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 calculate the mass of a star using redshift?

  1. Jan 30, 2015 #1
    Hi everyone,

    Is there a simple formula/equation for calculating the mass of a star simply by measuring it's redshift. I know there is a way to do it, but have been unable to find any clues on the web..

    Thanks for your help! :)
     
  2. jcsd
  3. Jan 30, 2015 #2

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    This depends on what you mean by redshift. There can be several sources of redshift, including cosmological, doppler, and gravitational.

    If you know or can neglect the effects of the other contributions, you can use the gravitational red shift to compute the mass of the star (or rather M/R, gravitational redshift depends on this quantity).
     
  4. Jan 30, 2015 #3
    Let's say it is gravitational redshift.. If you can, could you please also explain for the other two types of redshift?
     
  5. Jan 30, 2015 #4

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    The amount of red-shift is computed by looking at the position of spectral lines and comparing to the proper frequencies of these lines. You can then compute the gravitational mass using the predicted gravitational redshift and equaling it to the observed redshift. For details on how to compute the gravitational redshift, I recommend reading an introductory text on general relativity. There are several conceptual pitfalls and it really requires more careful writing than you will generally find in a forum. The same goes for cosmological redshift, while doppler shift is essentially a special relativistic effect. Neither the cosmological shift or doppler shift depend on the mass of the star. If you do not have access to an introductory textbook, I suggest starting at Wikipedia.
     
  6. Jan 30, 2015 #5
    I've been working on it, and I think this may work, M = z(rc^2)/G , where "z" is the redshift, do you think this would work, it seems to be giving around the correct result, but I would much prefer someone wiser to review and correct...
     
  7. Jan 30, 2015 #6

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Yes, this is true under some assumptions. Mainly, the Schwarzschild metric must be a good description of the space-time around the star and the radius of the star must be much larger than the Schwarzschild radius of an object with the same mass. For normal stars, this is generally a pretty good approximation.
     
  8. Jan 30, 2015 #7
    OK thank you for the help and advice :)
     
  9. Jan 31, 2015 #8

    Ken G

    User Avatar
    Gold Member

    And note you are actually getting M/r, not M, by observing z. That wouldn't work so well for giant stars, because M/r is very small and hard to detect, and it's not that helpful for main-sequence stars, because they all tend to have a similar M/r so you'd need to detect z very precisely to distinguish them, but the z would only be about 1 part in 100,000. But it is very handy for white dwarfs, because white dwarfs have a mass-radius relationship, such that r is proportional to M-1/3, so M/r is proportional to M4/3, so M is proportional to z to the 3/4, and z is much larger and easier to detect. So it's used as a good way to get the mass of a white dwarf.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: How to calculate the mass of a star using redshift?
Loading...