This chapter is a summary of our EBook Magnitude & Distance. Go there for more details on the Inverse Square Law, the magnitude scale and Distance Modulus.





Luminosity (as we also explained in our EBook Stellar Radiation) is a measure of the amount of energy that is emitted per unit of time. The physical quantity is Power which is expressed in the unit Watt or Joule per second (energy per second).

We are all familiar with the power of light bulbs that is expressed in Watt.



earth sun


In astronomy power is called luminosity and it is expressed in units of Lsun (the luminosity of our Sun as the reference).

Lsun = 3.83x1026 Watt or Joule/sec.

 This large number explains why the Watt is not a very practical unit for Luminosity in astronomy.