Rotation Curves Feature Summary

Type:
From Physics Research
Title:
Rotation Curves
Description:
This is NGC 3198, a spiral galaxy.  Astronomers originally assumed that most of the mass in this kind of galaxy was concentrated in the bright central region and, therefore, that the stars in the spiral arms moved in circular gravitational orbits--like the planets in the solar system.  But when astronomers measured how the stars move, they could not explain the rate of rotation based on the stars they could see in the galaxy ( Rotation Curves for results for NGC 3198). In particular, the mass of the stars was much too small to produce enough gravitational force to explain the observed rotation. They were forced to the conclusion that much of the mass in galaxies does not emit light (thus the name "dark matter").  

(This feature was updated on July 31, 2013.)
Image:
image credit: John Vickery and Jim Matthes/Adam Block/NOAO/AURA/NSF; <a href="http://www.noao.edu/outreach/aop/observers/n3198.html" target="_blank">image source</a>; <a href="http://www.physicstogo.org/images/features/ngc3198-large-8-04-08.jpg" target="_blank">larger image</a>
image credit: John Vickery and Jim Matthes/Adam Block/NOAO/AURA/NSF; image source; larger image
Image URL:
http://www.compadre.org/Informal/images/features/ngc3198-large-8-04-08.jpg
Featured:
September 1, 2008 - September 16, 2008