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?


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.

  • $\begingroup$ Where did you get the 24? $\endgroup$ – James 9779 Feb 1 '19 at 21:13
  • $\begingroup$ Nevermind! I'm sorry, I'm overthinking. This makes since and is much simpler to understand this way. $\endgroup$ – James 9779 Feb 1 '19 at 21:14
  • $\begingroup$ I was trying to use the Inverse Square Law which is much harder to understand. $\endgroup$ – James 9779 Feb 1 '19 at 21:16
  • $\begingroup$ 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. :-) $\endgroup$ – Chappo Hasn't Forgotten Monica Feb 2 '19 at 6:54

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