An astronomical object whose luminosity is known, or whose luminosity can be determined from something that is easily measured (e.g. the period of a Cepheid). Since you know the object's luminosity, by observing how bright it appears you can deduce its distance using the inverse-square law. Measurements like this are important to cosmology, since you have to measure extragalactic distances in order to find the Hubble Constant, Omega, and Lambda.

Typically, we know from physical arguments that all objects of a certain type should have the same luminosity. That type of object can become a standard candle, but first it must be calibrated; that is, we must measure the distance to at least one object of that type using some other method. That gives us the known luminosity we need, and thereafter we need only measure the brightness of objects of that type to figure out their distances.

The brighter something is, the farther away we can see it, so we would like to use the very brightest standard candles we can find. Unfortunately, the very brightest objects are also very rare, which means it is unlikely that there is one close by. That makes it difficult to calibrate them. We end up having to use fainter standard candles to calibrate brighter ones, to calibrate still brighter ones, and so on. The result is the extragalactic distance ladder.

Some examples of commonly used standard candles are Cepheid variable stars, Type Ia supernovae, and the third-brightest galaxy in a galaxy cluster.

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