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Spiral Galaxies

  1. Jan 8, 2010 #1

    This is my first post and excuse me I'm a bit dim (hence my username).

    So, I've had this question rattling around in my head for a while...

    There are a lot of pictures of so called "Spiral Galaxies" and according to Wikipedia the taxonomy of galaxy shapes was described by Edwin Hubble in 1936.

    My question is if the astrophysics community really considers them spirals, or is that just something they use when talking to us layman?

    I ask this because it seems to me that the spiral shape is an artifact of how we view the galaxy from distance, not that the matter is distributed as in the picture.

    If you look down on to a planar disc from a great height then it will take x amount of time for photons to reach the observer. It will take a greater time for photons further from the center of the disc (hypotenuse of a triangle and all that good stuff) to reach the observer.

    At galactic diameters, these distances will have a considerable effect on how the resulting image is received. A photon from the very edge of a galaxy may well have taken thousands of years extra to arrive at our observatory here on earth compared to one at the center of the galaxy.

    So, I suggest that what we see as a spiral galaxy could actually be just a single point with a couple of jets and the whole thing is rotating. We would see a spiral shape when actually it could be just a rotating X shape.

    Can someone please shed some light on this as it's been bugging me for a long time.

  2. jcsd
  3. Jan 8, 2010 #2


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    The short answer is: yes, galaxies are really shaped like that. Even the one we're in is a spiral.

    Your idea of a light from different parts of the galaxy may seem plausible on the surface but if you treid to think this through you'd soon see flaws in it.

    Here a few things to think of:

    1] If the light from nearer versus farther parts of a galaxy were causing us to view them differently, then it would make a BIG difference whether those galaxies were edge-on versus flat-on to us. But we see spiral galaxies from every conceivable angle, including edge-on.

    For example: it could never result in the very famous Sombrero Galaxy, which we see edge-on:

    or this spiral, seen edge-on:

    http://a52.g.akamaitech.net/f/52/827/1d/www.space.com/images/090901-ngc-4945-02.jpg [Broken]

    2] It is true that light from the edge of a galaxy might be a few thousand years older than the light from the centre of the galaxy, but galaxies shine for billions of years. We see light from an entire galaxy in one shot, even if some of that light is slightly older. A delay in some parts of the light would simply not produce a spiral pattern.
    Last edited by a moderator: May 4, 2017
  4. Jan 8, 2010 #3
    Thanks for the reply, especially the picture.

    I did try to think it through and to be honest I'm still not convinced by your assertions. I can come up with plausible galaxy structures that would show the same effect post temporal-distortion.

    Not that I'm disagreeing. In fact I'm sure you are right. I just would like to hear a more concrete explanation rather than a bolded assertion.
  5. Jan 8, 2010 #4


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    Let me put it the other way round for just a moment. Can your idea explain the spiral galaxies I linked to? How could a latency in the light rays result in a spiral bar in the foreground as well as a bar in the background?
  6. Jan 8, 2010 #5
    Well yes, it could be just a bunch of stars on the edge of a disc orbiting around. I don't see anything in the picture that requires it to be a spiral. Who knows, maybe some of them are going faster than the others but again I don't see that the exposure plate indicates the real structure.

    As far as viewing galaxies on the side, in fact the more they are viewed from the side the more distortion there will be because now you have to consider not only side-side distortion but also front to back (which could be horrendous). That would include our own.

    It's probably time for some math to calculate the expected time delays based on galaxies of known sizes and distance estimations. That would probably put the thing to bed, or at least put caps on the min/max distortions you might expect to see.

    I'll look at doing that tomorrow, it's 3am where I am so will have to wait.
  7. Jan 8, 2010 #6


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    But if your hypothesis is true, then all galaxies should suffer the same distortion. So how could we be seeing any galaxies other than spiral?

    Why don't any pictures of galaxies show this distortion you expect?

    No. Don't. Your hypothesis will cover much more ground as a thought-experiment for now. Numbers will merely cloud your vision and you'll lose sight of the bigger picture.

    Build a pretend model in your head or on paper on a human-scale (where light travels at, say, one foot per second), and see if there is any possible way that your beam of light could produce the image of a spiral like we see in the sky.
  8. Jan 8, 2010 #7


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    We see spiral galaxies in all different orientations from earth, including face and edge on. There is no difference in appearance based on orientation, so Dave is right. 100,000 years is a blink in galactic time scales, not nearly long enough for morphological effects to be significant.
  9. Jan 10, 2010 #8
    Does light bend with gravational fields?
    Does a lazer beam bend when traveling across long distance?
  10. Jan 10, 2010 #9


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    Yes, http://en.wikipedia.org/wiki/Gravitational_lens" [Broken], including laser light.
    Last edited by a moderator: May 4, 2017
  11. Jan 11, 2010 #10


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    No. Math really is the way to go here: If you do a quick google for the diameter of and distance from some of the nearer galaxies (try M51, for example...), you'll find that except for a very small handful of very near galaxies (such as M31 and M33), most of the brighter galaxies we see are tens of millions of light years away and their diameters are tens to hundreds of thousands of light years. So it is hundreds to thousands of times further to them than their diameters, making the time a photon takes to get to us from the edge very close to the time it take to get here from the center (when you view the face).

    Ie, M51 is 23 million light years away and a radius of 19,000 light years (via wiki), the angular radius is .0473 degrees and the distance to a point on the edge would be 22,999,992 light years, a difference of just 8 light years.
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