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Neutron stars and blue/red shifting

  1. Aug 6, 2009 #1
    I was wondering about the light emitted by one of these neutron stars. To my limited knowledge, neutron stars are among the discrete objects in the observable universe with the strongest gravitational and EM fields [black holes beat the living crap out of neutron stars, but that's besides the point here]. I don't know if clusters, galaxies and other bigger structures count as discrete. For the purpose of this question, the 'amount of gravity per unit volume' is where I'm going.

    Suppose we find a neutron star with an axis of rotation and magnetic poles nearly perpendicular to our line of sight (a pulsar pointing in a different direction, other than ours, that is). Regardless of the direction of rotation relative to our position: is the light emitted by this body, intrinsically blue/red shifted by the considerably high gravitational field present? Is this effect detectable in weaker, gravitationally speaking, stars?

    If so, is this a real measurement professional astronomers use to, say, calibrate the estimated value for the mass of a given neutron star? I understand it might be complicated to account for the shift caused by the expansion of space/time and the orbit of the star itself, but possible nonetheless. Especially if you do it for nearby neutron stars, you can pretty much tabulate a 'mass : shift' relationship. But I digress...

    Anyone care to shine a brighter light on this for me?

    Thanks in advance :)
  2. jcsd
  3. Aug 6, 2009 #2


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    Well the light will be gravitationally redshifted regardless of orientation, simply because that's a property of gravity. I don't know if this is used to estimate mass, though, as it would require a very detailed knowledge of the spectra of neutron stars (not sure we have this, maybe?). In theory though, I don't see why not. Like you said, you'd have to account for any cosmological redshift (likely nil, seeing as the neutron star is in our galaxy), shift due to proper motion (also like very small), and spectral line broadening due to the rotation of the star. But if you can do all that and get enough light from the star, sure.
  4. Aug 6, 2009 #3
    Indeed, I didn't think of that... What would the spectra of a neutron star would look like anyway? What kind of photons would be emitted of a giant ball of neutrons interacting in such a dense environment?

    Would there be any absorption lines considering that there are no 'elements' in the star itself (as we know them)?

    Plus, from what I know, neutron stars are not the only candidates for post-super-nova eerie thingies (quark stars, etc.). So yeah, I guess you're right, it might be too early to even be usable (that light shifting scenario I asked about in my previous post)

    Thanks for replying.
  5. Aug 6, 2009 #4


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    This paper might be a decent place to start:


    But in short:

    -NS spectra look very much like a black body with a harder spectrum
    -There can be absorption lines since there can be elements in the atmosphere
    -Astronomers certainly do compare observations do predicted spectra to infer things about the NS
    -Gravitational redshift is an important factor since the redshift factor z = GM/Rc^2 can be around 0.2 for typical neutron stars
    Last edited: Aug 6, 2009
  6. Aug 6, 2009 #5
    Thanks for the paper, I've lurking around that site today. I didn't know about it.
    *z = 0.2! So it is a very relevant effect. I'll read the paper I'm sure I'll find the answer [if not, I'll find something in arxiv.org.

    * BTW, not that I'm very familiar with common redshift values. I know that the farthest objects ever detected had a z around 6~7, so I'm guessing 0.2 is a lot for such a small and common object.

    Thanks for the replies, I have enough information to continue on my own.
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