# Computing luminosity from surface brightness

1. Mar 27, 2007

### v0id

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 $$I(0) = 15 \; mag \; arcsec^{-1}$$. I need the answer to be in multiples of the solar bolometric luminosity per square parsec.

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

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 $$d = 10 \; pc$$, 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 - $$\frac{4.83}{4 \pi R_{sun}^2}$$ where $$R_{sun}$$ 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