At large distances (up to about 1 billion light-years), astronomers can no longer use methods such as parallax or Cepheid variables. At such large distances, the parallax shift becomes too small and we can no longer even see individual stars in galaxies. Astronomers then turn to a series of methods that use "standard candles", that is, objects whose absolute magnitude is thought to be very well known. Then, by comparing the relative intensity of light observed from the object with that expected based on its assumed absolute magnitude, the inverse square law for light intensity can be used to infer the distance. The unique characteristics and enormous brightness of a certain type of supernova, the explosion which can occur at the end of the main sequence life of a massive star, can be used to determine distances beyond the reach of the previous methods.
There have been many measurements of the manner in which a supernova, whose distance to Earth is known (using one of the previous methods), increases its brightness and then dims into oblivion. There is one type (called type Ia) for which this brightening and dimming is very regular: when the maximum brightness at a distance of 1 light-year is calculated (using the known distance and the 1/distance2 rule), it is found to be about the same for all stars. Such Type Ia supernovae are then our standard candles.
If the distance to a far away galaxy is required, one must first locate a type Ia supernova in it (which do occur regularly) and then measure its observed brightness. Comparing this result with the known maximum brightness achieved by all such supernovae one can determine the distance to the galaxy in question (again using the 1/distance2 rule). Since supernovae are extremely bright, this method is useful to very large distances, up to one billion light-years.
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