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B How important is gravitational red/blueshift for stars?

  1. Apr 21, 2016 #1
    I know that the speed/acceleration of a star can be measured using redshift/blueshift caused by the motion of stars, but I just learned that gravitation also causes redshift and blueshift. Would this not cause some uncertainty into the determining how fast a star is moving away from us? If so, how do astronomers take it into account?

    Specifically what I mean is, light coming out of a "gravity well" is redshifted if I'm not mistaken, so if a star is moving toward us, it's light should be blueshifted, but the gravitation also pushes it to be redshifted. Presumably there would be a case where the effects could cancel. So how do you acquire accurate measurements?

  2. jcsd
  3. Apr 21, 2016 #2


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    We can calculate the expected red/blue shift due to gravity by assessing the mass of the stars in its vicinity. If there are no stars nearby then it red/blue shift will be entirely due to velocity.

    If it is in a massive body such as an elliptical galaxy, we can deduce the mass of the galaxy. We can also compare the overall velocity of the galaxy, so we can see an average shift on all stars in the cluster. Some stars will be moving toward us relative to the cluster's motion, while other will be moving away, give us the average movement of the cluster as a whole.

    We'll know from its orbital properties how fast a given should be moving within the cluster, so we can compare it's unique shift with that of the cluster.
  4. Apr 21, 2016 #3
    Thanks. I figured it was something like that. If I understand what you're saying, astronomers gain knowledge of the mass of the object by looking at objects around it, and then compensate accordingly. And further more, the average velocity of groups of bodies should make gravitational issues from individual bodies not very important. Am I on the right track?

    Also I'd bet gravitational redshift is mostly negligible in light of the accelerating universe. Would that be a reasonable guess?

    One final question: what about redshift caused by a bodies own gravitation? Could the light being produced by the star have a noticeable redshift caused by its own gravity, and if so is there anything beyond looking at the stars around it that is useful for compensating for that? Thanks again for the information.
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