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  1. #1
    Liam's Avatar
    Liam Guest

    Default dark matter and spiral galaxies

    Listened to an astronomy podcast:
    and the astronomer, or astrophysicist, mentioned one of the evidence
    for the existance of dark matter is the rotational rates of galaxies.
    She said that without dark matter then the centers of galaxies would
    rotate faster than the edges which would trail behind.
    So if you start with a straight line from the center of the galaxy to
    the edge, as the galaxy spun the line would begin to arc. But because
    of dark matter that imaginary line would remain straight as the galaxy
    But for me that begs the question, how did spiral galaxies form? That's
    something the podcast didn't address under this theory.
    So, can someone help point me to info that explains this appearant

  2. #2
    Bob May's Avatar
    Bob May Guest

    Default dark matter and spiral galaxies

    Start by taking any mass of gas and let it start coalescing into a galaxy.
    Any differences in the basic motion of the gas will turn into a rotational
    There's a simple little DOS program called GRAVITY that will show you how
    this happens. It supports up to 9 (from my memory of the program) masses
    but that will show the basics of what is happening. Email me if you can't
    find it on the web. It is a freeware program of small size and runs on any
    While nobody has figured out what the dark mass is, I suspect that one
    really good candidate would be the mass of light that surrounds a galaxy.

    Yeppie, Bush is such an idiot that He usually outwits
    everybody else. How dumb!

  3. #3
    N:dlzc D:aol T:com \(dlzc\)'s Avatar
    N:dlzc D:aol T:com \(dlzc\) Guest

    Default dark matter and spiral galaxies

    Dear Bob May:

    "Bob May" <> wrote in message

    The light would have to be closer to the galaxy center than the
    object "anomalously accelerated". Light is well distributed all
    around all objects... so its net contribution should be near
    zero. That is thinking though...

    David A. Smith

  4. #4
    canopus56's Avatar
    canopus56 Guest

    Default dark matter and spiral galaxies

    Liam wrote:

    This book available at your local library has a good overview:

    Waller, W.H. & Hodge, P.W. 2003. Galaxies and the Cosmic Frontier.
    Harvard Univ. Press.

    Ellipictal galaxies like M87 or the M32 dwarf satellite of the
    Andromeda Galaxy are spherical blobs.

    Stars orbit within them much in the same manner that electrons orbit
    around an electron. The orbits are randomnly distributed with respect
    to the center of the galaxy and their is no "mid-plane".

    Two galaxies then interact with each other. The outside gravity
    imparts an angular spin to your spherical blob galaxy and "turns on"
    star formation within the galaxy. The angular momentum effect is
    self-re-enforcing. Overtime, the spherical blob turns into a rotating
    flattened pancake.

    Neaby galaxy M51 in UMa is thought to have undergones changes and a
    recent burst of star formation as a result of its encounter with the
    fainter galaxy NGC5195 seen somewhat "behind" it.

    SEDS M51 pic showing NGC5195 in the background
    Hubble close-up of M51 showing high levels of star formation

    The evolutionary tracks by which the major galaxy types are created.
    in the 1920s and 30s, Edwin Hubble throught that elliptical blob
    galaxies "evolved" into lenticular pancake galaxies and then into
    spiral galaxies. He organized the major physical types of galaxies in
    that order into his famous "tuning fork" diagram:

    Although the tuning fork diagram is still used to mentally organize the
    taxomony of the galaxies in the celestial "zoo," galaxies are no
    longer thought to evolve in the order suggested by Hubble. For
    example, an elliptical galaxy could be created by the near passage of
    two spiral galaxies, one of which strips the outer gas and stars off
    the second galaxy.

    - Canopus56

  5. #5
    Impeach Bush's Avatar
    Impeach Bush Guest

    Default dark matter and spiral galaxies

    canopus56 wrote:

    nucleus ?

  6. #6
    Shawn's Avatar
    Shawn Guest

    Default dark matter and spiral galaxies

    canopus56 wrote:

    Assume you mean nucleus.
    You're implying that a star's orbit in the galaxy is determined by its
    quantum state?
    I suppose it might be, but quantum mechanics isn't the typical tool used
    to calculate stellar motion..


  7. #7
    Bob May's Avatar
    Bob May Guest

    Default dark matter and spiral galaxies

    The thing here is that whatever the dark mass is, it is reducing the
    attraction of the center of the galaxies by being outside of the central
    area that we see well. Light is pretty much everywhere and is emitted from
    the galaxies in large amounts. Interestingly, it is also transparent in
    that it doesn't affect other light by diminishing it. If you consider that
    light has mass by it's energy, it provides a continous presence everywhere
    and thus pulls out the visible masses from the center of the galaxy to the
    outer parts.
    The question is whether light has been checked for the cause of this dark
    mass problem.

    Yeppie, Bush is such an idiot that He usually outwits
    everybody else. How dumb!

  8. #8
    N:dlzc D:aol T:com \(dlzc\)'s Avatar
    N:dlzc D:aol T:com \(dlzc\) Guest

    Default dark matter and spiral galaxies

    Dear Bob May:

    "Bob May" <> wrote in message

    No, Dark Matter is distributed outside the center of a sprial
    galaxy, so that the rim doesn't spin off into open space at the
    rate we see spiral galaxies spinning. Dark Matter is required to
    assist the central core maintain its hold on the rim.

    A requirement of Dark Matter is that it doesn't interact with
    light. So sounds reasonable so far.

    No, but it contributes to gravitation by its energy...

    No. Since it is "present everywhere", it has no net
    contribution. Just like if you are in a tunnel through the
    center of the Earth, as you approach the center there is no net
    gravitational pull. Everything to your "left" is matched by an
    equal distribution on your "right".

    Well, Dark Matter has near zero net speed with respect to the
    normal matter it is travelling with. Can't say that about light.
    And what's more, the light represents mass "evaporating", so as
    it passes that which it once helped bind...

    David A. Smith



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