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M 110 - A Satellite of the Andromeda Galaxy M31
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M 110 - A Satellite of the Andromeda Galaxy, M31

February 19, 2005
Elliptical Galaxy M110 (NGC 205), type E6p, in Andromeda

Right Ascension: 00 : 40.4 (h:m)
Declination: +41 : 41 (deg:m)
Distance: 2900 (kly)
Visual Brightness: 8.5 (mag)
Apparent Dimension: 17x10 (arc min)

Discovered by Charles Messier in 1773.

M110 is the second brighter satellite galaxy of the Andromeda galaxy M31, together with M32, and thus a member of the Local Group.

Curiously, this galaxy was discovered by Charles Messier on August 10, 1773, as described in the Connaissance des Tems for 1801, and depicted on his fine drawing of the "Great Andromeda Nebula" and its companions published in 1807. However, Messier did never himself include this object in his catalog, due to unknown reasons, perhaps a certain sloppiness in recording. It was the last additional object, added finally by Kenneth Glyn Jones in 1966. Independent of Messier's discovery, Caroline Herschel discovered M110 on August 27, 1783, little more than 10 years after Messier, and William Herschel numbered it H V.18 when he cataloged it on October 5, 1784.

The small elliptical galaxy M110 is at about the same distance as the Andromeda galaxy M31, about 2.9 million light years, as confirmed by Walter Baade in 1944, when he resolved it into stars (Baade 1944). It is of Hubble type E5 or E6 and is designated "peculiar" because it shows some unusual dark structure (probably dust clouds). M110 is now often classified as a dwarf spheroidal galaxy, not a generic elliptical one (this would make it the first ever known dwarf spheroid, of course). However, as it is much brighter than typical dwarf spheroids, Sidney van dan Bergh has recently introduced the term "Spheroidal Galaxy" for this and similar galaxies, including Local Group members NGC 147 and NGC 185. M110's mass was estimated to be between 3.6 and 15 billion solar masses.

Apparently, despite its comparatively small size, this dwarf elliptical galaxy has also a remarkable system of 8 globular clusters in a halo around it. The brightest of them, G73, is of about 15th magnitude and thus within the reach of large amateur telescopes; Steve Gottlieb has observed it with a 44-cm telescope together with M31 globulars, and amateurs at the Ferguson Observatory near Kenwood, CA obtained a CCD image showing 7 of them with their 14-inch Newtonian and CB245 CCD camera.