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 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 at 21:14
  • $\begingroup$ I was trying to use the Inverse Square Law which is much harder to understand. $\endgroup$ – James 9779 Feb 1 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 Feb 2 at 6:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.