Satellite galaxies of the Milky Way

There are 61 small galaxies confirmed to be within 420 kiloparsecs (1.4 million light-years) of the Milky Way, but not all of them are necessarily in orbit, and some may themselves be in orbit of other satellite galaxies. The only ones visible to the naked eye are the Large and Small Magellanic Clouds, which have been observed since prehistory. Measurements with the Hubble Space Telescope in 2006 suggest the Magellanic Clouds may be moving too fast to be orbiting the Milky Way. Of the galaxies confirmed to be in orbit, the largest is the Sagittarius Dwarf Spheroidal Galaxy, which has a diameter of 2.6 kiloparsecs (8,500 ly) or roughly a twentieth that of the Milky Way.

Characteristics

Satellite galaxies that orbit from 1,000 ly (310 pc) of the edge of the disc of the Milky Way Galaxy to the edge of the dark matter halo of the Milky Way at from the center of the galaxy, are generally depleted in hydrogen gas compared to those that orbit more distantly. This is because of their interactions with the dense hot gas halo of the Milky Way that strip cold gas from the satellites. Satellites beyond that region still retain copious quantities of gas.