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Galactic Rotation / Evolution

  1. Feb 11, 2015 #1
    Greetings all,

    First time poster and an absolute neophyte, please forgive my utter cluelessness in advance.

    So I was reading "Why does the Milky Way rotate?" (http://phys.org/news/2015-02-milky-rotate.html), and it brought to mind a question that's niggled at me every so often when my thoughts turn to such things...

    The linked article above repeats a rotation figure for the Milky Way I've seen before - 200 million years or thereabouts per revolution. Spiral galaxies in general exhibit a great deal of intricate detail in their structures; density wave induced arms, well-defined core bulges, the list goes on - Andromeda makes a fine example.

    It's pretty difficult to read an article on astronomy without tripping over a figure of it being roughly 13.7 billion years since the big bang. The 400,000 or so years until "the era of recombination" and the first stars and galaxies began to form aside, the primordial mass that became the Milky Way galaxy has had time to complete something like 60 revolutions.

    The question: Has anybody ever studied the dynamics of spiral galaxy formation with regard to how many revolutions are required to evolve such detailed structures?

    Intuitively, something doesn't seem right - it's hard to envision 60 revolutions being enough for all the observed detailed structure to form, not to mention spiral galaxies merging to form hazy ellipticals. I can certainly be wrong about that, but can anyone point me to an analysis of that question?

    Thanks in advance.
  2. jcsd
  3. Feb 11, 2015 #2
    That's a fascinating question! I would recommend looking at the Hawking-Penrose Singularity Theorems. They may help you. Good luck!
  4. Feb 11, 2015 #3
    Maybe it is simply that I haven't been around for 200 Million years, but I don't see any correlation to planetary solar revolutions. Since everything about us is derived from the Sun, including climate and seasonal changes, it does seem safe to assume revolutions and resulting changes have governed the evolution of our planet and Us. I don't see a parallel to Sagittarius A. nor how revolutions around a galactic center "way out here in the boonies" has any important effect on us. Am I missing something?
  5. Feb 11, 2015 #4

    "...Am I missing something?"

    It seems so.

    Start with an amorphous blob of first generation stars, a lot of gas and perhaps a little dust (an "irregular" proto-galaxy?). Add some gravitationally-induced spin. Somehow, evolving that beginning mass into a highly structured, complex form like a spiral galaxy - even moreso a barred spiral - within a mere 60 revolutions seems a little quick.

    Just wondering if anyone had ever analyzed the dynamics - is 60 revolutions enough? That's the totality of it.
  6. Feb 11, 2015 #5


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    You shouldn't be thinking of galaxies as if they were rigid or even semi-rigid structures. The number for completed revolutions you came up with is not really telling you much about the dynamics of the galaxy - it's a number that tells you something about the behaviour of a particle (gas, star) at a particular point in the MW evolution, at a particular distance from the galactic centre.
    The gas and stars in the outer reaches will take a long time to complete a revolution, while the innermost bits will have spun many times. It just doesn't make much sense to talk of 'revolutions of a galaxy' - for most intents an purposes a galaxy is just a blob of gas floating in the void, flattened by its spin, with some ripples in it.
    But I understand you're asking when do the ripples first appear.

    The simulations show the emergence of spiral structure early on in the evolution. They say a picture is worth a thousand words, so I guess a video is worth close to a million:

    Press release article:
    http://www.mnf.uzh.ch/index.php?id=20&no_cache=1&L=1&tx_ttnews%5Btt_news%5D=576&type=98&print=1 [Broken]
    Last edited by a moderator: May 7, 2017
  7. Feb 12, 2015 #6

    That's exactly what I was looking for - Thank you!
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