The Luminosity of a Star

The Luminosity of a star depends on BOTH its temperature and its radius (surface area):

L is proportional to R2 T4.

  • A hotter star is more luminous than a cooler one of the same radius.

  • A bigger star is more luminous than a smaller one of the same temperature.

This will be important later as we will see that a cool (red) giant star is more luminous than the Sun because, even though it is cooler, it is much larger than the Sun.


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