Caption: Illustrated is the winding problem for the theory that spiral arms have fixed components.
The spiral arms of spiral galaxies CANNOT simply be a fixed set of stars and gas. If they were, the arms would wind tighter and tighter and merge after a few hundred million years (see CK-390; FK-568).
The orbital periods increase with distance from the galaxy centers, and so outer objects increasingly fall behind in the race.
Recall that 220 Myr is the orbital period of the Sun in the Milky Way (FK-565) and this period should be order of valid for most stars in the galactic disk of spirals.
BUT although the spiral arms of spiral galaxies CANNOT simply be a fixed set of stars and gas, the flocculent spiral arms of flocculent spiral galaxies form partially by winding. See spiral_arms_bars.html and Wikipedia: SSPSF model (stochastic self-propagating star formation model).
Credit/Permission: ©
David Jeffery,
2004 / Own work.
Image link: Itself.
Local file: local link: spiral_arm_winding_problem.html.
File: Galaxies file:
spiral_arm_winding_problem.html.