For larger distances run-of-the-mill stars are of no use: they are too dim. There are, however, some stars which at the end of their life blow themselves apart and, in doing so, become anomalously bright (out-shining a galaxy in many cases) for a brief period of time (less than a month); such an object is called a supernova (for more details see Sect. 9.3.4). The unique characteristics and enormous brightness of a certain type of supernova 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 l.y. is calculated (using the known distance and the 1/distance2 rule), it is found to be the same for all cases .
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 (at a 1 l.y. distance) 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 109 l.y.