Galaxy "rotation curves"
On the left, a spiral galaxy image, with
spiral arms delineated by so-called HII regions. On the right, the light
from a narrow strip running along the major axis of the galaxy has been
spread into a spectrum, between about 6500 and 6800 Angstroms. The rotation
of the galaxy is seen in the emission lines from H alpha at 6563 Angstroms
(the brightest line), as well as other fainter lines in this region due to
Nitrogen. The "S" shaped curve allows the rotation of the galaxy to be measured.