# Calculating the luminosity density of the universe

1. Mar 12, 2009

### IRNB

l = Phi*L*Gamma(a+2)

b)

The Sloan Digital Sky Survey (SDSS) has recently measured the following Schechter
parameters in the r passband: a = -1.16 +/- 0.03, M*= -20.80 +/- 0.03, Phi* = (1.50 +/-0.13). Given that the sun has absolute magnitude M = 4.62 in the SDSS r band, calculate the luminosity density in this band in solar units. Estimate the error on this quantity.
Gamma(0.84) = 1.122, Gamma(0.81) = 1.153, Gamma(0.87) = 1.094.

I done the entire question and then realised i had made a grave error. And this is what I think it is; I took L* as being -20.80 +/- 0.03 ie M*, and im pretty sure this is wrong. Luminosity confuses me and I'm not sure how to relate the absolute magnitude M* to the Luminosity L*. I know that L* is supposed to be in the order of 10^10. Can anyone out there help? Its kind of urgent (due in tomorrow!).

2. Mar 12, 2009

### IRNB

sorry i posted this in the wrong section. I've posted it again in the homework section. This can be deleted if need be.