21.3 Standard Candles
In astronomy, measuring the distances to stars and galaxies is essential for understanding the structure and expansion of the universe.
The standard candles are astronomical objects with known intrinsic brightness that serve as vital tools for measuring cosmic distances. By comparing how bright a standard candle appears from the Earth (apparent magnitude) to how bright it actually is, astronomers can calculate its distance using the inverse-square law of light. Examples of standard candles are Cepheid variable stars and Type Ia supernovae, both of which have predictable luminosities that make them reliable distance indicators.
(Fig. 21.2)
Tidbit!
lightbulbThe "standard candle" approach to distance measurement: If you know how bright the flame of a candle is, then by counting the number of photons received from a candle, you can infer the distance to the source. Light from a point source drops off according to the inverse square law, so a strictly geometrical relationship exists.