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: Computing luminosity from surface brightness

  1. Mar 27, 2007 #1
    1. The problem statement, all variables and given/known data
    I'm trying to find the central luminosity per square parsec of a galaxy with central surface brightness [tex]I(0) = 15 \; mag \; arcsec^{-1}[/tex]. I need the answer to be in multiples of the solar bolometric luminosity per square parsec.

    2. Relevant equations
    [tex]m_1 - m_2 = 100^{\frac{1}{5}}\log{\left(\frac{F_1}{F_2}\right)} = (2)\left(100^{\frac{1}{5}}\log{\left(\frac{d_1}{d_2}\right)}\right)[/tex]
    (the formula for absolute magnitude follows easily be letting [tex]d_2 = 10 \; pc[/tex])

    3. The attempt at a solution
    I know that surface brightness is independent of distance (for nearby objects, at least). Since 1 arcsec spans a distance of 10 A.U at [tex]d = 10 \; pc[/tex], finding the galaxy's magnitude per square parsec should be easy (computed to be 3.5236E-14). But if I try to find the Sun's magnitude per square parsec - [tex]\frac{4.83}{4 \pi R_{sun}^2}[/tex] where [tex]R_{sun}[/tex] is expressed in parsecs - I get very large number. This leads me to believe that my reasoning itself is faulty.

    What is the correct way to approach this problem?
    Last edited: Mar 27, 2007
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted