Why are heavenly bodies spherical?
Discussion
The very short answer is "falling".
Physicist would say "Because that shape minimises the objects gravitational potential energy".
Which is another way of saying, if you have a cloud of dust, gas, rock or whatever in space ... all of that stuff will naturally tend to fall toward the cloud's centre-of-gravity. As a result it tends to coallesce into a sphere. A sphere has the property that every point on its surface is equally close to its centre ... if you like, once an object has become as sphere, everything has fallen as far as it can.
Physicist would say "Because that shape minimises the objects gravitational potential energy".
Which is another way of saying, if you have a cloud of dust, gas, rock or whatever in space ... all of that stuff will naturally tend to fall toward the cloud's centre-of-gravity. As a result it tends to coallesce into a sphere. A sphere has the property that every point on its surface is equally close to its centre ... if you like, once an object has become as sphere, everything has fallen as far as it can.
Tch, you're all wrong... think about it!...
What shape do you make when you roll some dough between two hands... that's right, a sphere!... now... who made the planets?... that's right! God... how did he make them?... that's right, by rolling them in his hands!!
QED: Religion has the answer!
by the way... I was being sarcastic!
What shape do you make when you roll some dough between two hands... that's right, a sphere!... now... who made the planets?... that's right! God... how did he make them?... that's right, by rolling them in his hands!!
QED: Religion has the answer!
by the way... I was being sarcastic!
Frightens easy, does she?
scorp said:
Gravity and the fact most planets/stars/gas giants are mostly liquid.
I don't think spinning has anything to do with it (if anything it will cause distortions, like bulges at the equator)
>> Edited by scorp on Friday 22 July 10:11
Absolutely.
Gravity, while a very weak force at the molecular level (vis-a-vis strong- and weak- EM forces), acts FAR better at a distance. Thus, over astronomical timescales (literally and figuratively), an existing accumulation of H and HE will coalesce into a proto-star, gravity slowly taking more and more effect.
As the gases collapse into the centre of the proto-star under gravity, (I think it's) the coriolis force starts everything rotating. Over even longer, you will get a proto-star at the heart of a disc of dust and gas. Due to gravity, the heavier elements will "fall" faster to the centre, meaning the disc will typically have solid elements concentrated in close to the proto-star, and gaseous elements further out.
While this is happening, "clumps" within the disc of gas and dust will themselves exert more "gravity" on their surroundings, and will slowly aggregate and grow. Since they are orbiting the proto-star by this point, they will slowly "sweep" a clear path, collecting all the dust in and near their orbit to them. These will get larger and larger by aggregation.
What will also happen is some of these "planetesimals" will be attracted to each other and collide...again, over long periods of time, the resulting impacts will split apart the planetesimals but gravity will bring most of the parts back together to form one larger whole...
Meanwhile, back at the star, gravity has accumulated sufficient material for forces at the centre of things to cause a fusion reaction to commence, and the proto-star becomes a star and rapidly heats up.
This is how a solar-system will typically form...to specifically answer your initial question, gravity is a force which acts in proportion to distance, hence the further away from the centre of mass, the weaker the force. As all our "planetesimals" are either gaseous or volatile enough to be part-molten, and as we're acting over ultra-long periods of time, they will naturally tend towards sphericality as that makes the force acting to hold the planet together uniform across the body.
Rotation DOES cause "bulges" at the equator, as the centripetal force caused by the rotation counters part of the gravitational attraction. proportionately this force is very small, thankfully, unless the body is rotating at a very high rate...pulsars, for example, are thought to be far more ellipsoid than mainstream stars.
rsvmilly said:
www.lhup.edu/~dsimanek/fe-scidi.htm
Thanks Chim, but I think rsv's answer was more, ah, "enlightening". And a lot shorter!!!
Havoc, my physics A level seems a long time ago but from memory, I thought it was gravity on a molecular level that holds atoms together, overcoming their natural repulsion of the positively charged protons and the negatively charged electrons.
Splitting the atom involves moving the two just far enough apart for the gravity to no longer overcome this natural repulsion. They then fly off in opposite directions, releasing the massive amounts of energy that Einstein calculated as E=MC2.
Splitting the atom involves moving the two just far enough apart for the gravity to no longer overcome this natural repulsion. They then fly off in opposite directions, releasing the massive amounts of energy that Einstein calculated as E=MC2.
rsvmilly said:
Havoc, my physics A level seems a long time ago but from memory, I thought it was gravity on a molecular level that holds atoms together, overcoming their natural repulsion of the positively charged protons and the negatively charged electrons.
Splitting the atom involves moving the two just far enough apart for the gravity to no longer overcome this natural repulsion. They then fly off in opposite directions, releasing the massive amounts of energy that Einstein calculated as E=MC2.
I think your thinking of the strong nuclear force (which holds nuclei together). Gravity is far too weak at a sub-atomic level, it only effect super massive structures on massive scales.
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