1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Which star has the greater luminosity

  1. Jan 2, 2010 #1
    1. The problem statement, all variables and given/known data

    Question 1 - If two stars A of luminosity 9000 and B of luminosity 90 (both relative to the Sun) appear equally bright from the Earth, how much further away is A from B?

    Question 2 - Star A is twice as bright as Star B, but B is twice as far away as A. Which star has the greater luminosity, and by what ratio?

    Question 3 - If two stars, A and B, are the same distance away, but B is 20 times more luminous than A, how much brighter will B appear than A?

    I could use some assistance as to how I should set these out in accordance with

    brightness (proportional to) Luminosity / R^2


    2. Relevant equations

    See above

    3. The attempt at a solution


    Question 1 - Worked out to - by moving Star A 10 times further away we have diminished the apparent brightness of Star A by a factor 10 x10 = 100 so that it now matches the brightness of Star B.

    Question 2 - Worked out to - Luminosity of A
    Luminosity of B = (½)2 x 2 = ½

    This means that Star A really only has half the luminosity of Star B, but appears brighter because it is closer.
    Ratio 1:2

    Question 3 - Worked out to 1/20th .

    Just need assistance to set these out correctly - and also make sure that they are correct.
     
  2. jcsd
  3. Jan 3, 2010 #2
    Re: Luminosity

    Any thoughts?
     
  4. Jan 3, 2010 #3

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Re: Luminosity

    Looks good so far.

    So are you saying that B appears to be 20 times dimmer than A? I will suggest rethinking this one. Hint: distance is not a factor here, since it is the same for both stars.
     
  5. Jan 3, 2010 #4
    Re: Luminosity

    Zebra, you seem to be really close on the last one. When distance is constant, the apparent brightness is just proportional to the luminosity.
     
  6. Jan 4, 2010 #5
    Re: Luminosity

    Many thanks for the replies.

    Yes, I think I understand what you are saying about question 3.

    Again, many thanks for the replies.

    Cheers
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook