sagittarius Dwarf Elliptical Galaxy

The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center.[86] However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the Galactic Center. About 40% of the galaxy's clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation.[87] The globular clusters can follow rosette orbits about the Galaxy, in contrast to the elliptical orbit of a planet around a star.[88]
While the disk contains dust which obscures the view in some wavelengths, the halo component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo, as there is little gas cool enough to collapse into stars. Open clusters are also located primarily in the disk.
Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much further than previously thought,[89] the possibility of the disk of the Milky Way Galaxy extending further is apparent, and this is supported by evidence from the 2004 discovery of the Outer Arm extension of the Cygnus Arm.[80][90] With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the Galactic disk.
On January 9, 2006, Mario Juric and others of Princeton University announced that the Sloan Digital Sky Survey of the northern sky found a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Galaxy. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years (9 kpc) away