# The Luminosity of two stars with same brightness but at different PC away

Stars A and B are both equally bright as seen from Earth, but A is 120 pc away while B is 24 pc away. Which star has the greater luminosity? How many times brighter is it?

## 1 Answer

The absolute luminosity of A is bigger than B. Like two lamps. $$\frac{120}{24}=5,$$ so A is five times farther B, but as the luminosity decays with the square of the distance, then A is $$5^{2}=25$$ times more luminous.

• Where did you get the 24? – James 9779 Feb 1 '19 at 21:13
• Nevermind! I'm sorry, I'm overthinking. This makes since and is much simpler to understand this way. – James 9779 Feb 1 '19 at 21:14
• I was trying to use the Inverse Square Law which is much harder to understand. – James 9779 Feb 1 '19 at 21:16
• James, you might also like to read Wikipedia's page on absolute magnitude which is about being able to compare the luminosity of stars at different distances. :-) – Chappo Hasn't Forgotten Monica Feb 2 '19 at 6:54