The Andromeda Galaxy - Two Frame Mosaic
Known to Al-Sufi about AD 905.
M31 is the famous Andromeda galaxy, our nearest large neighbor galaxy, forming the Local Group of galaxies together with its companions (including M32 and M110, two bright dwarf elliptical galaxies; M32 seen just up and left of the nucleus and M110 in the bottom right of the image).
Visible to the naked eye even under moderate conditions, this object was known as the "little cloud" to the Persian astronomer Abd-al-Rahman Al-Sufi, who described and depicted 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 starmap of 1500. Charles Messier, who cataloged it on August 3, 1764, was obviously unaware of these early reports.
It was longly 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).
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 had the highest
velocity ever measured, about 300 km/sec in approach. This already
pointed to the extra-galactic nature of this object.
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. Hubble published his epochal study of the Andromeda "nebula" as an extragalactic stellar system (galaxy) in 1929 (Hubble 1929).
In modern times, the Andromeda galaxy is certainly the most studied "external" galaxy. It is of particular interest because it allows studies of all the features of a galaxy from outside which we also find in The Milky Way but cannot observe as the greatest part of our Galaxy is hidden by interstellar dust.
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.
The Andromeda Galaxy is in notable interaction with its companion M32, which is apparently responsible for a considerable amount of disturbance in the spiral structure of M31. The arms of neutral hydrogen are displaced from those consisted of stars by 4000 light years, and cannot be continuously followed in the area closest to its smaller neighbor. Computer simulations have shown that the disturbances can be modelled by a recent close encounter with a small companion of the mass of M32. Very probably, M32 has also suffered from this encounter by losing many stars which are now spread in Andromeda's halo.
Despite the large amount of knowledge we now have about the Andromeda Galaxy, its distance, though among the best known intergalactic distances, is not really well-known. It is usually given between 2.4 and 2.9 million light-years.
Under "normal" viewing conditions, the apparent size of the visible Andromeda Galaxy is about 3 x 1 degrees. At its distance of 2.9 million light years this galaxy is therefore more than twice as large as our own Milky Way galaxy ! Its mass was estimated at 300 to 400 billion times that of the sun. Compared to the newer estimates for our Milky Way galaxy, this is considerably less than the mass of our galaxy, implying that the Milky Way may be much denser than M31. These results are confirmed by new estimates of the total halo masses, which turn out to be about 1.23 trillion solar masses for M31, compared to 1.9 trillion for the Milky Way (Evans and Wilkinson, 2000).===============