Teach Astronomy - Velocity Dispersions скачать в хорошем качестве

Teach Astronomy - Velocity Dispersions

http://www.teachastronomy.com/ The best way to measure the mass of an elliptical galaxy is by the velocity dispersion of its stars. Velocity dispersion is basically the spread in velocities of the stars in a galaxy. In an elliptical galaxy the stars are heavily concentrated at the center, and individual stars cannot of course be resolved. When spectroscopy is done on the nucleus of a galaxy, the individual motions of individual stars are not followed either, but the astronomers measure a statistical average of the combined motions of very many stars that are on elliptical orbits in and out of the center of the galaxy. Thus the Doppler spread of the absorption lines in the spectrum is an indication of the range of motions of the stars in the galaxy, and that range of motions is a measure of the total mass of the galaxy. Using the velocity dispersion, it's possible to show that most of the mass of elliptical galaxies is contained in a dark halo that goes well beyond the visible light of the stars.