M 31
M 31
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), 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 Al-Sufi, who observed it as early as 905 AD (described 964 AD in his Book of Fixed Stars). Charles Messier was obviously unaware of this early report and ascribed its discovery to Simon Marius, who was the first to give a telescopic description in 1612. It was longly believed that the "Great Andromeda Nebula" was one of the closest nebulae (William Herschel believed, wrongly of course, that its distance would "not exceed 2000 times the distance of Sirius" (17,000 light years).
It was William Huggins, the pioneer of spectroscopy, who noted the difference between gaseous nebula with their line spectra and those "nebulae" with continuous spectra, which we now know as galaxies.
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 Jeff Kanipe's article in Astronomy, November 1995, p.), galactic nucleus, companion galaxies, and more.
Some of these features 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. The better equipped amatuer can see much more, e.g. the brightest globular cluster, G1, with telescopes starting at 10-inch aperture (see Leos Ondra's article in Sky & Telescope, November 1995, p. 68-69). 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.
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, corresponding to a disk diameter of roughly 200,000 light years at its distance of 2.2 to 2.4 million light years, so that this galaxy is about double as large as our own Milky Way galaxy !
The Hubble Space Telescope has revealed that the Andromeda galaxy M31 has a double nucleus, probably because it has "eaten" a smaller galaxy which once intruded its core.
| Right ascension | 00 : 40.0 (hours : minutes) |
|---|---|
| Declination | +41 : 00 (degrees : minutes) |
| Distance | 2200.0 (light-years*10^3) |
| Visual magnitude | 4.8 |
Experimental web page: How To Find M31
