<P> Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago . However, several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped thick disk of the Milky Way . </P> <P> 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 . However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light - years from the Galactic Center . About 40% of the Milky Way's clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation . The globular clusters can follow rosette orbits about the Milky Way, in contrast to the elliptical orbit of a planet around a star . </P> <P> Although the disk contains dust that 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 . </P> <P> 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, the possibility of the disk of the Milky Way extending further is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the Cygnus Arm and of a similar extension of the Scutum - Centaurus Arm . 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 . </P>

Where does most star formation take place in the milky way