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  1. #1
    lanky_lx5's Avatar
    lanky_lx5 Guest

    Default Spiral Arms of the Milky Way


    Our sun and other stars in the spiral arm we are in, (Orion's Arm),
    alternately pass through other spiral arms and inter arm regions . So my
    question is, do regions where stars form in, like the Orion Nebula, move
    in the same way as the sun?

    If the stars are dynamic in position over time, does this mean Orion's
    belt and stars like Theta, at the heart of Orion, will someday be part
    of another arm within our galaxy?

  2. #2
    Davoud's Avatar
    Davoud Guest

    Default Spiral Arms of the Milky Way

    lanky_lx5 wrote:

    Roughly speaking, yes. The Galaxy is known to be rotating and we and
    Orion are rotating with it in such a way that the Orion Nebula will
    appear to be where we see it now for a long time to come.

    In the Universe there are motions within motions within motions -- more
    localized motions matter in this case. Due to perturbations by other
    masses (stars, gas clouds, dark matter (maybe), the local motion of a
    given body will likely be less regular than its larger motions
    (rotation with the rest of the stars of the Galaxy, movement of the
    Galaxy itself within the Local Group, and on up the hierarchy.) The
    constellations are ephemeral; Orion has looked the way it does today
    for a very long time, but in the distant past a hypothetical observer
    on Earth would have seen it differently; indeed, if the observer lived
    long enough he would have seen Orion taking shape. This goes for the
    future, as well; Orion (and the rest of the sky) will look pretty much
    the same for a very long time, but not forever. If our observer
    continues his very long life he will see Orion break up.

    Possibly the most visible short term (circa 26,000 yrs) change is that
    the pole star is inconstant due by precession of the Earth's polar
    axis. It has not always been Polaris, and it will not always be
    Polaris. Please see "Precession" at
    <> for a good
    explanation of some of this motion. This motion, and the motions of
    nearby stars are easily detectable in the course just a few years --
    well within a human lifetime (also see "Parallax" at the above URL.)

    None of this will require you to adjust your polar alignment in the
    course of a night's observing, however...


    usenet *at* davidillig dawt com

  3. #3
    Paul Schlyter's Avatar
    Paul Schlyter Guest

    Default Spiral Arms of the Milky Way

    In article <>, Davoud <> wrote:

    Perhaps we should but some numbers on the time scales here, because
    what's "very long" from a human time perspective will be just a blink
    of an eye in a cosmic time perspective.

    A few millennia will be enough to notice that some stars are moving,
    even without special instruments. That's how the first star was
    detected to have a proper motion: the star Arcturus was noticed to be
    situated one or two degrees differently relative to the background
    stars a few centuries ago, compared to the time of the ancient Greeks.

    After tens of millennia, many constellations will have changed their
    shapes quite noticeably.

    After hundreds of millennia, most of today's constellations will be
    recognizable only with difficulty - some not at all.

    And after millions of years or more, today's constellations will be
    almost completely unrecognizable. Some of the more massive stars
    will by then even have evolved significantly - Betelgeuze may have
    gone nova and then become a white dwarf, for instance.

    Consider a swarm of mosqitoes flying by you. Also, imagine that you
    were a creature with an extremely short life time, living for perhaps
    only 1/100 second or so. During your very short life you see this
    swarm of mosqitoes as more or less stationary - perhaps you even
    recognize patterns among their apparent positions, and invent
    "constellations" of mosqitoes to be able to recognize them. These
    constellations would be there all through your life and a number of
    generations afterwards. But after hundreds of generations (i.e. after
    a few seconds in real time), the mosquitoes will have moved quite
    noticeably, and those "constellations" will have changed, or even
    vanished, just to be replaced by new constellations, invented by
    future generations.

    You can view the sky in a similar way: there's a huge of stars there,
    all moving relative to one another and relative to the Sun. Now,
    humans live some 100 years instead of 1/100 of a second - but otoh
    stars live and move over a much longer time scale. So the effect will
    be similar: our constellations are nothing but a snapshot of this
    swarm of stars, or, more correctly, of that part of the swarm which
    happens to be close enough to our own solar system fo the moment.
    These constellations will vanish after a suffucuently long time
    (thousands of human generations), and perhaps our descendants in the
    distant future will invent other constellations of their own?

    Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
    e-mail: pausch at stockholm dot bostream dot se

  4. #4
    Ernie Wright's Avatar
    Ernie Wright Guest

    Default Halley, Ptolemy, proper motion

    Paul Schlyter wrote:

    It's true that Halley cited the position of Arcturus (and of Sirius and
    Procyon) in the Almagest as support for the idea of proper motion, but
    those positions really weren't accurate enough to constitute evidence.
    The positions for both Arcturus and Procyon are wrong by half a degree
    of latitude, and this is about average for the Almagest star catalog.

    It's more accurate to say that Halley became convinced that proper
    motion occurred, and that he looked for and claimed to find evidence of
    this in the Almagest star catalog. A more convincing case was built,
    by Halley and others, using data from much more recent and accurate
    observations, starting with those of Tycho.

    A fine point and not very germaine to the OP's question, but...

    - Ernie



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