How Far?
Astronomers use several techniques to measure the distances to galaxies:
Cepheid Variables
These large, bright stars pulsate like beating hearts. How quickly they pulse reveals their true brightness, so comparing how bright they appear from Earth to their true brightness reveals their distance. These stars measure distances to the nearest galaxies.
Supernovae
Type Ia supernovae, which are powerful exploding stars, shine at the same brightness. As with Cepheids, measuring how bright they appear in our sky reveals their distances, and thus the distances of their galaxies. But supernova explosions are not common, so only a few galaxies can be measured with this technique.
Redshift
For distant galaxies with no supernovae, astronomers measure how much their light is "stretched" by the expansion of the universe. This can reveal the distances to the most remote galaxies.
This document was last modified: October 19 2009.
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