Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: lenadorap on 11/03/2010 14:32:30
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Hi
Can anyone explain why the galaxies show the pattern of vortex?
lenadorap
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Firstly spiral galaxies are gravitationally bound structures with many items orbiting the common centre of gravity and are therefore rotating. (note big elliptical and spherical galaxies are much more complex structures formed from the mergers of many galaxies and may nor be rotating in the conventional sense).
Secondly the presence of dark matter considerably reduces the shear associated with orbits at different distances from the centre of mass. This was a serious problem back in the 60's before dark matter was agreed and proved by measurements of rotation speeds.
Finally gravitational disturbances of a galaxy tend to result in episodes of star formation which themselves tend to trigger further star formation. Like a slow moving shock wave. This coupled with a small amount of orbital shear produces spiral arms.
In effect spiral arms are shock waves of star formation propagating through a galaxy that are distorted by the gravitational shear of the orbital velocities in the galaxy.
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In addition to Soul Surfer's comments, only typical spiral type galaxies have a 'vortex' structure. Elliptical and irregular galaxies don't have 'arm' structures, and even in some types of spiral galaxy the 'arms' aren't twisted around the core i.e. barred spirals. Although relatively rare, some spiral and elliptical galaxies also have rings too.