Messier 31

Spiral Galaxy M31 (NGC 224), type Sb, in Andromeda

Andromeda Galaxy

Known to Al-Sufi about AD 905.

Messier 31 (M31, NGC 224) 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), our Milky Way and its companions, M33, and others.

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 this 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 (Ismail Bouillaud), 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 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); 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 (Huggins and Miller 1864).

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, i.e. heliocentric motion, not that w.r.t. the Milky Way's Galactic Center. 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 "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 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 (see e.g. Jeff Kanipe's article in Astronomy, November 1995, p. 46), 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 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.

The brightest globular cluster of the Andromeda Galaxy M31, G1, is also the most luminous globular in the Local Group of Galaxies; its apparent visual brightness from Earth is still about 13.72 magnitudes. It outshines even the brightest globular in our Milky Way, Omega Centauri, and can be glimpsed even by better equipped amateurs under very favorable conditions, with telescopes starting at 10-inch aperture (see Leos Ondra's article in Sky & Telescope, November 1995, p. 68-69). The Hubble Space Telescope was used to investigate globular cluster G1 in mid-1994 (published April 1996). While the easiest, G1 is not the only M31 globular cluster which is in the reach of large amateur telescopes: Amateur Steve Gottlieb has observed 18 globular clusters of M31 with a 44cm telescope. With their 14-inch Newton and CB245 CCD camera, observers of the Ferguson Observatory near Kenwood, CA have photographed G1 and four fainter M31 globulars. Barmby et.al (2000) have found 435 globular cluster candidates in M31, and estimate the total number at 450 +/- 100.

The astrophotographer is even better off, as he can gather the fainter light of the fine detail in the spiral arms, as in our image: Amateurs can obtain most striking pictures even with inexpensive equipment, from wide-field exposures to detailed close-ups. Also in photography, better equipment pays off, as is demonstrated by our image, which was obtained by (and is courtesy of) Texas amateur Jason Ware, with a 6-inch refractor. More information on this image is available.

The brightest star cloud in the Andromeda galaxy M31 has been assigned an own NGC number: NGC 206, because William Herschel had taken it into his catalog as H V.36 on the grounds of his discovery observation of October 17, 1786. It is the bright star cloud at the upper left, just below a conspicuous dark nebula, in our photograph (very conspicuous in the larger photo).

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. While it is well established that M31 is about 15-16 times further away than the Large Magellanic Cloud (LMC), the absolute value of this measure is still uncertain, and in current sources, usually given between 2.4 and 2.9 million light-years - a consequence of the uncertainty in the LMC distance and thus the overall intergalactic distance scale. E.g., the semi-recent correction from data by ESA's astrometrical satellite Hipparcos has pushed this value up by more than 10 percent, from about 2.4-2.5 to the about 2.9 million light-years we use here.

Under "normal" viewing conditions, the apparent size of the visible Andromeda Galaxy is about 3 x 1 degrees (our acurate value, given above, is 178x63 arc minutes, while NED gives 190x60'). Careful estimates of its angular diameter, performed with 2-inch binoculars, by the French astronomer Robert Jonckhere in 1952-1953, revealed an extension of 5.2 times 1.1 degrees (reported by Mallas), corresponding to a disk diameter of over 250,000 light years at its distance of 2.9 million light years, so that this galaxy is more than double 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).

The Hubble Space Telescope has revealed that the Andromeda galaxy M31 has a double nucleus. This suggests that either it has actually two bright nuclei, probably because it has "eaten" a smaller galaxy which once intruded its core, or parts of its only one core are obscured by dark material, probably dust. In the first case, this second nucleus may be a remainder of a possibly violent dynamical encountering event in the earlier history of the Local Group. In the second case, the duplicity of Andromeda's nucleus would be an illusion causes by a dark dust cloud obstructing parts of a single nucleus in the center of M31.

Up to now, only one supernova has been recorded in the Andromeda galaxy, the Supernova 1885, also designated S Andromedae. This was the first supernova discovered beyond our Milky Way galaxy, on August 20, 1885, by Ernst Hartwig (1851-1923) at Dorpat Observatory in Estonia. It reached mag 6 between August 17 and 20, and it was independently found by several observers. However, only Hartwig realized its significance. It faded to mag 16 in February 1890.

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