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Question on Red shift applied to objects within the same galaxy

  1. Feb 28, 2013 #1
    Hi everyone,

    I signed up just to ask you guys a quick question, as most of you will know infinitely more than I do on this subject. I'm not an astrophysicist, I have a strong interest in the universe and cosmology, but it's more of a hobby for me than a central focus, so I apologise in advance if this seems very elementary and obvious. Anyway, I just wanted to clear something up for a science fiction story I'm currently planning.

    I know red shift is used to calculate the speed at which different galaxies are moving away from our own - but does that also apply to stellar objects within the same galaxy? I know that stellar bodies within the same galaxy all orbit a central point at the galactic core, but does that hold everything in place? For example - would a star say, 5,000 light years from Earth have the same red shift now as thousands of years ago?

    Again, apologies if this seems very basic and obvious, but I thought it best to get clarification on this.
  2. jcsd
  3. Feb 28, 2013 #2


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    Science Advisor

    The blue/red shift of an object will tell you how fast it is moving toward or away from us, even inside our own galaxy. Hubble's law does not apply for objects so close, and the motion of objects within our galaxy depends on the details of their orbital motion in the potential of the galaxy, as you said. This orbital motion does not "hold everything together", so the galaxy does not rotate as a rigid body, for two reasons. First, objects closer to the center are orbiting faster, so even if all of the orbits were circular, stars would shift relative to each other over time. Second, the orbits are generally not circular. So if you plotted the trajectories of all of the stars, it would look like a big tangled plate of spaghetti. Even the orbit of a single star does not close on itself, so it does not follow the same path on successive orbits.
  4. Feb 28, 2013 #3


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    Staff: Mentor

    Yes. The redshift can be measured and used to figure out the motion away from or towards ourselves. Remember that redshift works the same for both galaxies moving away from us and for stars and other objects moving away from us as well. It is only the cause that is different.

    No, stars are constantly passing and being passed, interacting with each other and everything else. They even oscillate up and down through the spiral arm they occupy, leading to a motion in the z axis as well. It's a big mess out there!
  5. Sep 17, 2013 #4
    Thank you for your responses, it has been very helpful to me!
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