Teach Astronomy - Rotation Curves скачать в хорошем качестве

Teach Astronomy - Rotation Curves

http://www.teachastronomy.com/ The best way to measure the mass of any spiral galaxy is a rotation curve. In this observational technique the slit of a spectrograph is laid across the major axis of a galaxy, and astronomers measure the circular velocity of stars or gas in the disk as a function from the center of the galaxy. What is typically seen is a velocity that rises rapidly to several hundred kilometers per second and then remains flat to the outer visible edge of the disk. This indicates that ninety percent or more of the mass of the galaxy is contained in a dark halo and not in the visible material of the disk matter. Rotation curves can only be measured for spiral galaxies with circular rotation, and they can only be measured for spirals where the inclination is high, that is preferentially edge-on rather than face-on. Also, the rotation curve can only be measured out to the edge of the visible disk, and indirect measurements indicate that much of the mass of a galaxy lies beyond this in a dark halo.