity and found that at about 300 kilometres per second, it was the fastest moving astronomical object ever measured.
Around this time there was a great debate surrounding the nature of the Milky Way, spiral nebula like M31 and the dimensions of the Universe. One camp believed in an island universe hypothesis, which held that spiral nebulae were actually independent galaxies, while the other side believed they were located within our galaxy. The debate was settled in 1923 when Edwin Hubble found the first Cepheid variables in M31. He subsequently measured their brightness variation, calculated the distance of M31 and hence its true nature as an independent galaxy.
Hubble's original measurement of M31 was about 750,000 light-years. At that distance, there is no doubt that M31 is a galaxy in its own right but it would be intrinsically much smaller in size than the Milky Way. It was not until 1953, when observations made using the largest telescope in the World - the 200-inch Palomar observatory - determined that there are two types of Cepheid variables and the true distance to the Andromeda Galaxy is more than double Hubble's original estimate.
Modern measurements place M31 at 2.54 Million light-years. This corresponds to an intrinsic diameter of 140,000 light-years, which is much larger than the 100,000 light-years of the Milky Way. It's estimated that M31 may contain up to 1 trillion stars (1x1012).
The galaxy is best seen from Northern Hemisphere latitudes during the months of October, November and December.
M31 is located near the center of the constellation of Andromeda and to the northeast of the Great Square of Pegasus. Of the four stars of the square, only three of them actually belong to Pegasus. The northeastern star and brightest of the four, Alpheratz (α And - mag. +2.1) is part of neighbouring Andromeda. Located 7 degrees northeast of Alpheratz is δ And (mag. +3.3) and a further 8 degrees northeast is Mirach (β And - mag. +2.1). The Andromeda galaxy is 8 degrees northwest of Mirach, at the end of a line connecting Mirach with μ And and ν And.
To the naked eye, M31 appears as a large easily visible fuzzy patch. Through 10x50 binoculars, it looks like an oval shaped small cloud or detached part of the Milky Way. An 80mm (3.1-inch) telescope shows subtle details, with the centre pronounced. Even at low magnifications the galaxy appears large, filling most of the eyepiece field of view. With averted vision, some spiral dark lanes swirling around the core are detectable. Small telescopes also show M32 and M110, the two main satellite galaxies of M31. Through a 200mm (8-inch) scope, M31 is a spectacular sight. At low magnifications, it exceeds the eyepiece field of view with the centre extremely bright. A large elongated diffuse mist with prominent dark dust lanes superimposed can be seen. Very large amateur scopes show more intrigue details, but the true beauty of M31 is revealed in images and long exposure photographs where the vast expanse of this magnificent spiral is apparent.
In total, M31 covers more than 3.0 x 1.0 degrees of apparent sky, which corresponds to an apparent diameter of over six times that of the full Moon.
M31 Data Table
|Object Type||Spiral galaxy|
|Distance (light-years)||2.54 Million|
|RA (J2000)||00h 42m 44s|
|DEC (J2000)||+41d 16m 06s|
|Apparent Size (arc mins)||189 x 62|
|Number of Stars||1 Trillion|
|Notable Feature||Most distance object that's easily visible to the naked eye|