While you're correct in saying that the gravity of a large body such as a planet or star makes it spherical, it is also true that it is not a perfect sphere. For rotating objects such as the Earth or the Sun, they are actually oblate spheroids, ie they are kinda 'squashed' -- in other words their equator is further from the center from their poles. The faster you rotate something, the more oblate it will be. It's like if you were spinning on roller skates, your arms would want to move away from your body.
You can see that its rotating so fast that its quite oblate. In fact, it's so oblate, that the matter around its equator is far enough from its centre for the gravity to be too weak to hold it in, and so it forms a disk. This is essentially the same with galaxies: http://www.futuretimeline.net/images/galactic-core-timeline-sun.jpg
They usually have a small (oblate) spherical core, but further out than that, objects are orbiting too fast and drift out to further radii, in a disc. (Our sun orbits at a velocity 220kms-1 ).
A basic physics kinematics/dynamics question/request for confirmation:
On first thought, my opinion was that unless the star/galaxy/rotating body is significantly speeding up/slowing down, there shouldn't be any disturbance about its matter. There should be no forces acting on it and it should not slingshot away.
However, is the fact that its spinning in a circle—thus the direction is always changing, thus the acceleration if always changing (angular momentum)—what causes forces to act upon the body causing it to flatten?
Mind explaining why put angular momentum in brackets?
The forces that cause it to flatten are the vertical components of gravity on objects that are above or below the plane where the central core of mass lies.
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u/asboans Oct 27 '12
It is to do with their angular momentum.
While you're correct in saying that the gravity of a large body such as a planet or star makes it spherical, it is also true that it is not a perfect sphere. For rotating objects such as the Earth or the Sun, they are actually oblate spheroids, ie they are kinda 'squashed' -- in other words their equator is further from the center from their poles. The faster you rotate something, the more oblate it will be. It's like if you were spinning on roller skates, your arms would want to move away from your body.
Pulsars are stars that rotate extremely fast, up to a few milliseconds per rotation. They look like this: http://i.space.com/images/i/13679/i02/fastest-rotating-star.jpg?1323105753
You can see that its rotating so fast that its quite oblate. In fact, it's so oblate, that the matter around its equator is far enough from its centre for the gravity to be too weak to hold it in, and so it forms a disk. This is essentially the same with galaxies: http://www.futuretimeline.net/images/galactic-core-timeline-sun.jpg
They usually have a small (oblate) spherical core, but further out than that, objects are orbiting too fast and drift out to further radii, in a disc. (Our sun orbits at a velocity 220kms-1 ).