Editing DarkMatter/CK2015 (section)

==Implications for Stellar-Dynamic Systems==
I think that one key implication of the CK15 work is that old ''stellar''-dynamical systems should not exhibit flat rotation curves.  [This does not immediately contradict observations because it is extremely difficult to measure orbital velocities of individual stars; observers tend to be able to only measure mean velocity dispersions and "average" rotational velocities.]  But, at the same time, the CK15 result should predict that the stars in these old stellar-dynamic systems should not be on circular orbits.  They should be far from it!  When a star is formed, it is ''injected'' into the galaxy with an initial velocity (magnitude and direction) that is identical to the velocity of the H-II region from which it formed; but it no longer feels the radial pressure gradient of the underlying gaseous disk, so its orbit cannot remain circular.  It ''must'' develop a, perhaps considerable, radial component to its motion.

The farther you move away from the center of the galaxy, the larger the discrepancy will be between the orbital velocity of the gas (which is obeying a flat rotation curve) and the circular velocity that would be needed in order for a star (that feels no gas pressure) to stay in a circular orbit.  Hence, stars that are formed progressively farther and farther out in a gaseous disk must develop progressively larger and larger ''radial'' velocity components.  In the terms used by the stellar-dynamic community, such stellar-dynamic systems will be "hot" &#8212; because they have highly noncircular orbits &#8212; even though they still have very small '''vertical''' velocity components.

Prediction:  Stellar disks that are "radially hot" but "vertically cold" when they form, cannot remain that way.  Equipartition of energy demands that they will have to "puff up" &#8212; and, correspondingly shrink radially &#8212; to the extent where the kinetic energy in vertical motion is comparable to the kinetic energy in non-circular radial motion.  I think that this can very naturally lead to an explanation for the entire Hubble sequence!  For example:  Late-type galaxies (those with small bulges and vertically-thin, gas-rich disks) will naturally evolve into stellar-dynamic (low amount of gas) systems that are quite thick; the time-scale of this evolution will be some combination of the star formation rate (gas disappearing) and the equipartition time-scale for stellar-dynamic systems.  And how vertically thick the stellar disk becomes should reflect how extreme the mismatch was between the injection velocity when the star was born (from the flat-rotation-curve disk) and the required Keplerian circular velocity.