# Luminosity and brightness

### Luminosity

Luminosity is a measure of the total energy given output by a star at all wavelengths from gamma radiation to radio waves. For example the Sun gives out about 500 million million million MJ of energy every second so its luminosity is 500 million million million MJ.

The luminosity depends on:
(a) the size of the star
(b) the temperature of the star

(a) for a star with a certain surface temperature the bigger the star the more energy it gives out. A star with double the radius of another one will have an area four times as great and so have a luminosity four times greater than the first star
(b) for a star of a certain size the hotter the star the more energy it gives out and so the greater its luminosity. A star with a temperature of double another one will have a luminosity sixteen times greater.

### Brightness

The brightness is how bright a star appears when seen from the Earth. This depends on:
(a) the actual luminosity of the star
(b) the distance of the star from the observer on the Earth

If we have two stars of the same luminosity with one star double the distance of the other from the Earth the closer star will look four times brighter. It obeys the inverse square law.

The photograph shows the Pleiades star cluster. The brighter stars look about the same brightness – in fact they are. They are all part of the cluster and about the same distance from the Earth. However some of the background stars may be just as bright – they don't look it because they are much further away.

I am very grateful to Marcos Mataratzis & Vivek Hira for the permission to use their photograph.

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