The Milky Way has several smaller galaxies gravitationally bound to it, as part of the Milky Way subgroup, which is part of the local galaxy cluster, the Local Group.
There are 61 small galaxies confirmed to be within 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 or roughly a twentieth that of the Milky Way.
Satellite galaxies that orbit from 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.
The Milky Way's satellite galaxies include the following:
<imagemap> Image:Satellite_Galaxies.svg|frame|center|Milky Way's satellite galaxies (clickable map)
rect 289 219 352 251 Milky Way rect 319 252 380 281 Sagittarius Dwarf Elliptical Galaxy rect 187 81 229 113 Sextans Dwarf rect 168 273 249 318 Large Magellanic Cloud rect 229 326 288 368 Small Magellanic Cloud rect 297 376 352 407 Sculptor Dwarf rect 183 446 234 476 Fornax Dwarf rect 107 297 150 332 Carina Dwarf rect 296 107 339 144 Bootes Dwarf rect 336 180 408 196 Ursa Major II rect 357 40 423 59 Ursa Major I rect 370 110 428 142 Ursa Minor Dwarf rect 430 119 470 154 Draco Dwarf </imagemap>
The Sagittarius Dwarf Spheroidal Galaxy is currently in the process of being consumed by the Milky Way and is expected to pass through it within the next 100 million years. The Sagittarius Stream is a stream of stars in polar orbit around the Milky Way leeched from the Sagittarius Dwarf. The Virgo Stellar Stream is a stream of stars that is believed to have once been an orbiting dwarf galaxy that has been completely distended by the Milky Way's gravity.