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I Objective classification of spiral galaxies

  1. Nov 23, 2016 #1
    I have question that has been troubling me and I was hoping someone may be able to help me.

    I've always thought the Hubble classification of galaxies was a bit subjective. I was recently looking at some images online of galaxies and I was wondering how you might objectively observe and compare the morphological types of galaxies, from CCD images.

    With Elliptical galaxies you can derive the En number where the number n describes the apparent axial ratio (b/a) by the formula: n=10 [1 – (b/a)], (1) where a and b represent the length of the semi‐major and semi‐minor axis, respectively, of the galaxy. So that's pretty objective.

    However how do you objectively do the same for Spiral galaxies? After numerous searches I haven't found a method that is similar to the one for elliptical galaxies above.
  2. jcsd
  3. Nov 23, 2016 #2


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    The Hubble classification is quite subjective. If you want a more objective classification scheme, try looking at these color-mass diagrams, which effectively separate galaxies into ellipticals and spirals.
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