-----------------------------------------------------------
http://en.wikipedia.org/wiki/Spiral_galaxies wrote: Origin of the spiral structure
<<The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realised that the idea of stars arranged permanently in a spiral shape was untenable due to the "winding dilemma". Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy, a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem. Or, the stars on the outermost edge of the galaxy would have to move faster than those near the center, as the galaxy rotates. Neither behaviour is observed.
There are two leading hypotheses or models for the spiral structures of galaxies:
* Star formation caused by density waves in the galactic disk of the galaxy.
* The SSPSF model - Star formation caused by shock waves in the interstellar medium.
These different hypotheses do not have to be mutually-exclusive, as they may explain different types of spiral arms.
------------------------------------------------------------
. Density waves model
Bertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy’s stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms.
...........................................................
. Historical theory of Lin and Shu
The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964.
* They suggested that the spiral arms were manifestations of spiral density waves.
* They assumed that the stars travel in slightly elliptical orbits and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic centre. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.
------------------------------------------------------------
. Star formation caused by density waves
The following hypotheses exist for star formation caused by density waves:
* As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse (the Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.
* As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
* As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars.
...........................................................
. More young stars in spiral arms
The arms appear brighter because there are more young stars (hence more massive, bright stars). These massive, bright stars also die out quickly, which would leave just the (darker) background stellar distribution behind the waves, hence making the waves visible.
While stars, therefore, do not remain forever in the position that we now see them in, they also do not follow the arms. The arms simply appear to pass through the stars as the stars travel in their orbits.
------------------------------------------------------------
. Alignment of spin axis with cosmic voids
Recent results suggest that the orientation of the spin axis of spiral galaxies is not a chance result, but instead they are preferentially aligned along the surface of cosmic voids. That is, spiral galaxies tend to be oriented at a high angle of inclination relative to the large-scale structure of the surroundings. They have been described as lining up like "beads on a string," with their axis of rotation following the filaments around the edges of the voids.>>