somewhat more complex than a rotating disk. Gas is not solid, so it does not rotate like an object. Like everything else in a galaxy, H I gas is bound by gravity, moving in an orbit around the galaxyΓÇÖs center. The rotational speed of the gas rises rapidly in the inner portions of the galaxy, peaking at about 150 kilometers per second at a radius of 10,000 light years from the center. Beyond this radius the gas orbits more slowly all the way out to the edge of the H I disk, 40,000 light years from the nucleus. From these rotational speeds astronomers can deduce how much matter must be in the galaxy. ΓÇ£FlatΓÇ¥ rotation curves like this one indicate that galaxies must have large halos of unseen matter that extend well beyond their observed edges.
