APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

[neufer]

Exaggerated diagram illustrating Lin and Shu's explanation of spiral arms in terms of symmetrical elliptical orbits.

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ok, then a galactic bar is such a stick-shaped region of dense stellar population that happens to stand still in a rotating reference system.

It should be noted that while the quasi-harmonic potential well that seems to exist in the inner ~20% of disc galaxies does indeed allow for circular orbits of constant orbital period T it also allows for symmetrical elliptical orbits of constant orbital period T. Combined with the Lin/Shu model of such fixed symmetrical elliptical orbits generates both fixed non-rotating spiral arms as well as fixed non-rotating (Matryoshka doll) bar structures. Outside of the inner ~20% quasi-harmonic potential well of disc galaxies Lin/Shu elliptical orbits precess in the direction of orbital motion resulting in precessing spiral arms in that direction.

[VictorBorun]

Like a pendulum!
A bunch of pendulums with the same fixed pivot, the same length of their massless rods or strings and the same equilibrium position (in the center of the galaxy).
Those pendulums (stars, nebulae) can harmonically oscilate on a line through the center, on a circle orbit or on a symmetrical elliptical orbit.

[VictorBorun]

So the recipe for a bar is:
1) a viscous gas disk to make stars in it
2) a non-colliding dark particles globe of uniform density to make a harmonic gravity well
3) some billions of years to make a bar with orbital resonance

[VictorBorun]

a stellar globular cluster meets just 2 of 3 conditions:
2) a non-colliding particles (stars with no interstellar gas) globe of uniform density to make a harmonic gravity well
3) some billions of years to make a bar with orbital resonance

an outer part of galaxy meets just 2 of 3 conditions:
1) a viscous gas disk to make stars in it
3) some billions of years to make a bar with orbital resonance