Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The shape of galaxies

  1. Nov 5, 2014 #1
    This may be a stupid question but it has been bothering me for a while.

    Why do galaxies look the way they do in a spiral?

    If a typical galaxy is 100k light years across is can take that much longer before the light from the far side to reach the telescope. Unless it is face on of course.
    If the galaxy is spinning, by the time the light from the far side gets to us surely it would be distorted and it wouldn't look like a nice spiral?

    Could this partially explain some elliptical galaxies?

  2. jcsd
  3. Nov 5, 2014 #2


    User Avatar
    Gold Member
    2016 Award

    Given how far away almost all galaxies ARE from us, why would you expect something as trivial as 100,000 light years to matter?
  4. Nov 5, 2014 #3


    User Avatar
    Science Advisor

    Hi Jake,

    Your question begins with spiral galaxies, and ends with ellipticals. It's not clear what exactly you're aiming for.
    But let's assume it's just about the general distorting effect of light arrival delay.

    What phinds said seems like a red herring to me, with little relevance to the question, but perhaps he can elaborate.

    So, when faced with this kind of question, it's best to approach it as a Fermi problem(http://en.wikipedia.org/wiki/Fermi_problem).

    Assume a Milky Way-like galaxy, seen close to but not exactly edge-on, so that the distance between the edges is about 100kly, but the structure can be discerned. Say, inclination5 in the Yerkes classification scheme(like Andromeda galaxy).

    The escape velocity at the edge of the galaxy would be about 300km/s, so the stars near the edge can't move faster than that. That's 0.001% of the speed of light. In the 100ky it takes for the light from the far edge to pass the near edge, the near edge stars will have moved by 100ly. That is 100/600000 of a circumference of a 100kly-wide galaxy. That's 0.06 degree of a distortion from whatever shape the galaxy otherwise has got.

    As you can see, it's easy to find out that the effect is negligible, even without any detailed knowledge of the subject.

    Additionally, if you read a bit more on elliptical galaxies, you'll realise that they're actually mostly shaped like an ellipsoid, not like an ellipse. That is, unlike spirals, they're more three-dimensional. No distortion in one plane could ever make them look like that.
  5. Nov 5, 2014 #4


    User Avatar
    Gold Member
    2016 Award

    Your specific analysis of WHY it is negligible is the core of what I mean when I said it wouldn't matter. I was coming at it from the point of view that if a galaxy is, say, a billion light years away as measured to the near side, then the 1.0000 billion years for the light to reach us from the near side is not significantly different from the 1.0001 billion years for the light to reach us from the far side. And of course, the difference diminishes with further distance.
  6. Nov 5, 2014 #5


    User Avatar
    Science Advisor

    I must say I don't see how it's obvious, let alone true. I suspect you're thinking of some other form of distortion than what I think OP is talking about, which is basically the fact that at any given moment we see different parts of extended objects at different "age", and for rotating edge-on objects this translates to angular positions of different parts not matching those we'd see when viewed face-on.
    To use another example, a 1ly long straight rod rotating at some significant fraction of c (meaning the tangential velocity of its ends) would look more and more bent the less face-on the plane of rotation is aligned.
    Here, only the difference between time of emission from the far and near ends matters, the remaining distance to the observer is of no cosequence.

    Do straighten out my thinking if I misunderstood.
    Last edited: Nov 5, 2014
  7. Nov 5, 2014 #6


    User Avatar
    Gold Member
    2016 Award

    OOPS ... you're thinking logically and I'm ... well, I'm not sure WHAT I'm doing. I'll be quiet now o:)
  8. Nov 5, 2014 #7


    User Avatar
    Science Advisor
    Gold Member

    Do you realise it takes the galaxy a long time to spin one revolution, particularly the outer regions of the spiral arms ?
    offhand it takes some 250 million years for our solar system to do one full revolution around our Milky Way galaxy

    Did you know astronomers can see and measure the Doppler shift difference in the rotation of a galaxy ?


    The blue region is moving towards us, the red region is moving away from us

  9. Nov 8, 2014 #8
    Thankyou. I did know they were spinning but didn't realise how slowly.

    So does the math work out? They are spinning so slowly it doesn't effect the look at all? Is this the same in all cases?
  10. Nov 9, 2014 #9


    User Avatar
    Science Advisor

    Is the math in post #3 not to your satisfaction?
  11. Nov 9, 2014 #10
    Sorry Bandersnach I didn't see the other posts and I thought there was only one reply. Didn't get notification of the others!?

    Your answer was just what I wanted. I thought that must be the case but didn't understand why.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: The shape of galaxies
  1. Shapes of Galaxies (Replies: 2)

  2. Speed of galaxies (Replies: 12)