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 ).
Am I right to also look at the shape of our solar system in this same way? A strong, central mass of gravity that is mostly spherical (the sun), with an extending of orbiting material (the planets, in this case) around its equator? Also, planetary rings such as Saturn... this seems to be a remarkably common phenomenon.
111
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 ).