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Velocity of a galaxy, is it the rotating speed around itself?

  1. Jan 22, 2005 #1
    good day,
    well the coming fall i'm entering the astronomy faculty
    and i started from now to study the NGC and M objects....
    while i was searching for the names and numbers on the web..
    i have found several descriptions, which i didn't understand:
    1) when they display the distance in LY, is it the distance from the center of the galaxy through the center of ours or what?
    2) when they talk about the velocity of a galaxy, is it the rotating speed around itself?
    3) when they talk about brightness, the unit used is MAG, can anyone explain the meaning and the use of this parameter?
    4) i do know galaxies have centers of gravity, why clusters don't.. if they do have then, they should have also a rotating speed?

    i think that's it for now.. :)
    it sure is beginner's basic info
    but i would appreciate your responses
  2. jcsd
  3. Jan 22, 2005 #2


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    1) The distance is measured from US (Earth) and some closer objects can be very accurate, but the precision drops as distance increases. For M13, as example, it is usually listed at ~22,500 LY. That is from us, not the galactic center. M31 (Andromeda galaxy) used to be listed as ~2.2 million LY, but has been revised to ~2.9 million LY. At that distance, from us or our galaxy center really wouldn't matter.

    2) "The velocity of a galaxy" would almost always mean its radial velocity toward or away from us, not its rotation speed.

    3) MAG is magnitude. There is "apparent" and "absolute". see: http://www.astronomynotes.com/starprop/s4.htm for both.

    4) What kind of clusters? Open clusters or Galactic (globular) clusters? Any associated cluster will have a "center of gravity", it just depends on whether the gravity vs. the velocity of stars is enough to hold the association (cluster) together. Open clusters usually disassociate and Globular clusters usually stay "bound" for billions of years and do rotate as well as revolve around their host galaxy.
    Last edited: Jan 22, 2005
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