Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How to find mass of giant star?

  1. Apr 22, 2012 #1
    How would someone find the mass of an AGN when a solar luminosity is given? I know you can't simply use the mass-luminosity relationship since that only applies to main sequence stars, right?

  2. jcsd
  3. Apr 22, 2012 #2
    Do you mean AGB star? And do you mean that its luminosity is given in units of the solar luminosity?
  4. Apr 22, 2012 #3
    Sorry. I meant to say galaxy instead of star.

    AGN = Active Galactic Nuclei

    I am given a luminosity of 12 trillion solar luminosities, and I need to find its mass.
    Last edited: Apr 22, 2012
  5. Apr 22, 2012 #4
    are those the *only* two things?
  6. Apr 22, 2012 #5
    good luck,man!http://www.hergoods.info/avatar1.jpg [Broken]
    Last edited by a moderator: May 5, 2017
  7. Apr 22, 2012 #6


    User Avatar
    Science Advisor
    Gold Member

    Since luminosity is essentially a function of stellar mass, it should be possible to roughly approximate the mass of a galaxy if you know its distance and gross luminosity. This, however, is not a trivial matter. For starters, you would have to integrate its luminosity over its angular size. Nobody really spends much time trying to do this because there are easier ways to get a more accurate estimate.
  8. Apr 23, 2012 #7
    One way to do it is if you know the distribution of stars in the galaxy, assuming they are all main sequence stars (this is basically its initial mass function). Then you could integrate the distribution so that it is normalized to that total luminosity. I recall that if this is done for galaxies like ours, you get a roughly linear relationship between luminosity and mass, and the proportionality constant is of order 1 to 10. But I have never done the calculation. Furthermore, if the central engine is actually currently active, then that seriously complicates matters.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook