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.
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.