The luminosity (brightness) L, radius R, and temperature T of a star are related by the formula:
However, we can simplify the formula when comparing two stars by taking
ratios. If we denote the luminosity, radius and temperature of the sun
by 
, 
, and 
, respectively, we
can set up a ratio:
Example:
If another star has the same radius as the Sun, but a temperature twice
that of the sun, how many times more luminous is this star? Here we
do not need any absolute measures of temperature or luminosity, only
ratios. The ratio (
) = 2 and the ratio (
) = 1.
Thus (
) = 
= 16. The second star is 16 times as
bright as the Sun.
Next: Masses of Binary Stars
