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Thread: Spiral arms and Density Waves

  1. #1
    Join Date
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    Spiral arms and Density Waves

    In the following article it is stated:

    http://www.astronomy.ohio-state.edu/...4/spirals.html

    "What are Spiral Arms?
    Spiral Arms are Density Waves that pass through the general disk of stars and gas.
    Density Waves are a kind of orbital traffic jam

    Orbits crowd together in the arms, stars pile up and make the regions look brighter.
    Gas clouds pile up, collide, fragment, and form new stars.
    O&B Stars ionize leftover gas (HII Regions), then die before moving far from the waves.
    Kalnaj Spiral Schematic
    Density Waves
    Density waves pass through the disk like water waves pass over the ocean.
    Stars move through the spiral arms.
    Gas clouds try to move through, but some are induced to form stars (collision or compression)
    We are not sure how the waves are excited:

    Tidal disturbance from a nearby companion?
    Excited by a stellar bar in the central regions?
    Both of these have been implicated, and there are strong (but not conclusive) arguments for each. Some computer simulations suggest that spiral structure may be transitory, but given how many disk-shaped galaxies show at least some spiral pattern, it must recur frequently."

    1. With regards to spiral arms:
    It is stated that: "O&B Stars ionize leftover gas (HII Regions), then die before moving far from the waves."

    A. Does it mean that if our Sun will move away from the spiral arm wave it should die?
    B. Could it be that our Sun was at some point of time (from its first day) outside the Arm?
    C. If it could be outside the arm, why other stars couldn't live outside the arm? Why do we kill them all as they move outside? Why do we claim: "then die before moving far from the waves"?
    D. Does it mean that there are no stars between the arms? If there is - would you kindly show just few examples of stars which are located between Orion Arm and the nearby arm? Not a group of stars - but only one star alone between the arms (in the disc).

    2. With regards to the density wave:
    A. It is stated: "Density Waves are a kind of orbital traffic jam".
    However, traffic jam is a random phenomenon. so why we couldn't get different variation of arm shape? Why not stright arm or even be a ZiG Zag arm?
    In about 70% of all the galaxies, we see a nice spiral arms. Therefore, don't you think that there must be a key idea in this structure? Could it be that our scientists have missed something?

    B. It is also stated that: "we are not sure how the waves are excited: Tidal disturbance from a nearby companion? or Excited by a stellar bar in the central regions?"
    If they don't know, could it be that we have a problem with this hypothesis? Why they are not willing to get the real answer for that?
    Last edited by Dave Lee; 2018-May-07 at 04:27 PM.

  2. #2
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