Messier 110, or M110, also known as NGC 205, is a dwarf elliptical galaxy that is a satellite of the Andromeda Galaxy in the Local Group.^[9]

Charles Messier never included the galaxy in his list, but it was depicted by him, together with M32, on his drawing of "Nébuleuse D'Andromède", later known as the Andromeda Galaxy. A label of the drawing indicates that Messier first saw the object in 1773.^{[lower-alpha 1][10]} M110 was independently discovered by Caroline Herschel on August 27, 1783; her brother William Herschel described her discovery in 1785.^{[lower-alpha 2][10][11]} The suggestion to assign the galaxy a Messier number was made by Kenneth Glyn Jones in 1967,^[12] making it the last member of the Messier List.

This galaxy has a morphological classification of pec dE5, indicating a dwarf elliptical galaxy with a flattening of 50%. It is designated peculiar (pec) due to patches of dust and young blue stars near its center.^[13] This is unusual for dwarf elliptical galaxies in general,^[9] and the reason is unclear.^[13] Unlike M32, M110 lacks evidence for a supermassive black hole at its center.^[14]

The interstellar dust in M110 has a mass of (1.1–1.8)×10⁴ M_☉ with a temperature of 18–22 K, and the interstellar gas has (4–7)×10⁶ M_☉. The inner region has sweeping deficiencies in its interstellar medium IM, most likely expelled by supernova explosions. Tidal interactions with M31 may have stripped away a significant fraction of the expelled gas and dust, leaving the galaxy as a whole, as it presents, deficient in its IM density.^[15]

Novae have been detected in this galaxy, including one discovered in 1999,^[16] and another in 2002. The latter, designated EQ J004015.8+414420, had also been captured in images taken by the Sloan Digital Sky Survey (SDSS) that October.^[17]

About half of the Andromeda's satellite galaxies are orbiting it along a highly flattened plane, with 14 out of 16 following the same sense of rotation. One theory proposes that these 16 once belonged to a subhalo surrounding M110, then the group was broken up by tidal forces during a close encounter with Andromeda.^[18]

• List of Messier objects