Messier 110 (NGC 205) is the second bright satellite galaxy of the Andromeda galaxy M 31; the other is M 32.
Discovery and Observation
M 110 was discovered by Charles Messier on August 10, 1773, as depicted on his fine 1807 drawing of the "Great Andromeda Nebula". However, Messier did not himself include this object in his catalog. Caroline Herschel independently discovered M 110 in 1783, and assigned it No. 9 in her small list. William Herschel numbered it H V.18. It was added by Kenneth Glyn Jones in 1966 as M 110, the last of the Messier objects.
Even though outclassed by the nearby Andromeda galaxy, NGC 205 is a fine object. This elliptical galaxy is fairly bright, very large, elongated 10' x 4' with a broad prominent core but no stellar nucleus. Numerous faint stars (not true members of the galaxy) are visible in its halo, the most noticeable being a 14th magnitude object on the southeast edge of the halo.
Properties and Structure
M 110 is at about the same distance as the Andromeda galaxy M 31, about 2.5 million light years, as confirmed by Walter Baade in 1944, when he resolved both into stars. M 110 is only about 12,000 light-years across, less than one-tenth the Andromeda Galaxy's span of 150,000 light years. Despite its comparatively small size, this galaxy has a remarkable system of 8 globular clusters in a halo around it. The brightest, G73, is of about 15th magnitude and thus within the reach of large amateur telescopes.
M 110 contains some dust and hints of recent star formation, which is unusual for dwarf elliptical galaxies. M 110 is of Hubble type E5 or E6 and is designated "peculiar" because it shows some unusual dark structure (probably dust clouds). M 110 is now classified as a dwarf spheroidal galaxy, rather than an elliptical one. However, it is much brighter than typical dwarf spheroids, and contains between 3.6 and 15 billion solar masses. The structure of NGC 205 shows an odd twist caused by the gravitational perturbations on it by M31. Many hot, young O and B type stars are mixed with the older stars typical of elliptical systems, suggesting past interactions between the two galaxies.