INVERSE SQUARE LAW

Luminosity is an intrinsic property of the star: How many Watts?

Brightness depends on the observer: How bright does the star appear?








To measure a star's luminosity an astronomer must

  1. Measure the brightness of the star (how many photons per second in the telescope)
  2. Measure the distance to the star




or




If an astronomer knows the luminosity, one can work backwards to get the distance!


Standard Candle Method