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Is the Parenago Discontinuity a real thing?

  1. Sep 10, 2017 #1
    Recently I came across a discussion on a Pantheist forum concerning the movement of cooler Stars & the Parenago Discontinuity. The proposal was that this disconinuity could be used as a way of testing whether cooler Star have self determination... yeah yeah I know, I'm not asking here for a debate about Panpsychism but some clarification about whether the Parenago Discontinuity is an actual thing in the first place.

    I'm aware that Pavel Petrovich Parenago was an eminant Soviet Scientist who was interested in the kinematics of the Milky Way but have yet to fing reference to the Parenago Discontinuity. Any help much apreciated.
  2. jcsd
  3. Sep 10, 2017 #2
    Parenago's Discontinuity is mentioned briefly on page 428 of Galactic Astronomy: Structure and Kinematics by Dmitri Mihalas and James Binney, second edition.

    Also see pages 93-100 of The Alchemy of the Heavens: Searching for Meaning in the Milky Way by Ken Croswell.

    A quick summary: As described in The Alchemy of the Heavens, during the 1910s, astronomers first became aware that blue stars have a lower velocity dispersion than yellow stars, which have a lower velocity dispersion than red ones.

    With a modern knowledge of stellar evolution and Galactic structure, we now recognize that this result means young thin disk stars (younger than a billion years) have lower velocity dispersions than old thin disk stars. This means that young thin disk stars have more circular orbits around the center of the Galaxy than do old thin disk stars. The young stars also stay closer to the Galactic plane.

    Parenago thought there was such a difference between the early-type stars (O, B, A, F up to F5) and the late-type stars (late F, G, K, and M) that there was an actual discontinuity between the two groups of stars. Today, however, most astronomers think otherwise. Instead, giant molecular clouds scatter young stars to and fro, explaining why those stars that have made many orbits around the Galaxy--i.e., old thin disk stars--have higher velocity dispersions, greater distances from the Galactic plane, and more elliptical orbits than younger stars, an idea that was proposed by Lyman Spitzer, Jr., and Martin Schwarzschild, both of whom Ken Croswell interviewed for his book, in the 1950s.
  4. Sep 10, 2017 #3


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    There is also Secular Evolution of the Galactic Disk by James Binney.
    It is just a natural phenomenon. Figure 1 shows the velocity curve as function of B-V.
  5. Sep 11, 2017 #4
    Two possible explanations:
    1) All Milky Way stars were formed in a thin disc. However, some mechanism has operated and is operating to scatter stars into crown and thick disc over time
    2) In young Milky Way, stars formed everywhere around crown. However, as Milky Way aged, star formation ended first in crown, then in thick disc, and only continues in thin disc.

    Which of these explanations is right?
  6. Sep 11, 2017 #5
    Thanks for the informative replies, I have some reading to do....
  7. Sep 12, 2017 #6
    This has nothing to do with the thick disk, but explanation 1 is correct for the thin disk.

    The thin disk has two subpopulations: the young thin disk and the old thin disk. Scattering by giant molecular clouds (and spiral arms) explains why the old thin disk has a larger scale height, greater velocity dispersion, and more elliptical orbits than the young thin disk.
  8. Sep 12, 2017 #7
    When you talk about velocity dispersion are you talking about deceleration?
  9. Sep 13, 2017 #8

    As defined on page 286 of Ken Croswell's The Alchemy of the Heavens:

    Velocity Dispersion. The spread of a velocity distribution--that is, how stars move relative to one another. Technically, the velocity dispersion is the standard deviation of the velocity distribution. Stars with similar velocities have a low velocity dispersion, whereas stars with wildly different velocities have a high velocity dispersion.

    As an example, blue main-sequence stars, which are part of the young thin disk, have a low velocity dispersion (6 to 10 km/sec, according to page 423 of the Mihalas and Binney book), whereas red dwarfs, which are mostly members of the old thin disk, have a higher velocity dispersion (16 to 32 km/sec).
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