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The Question

(Submitted January 03, 2007)

How can one calculate the distance to other galaxies? Do you have an example to give me bettter understanding in this matter?

The Answer

To determine the distance a galaxy is from Earth, astronomers look for celestial objects within that galaxy they can determine the luminosity (or absolute magnitude) of. These objects are known as standard candles. Once the luminosity is known from observation they can make a measurement of how bright the object appears here on earth and determine the distance. If one has two objects of the same luminosity, the one further away will appear less bright. A good discussion of how the distance can be calculated knowing the absolute magnitude of the star is available here:
http://ceres.hsc.edu/homepages/classes/astronomy/spring99/Mathematics/sec19.html

The key to making the distance measurement is to find and use a standard candle. One such class of objects are Cepheid variable stars. These are stars whose luminosity varies periodically due to ionization of helium in the star's atmosphere followed by expansion and deionization. Observation has shown there is a relation between the stars absolute magnitude and the pulsation period. Another class of stellar objects that are used as standard candles are Type Ia supernovae which are explosions of white dwarf stars in a binary star system. These explosions always release roughly the same amount of energy and have a known peak magnitude which allows them to be used as a standard candle. There's a good discussion of these and other standard candles here:
http://universe-review.ca/R02-07-candle.htm

Hope this helps
Jason and Koji
for "Ask an Astrophysicist"

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