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A standard candle is any type of astronomical object whose brightness is known or can be determined irrespective of distance. When an instance is discovered, its distance can be determined by its brightness, also offering the distance to objects determined to be near it or surrounding it. For example, a galaxy's distance can be determined if a Type Ia supernova is observed within it: the length of features of such a supernova's light curve are related to its brightness, and by comparing that to how bright it appears from Earth, the distance to the supernova (and galaxy) can be calculated. Standard candles are keys to the cosmic distance ladder. Very bright sources or transients have the potential to be very useful standard candles: the brighter the object, the longer the distances that it can help determine: gamma-ray bursts (GRBs) are of interest if a means is developed to determine their brightness because some are from extremely high redshifts. Three standard candles have a long history of development and use:
Having numerous independent standard candles allows more cross-checking of methods and results. Some of the others (some of these are used in specific circumstances and some are less secure):
The latter are sometimes referred to as scaling relations, the scale of the object has a mathematical relation with some observable quantity.