# How do you determine which star, between two, is more luminous?

For example, if star 1 is 4x bigger than star 2 but both are the exact same temperature how do you determine which is more luminous?

• Consider Stefan–Boltzmann law. But the problem as stated seems highly artificial. – Stan Liou Jul 16 '15 at 18:47
• It's a conceptual question I pulled from my astronomy text book. The link below states that in order to determine the luminosity of a star that is twice as large as the sun you would use the following equation: L = (2 R⊙)2(T⊙)4 = 4L⊙ (the Hertzsprung-Russell Diagram). How do I apply this to my question? astro.unl.edu/naap/hr/hr_background2.html – Jay Jul 16 '15 at 19:07
• Just use 4 times the radius instead of two times the radius. I can't understand your difficulty, star 1 is $4^2$ times as luminous. – ProfRob Jul 16 '15 at 20:31

One way to determine the luminosity of a star is to first determine its distance, d. Then one can use the formula, Luminosity = Flux * 4 * Pi * d$^2$. Flux is related to the apparent brightness that is measured with a telescope (and CCD or photometer). mag = -2.5 * Log(Flux) + c.