The Milky Way

The Milky Way is a spiral galaxy that has several distinct components:

  • The disk extends out to a radius of about 25 kpc. It is dominated by stars; the density of the stars falls off with radius almost exponentially. The disk contains most of the light, the interstellar gas and the dust in the Galaxy, and probably 10 to 25% of the total mass.

  • The bulge is the spheroidal component located within the inner 2 to 3 kpc. It may be dominated by a bar, although that is still somewhat uncertain. The bulge of the Milky Way probably contains less than 10% of the mass (and the light) of the Galaxy.

  • The halo is the spheroidal component that extends far beyond the disk and the bulge, probably greater than 100 kpc. The globular clusters are found in the halo and outline its distribution. Within the halo, the density falls off roughly as R-2. It is believed that the halo contains most of the mass of the Galaxy in the form of dark matter.


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