IC 10

IC 10 is an irregular galaxy in the constellation Cassiopeia. 
It was discovered in 1887. Edwin Hubble suspected it might belong to the Local Group of galaxies, but this was uncertain for decades. The radial velocity of IC 10 was measured in 1962. It is approaching the Milky Way at about 350 km/s. This strengthened the evidence for its being in the Local Group. Its membership in the group was finally confirmed in 1996 by measurements of its distance by observations of Cepheids. Despite its closeness, the galaxy is rather difficult to study because it lies near the galactic plane of the Milky Way. It is heavily obscured by interstellar matter.
The apparent distance between IC 10 and the Andromeda Galaxy is about the same as the apparent distance between the Andromeda Galaxy and the Triangulum Galaxy, which suggests that IC 10 may belong to the M31 subgroup.
IC 10 is the only known starburst galaxy in the Local Group of galaxies. It has many more Wolf-Rayet stars per square kiloparsec (5.1 stars/kpc²) than the Large Magellanic Cloud (2.0 stars/kpc²) or the Small Magellanic Cloud (0.9 stars/kpc²). Although the galaxy has a luminosity similar to the SMC, it is much smaller. The evolutionary status of the Wolf-Rayet stars suggests they all formed in a relatively short period. The galaxy produces stars at the rate of 0.04–0.08 solar masses per year, which means that the gas supply in the galaxy can last for a few billion years longer.
Observations of IC 10 in the far-infrared show that the dust in this mild starburst galaxy is deficient in small grains. Perhaps small grains were destroyed by strong ultraviolet radiation in the areas around the hot luminous stars.
The galaxy has a huge envelope of hydrogen gas, with an apparent size measuring 68′ × 80′, which is far larger than the apparent size of the galaxy in visible light (5.5′ × 7.0′). IC 10 is also unusual in the respect that the visible part of the galaxy seems to rotate in a different direction than the outer envelope. It has an H II nucleus.