galactic rotation

galactic rotation

The rotation of the Galaxy about its center, all its components sharing in this rotation to varying degrees. The predominant motion in the galactic disk is circular and parallel to the galactic plane. The orbital speed is determined by the mass within a star's orbit, not by the Galaxy's total mass, so the stars' rotation speeds do not follow Kepler's laws. The rotation speed increases from near zero to 150 km s–1 in the first kiloparsec of the Galaxy's radius, then increases more gradually to a peak near the Sun's orbit (at a radius of about 10 kpc). Farther away it may fall off gradually or remain roughly constant. The rotation velocity of the local standard of rest in the galactic plane was formerly taken to be 250 km s–1, but the accepted value is now 220 km s–1 as a result of the Galaxy's massive dark halo. Objects in the galactic halo display much more random motions than those in the disk and the system as a whole has only a small residual rotation with respect to the galactic center. See also rotation curve.

galactic rotation

[gə′lak·tik rō′tā·shən] (astronomy) The rotation of the Milky Way about an axis through the center and perpendicular to the plane of the Galaxy; the rotation is apparent from the highly flattened shape and from relative stellar motion.