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Gravitation At The Center Of A Star


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#1    The Lone Ranger

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Posted 13 April 2012 - 08:08 PM

If i am correct:
According to science astronomical objects can orbit around a star. This can be caused because the star has a bigger mass an therefore a bigger gravity than its surouding  (smaller) astronomical objects causing it to be pulled into the bigger astronomical objects orbit. The star can prevent its own collapse by fusion. The fusion of atoms happens under high temperature and the atoms collide in high speed. A lot of energy is released by this fusion. Gravity pulls object to a point, so the objects in space are being pulled to a point. So planets are being pulled towards a star. So a star is being pulled towards itself and in order to prevent collapse fusion takes place. so the star pulls itself towards the center so counterweight or a force that works the other way has to be there and this can happen in the form of fusion.

So i was thinking what is is that is at the center of the star? if a star would explode would there still be something pulling objects towards it. and at the point when a planatery system is created what is at the center of a star, what would happen if a object would be at the precise center? is there something pulling everything towards that exact point, the center at the planatery system?

if a made a mistake in the explanation: please correct.

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#2    Mentalcase

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Posted 13 April 2012 - 09:10 PM

The answer depends on the type of star.

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#3    The Lone Ranger

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Posted 13 April 2012 - 09:36 PM

Im talking about an object and the effect, not the elements in a star. elements can fuse into heavier elements.
But if an astronomical objects got to close to a black hole it would get sucked it. what would happen if a astronomical objects got to close to the point at the center of an planatery system (whether the star is there or not).

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#4    Mentalcase

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Posted 13 April 2012 - 09:39 PM

View PostThe Lone Ranger, on 13 April 2012 - 09:36 PM, said:

Im talking about an object and the effect, not the elements in a star. elements can fuse into heavier elements.
But if an astronomical objects got to close to a black hole it would get sucked it. what would happen if a astronomical objects got to close to the point at the center of an planatery system (whether the star is there or not).

It would simply meld with the object. Adding even more material to burn.

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#5    Mentalcase

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Posted 13 April 2012 - 09:41 PM

There would be no planetary system without the star, unless you consider a lone planet with moons. Which is certainly possible.

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#6    sepulchrave

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Posted 13 April 2012 - 09:52 PM

View PostThe Lone Ranger, on 13 April 2012 - 08:08 PM, said:

If i am correct:
According to science astronomical objects can orbit around a star. This can be caused because the star has a bigger mass an therefore a bigger gravity than its surouding  (smaller) astronomical objects causing it to be pulled into the bigger astronomical objects orbit.
You are sort of correct. Any two objects under gravitational attraction will orbit their mutual centre of gravity. For two equal mass objects, this point will be roughly half-way between the two; while for a star and a planet this point will be almost exactly at the centre of the star.

So the ``typical'' picture of orbits; where one object circles another, only works when the latter is many many times more massive than the former.

But technically any two objects can orbit each other, these orbits just might not be nice circles or ellipses.

View PostThe Lone Ranger, on 13 April 2012 - 08:08 PM, said:

The star can prevent its own collapse by fusion. The fusion of atoms happens under high temperature and the atoms collide in high speed. A lot of energy is released by this fusion. Gravity pulls object to a point, so the objects in space are being pulled to a point. So planets are being pulled towards a star. So a star is being pulled towards itself and in order to prevent collapse fusion takes place. so the star pulls itself towards the center so counterweight or a force that works the other way has to be there and this can happen in the form of fusion.

So i was thinking what is is that is at the center of the star? if a star would explode would there still be something pulling objects towards it. and at the point when a planatery system is created what is at the center of a star, what would happen if a object would be at the precise center? is there something pulling everything towards that exact point, the center at the planatery system?

if a made a mistake in the explanation: please correct.
A distribution of mass exerts a gravitational field that effectively pulls objects towards that distribution's centre of gravity.

The centre of gravity is a ``special'' point, but only because of the distribution of mass.

Internal forces cannot change the location of the centre of gravity*, so if a star explodes the net gravitational force still pulls objects towards the original centre.

HOWEVER: This only really works for objects outside the star.

For a spherical distribution of mass, the strength of gravity at a distance d from the centre is proportional only to the quantity of mass between that point and the centre. So at the centre of the star, the gravity is zero (the pressure, however, is definitely not). You may find this graph of the gravitational strength of the Earth helpful, notice how as you approach the core of the Earth the acceleration of gravity goes to zero.

The centre of gravity is ``special'' because it can only be changed by external influences. However that is the only reason; there isn't any thing special at the centre of gravity, and because it is the centre the gravity at that point is zero (gravity pulls you in a particular direction; which direction can you get pulled if you are at the centre?).

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[*] Technically internal forces cannot change the location of the centre of mass, which is not always the centre of gravity, but for our purposes here they can be considered the same thing.


#7    The Lone Ranger

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Posted 13 April 2012 - 09:59 PM

So without a star there would be no planatary system?
but what is at that exact center, the star is fusing to counter balance against collapse. but what is it going against. what wil happens if a object interact at that center?

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#8    The Lone Ranger

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Posted 13 April 2012 - 10:53 PM

View Postsepulchrave, on 13 April 2012 - 09:52 PM, said:

You are sort of correct. Any two objects under gravitational attraction will orbit their mutual centre of gravity. For two equal mass objects, this point will be roughly half-way between the two; while for a star and a planet this point will be almost exactly at the centre of the star.

So the ``typical'' picture of orbits; where one object circles another, only works when the latter is many many times more massive than the former.

But technically any two objects can orbit each other, these orbits just might not be nice circles or ellipses.


A distribution of mass exerts a gravitational field that effectively pulls objects towards that distribution's centre of gravity.

The centre of gravity is a ``special'' point, but only because of the distribution of mass.

Internal forces cannot change the location of the centre of gravity*, so if a star explodes the net gravitational force still pulls objects towards the original centre.

HOWEVER: This only really works for objects outside the star.

For a spherical distribution of mass, the strength of gravity at a distance d from the centre is proportional only to the quantity of mass between that point and the centre. So at the centre of the star, the gravity is zero (the pressure, however, is definitely not). You may find this graph of the gravitational strength of the Earth helpful, notice how as you approach the core of the Earth the acceleration of gravity goes to zero.

The centre of gravity is ``special'' because it can only be changed by external influences. However that is the only reason; there isn't any thing special at the centre of gravity, and because it is the centre the gravity at that point is zero (gravity pulls you in a particular direction; which direction can you get pulled if you are at the centre?).

-------------
[*] Technically internal forces cannot change the location of the centre of mass, which is not always the centre of gravity, but for our purposes here they can be considered the same thing.

That is really helpfull. Took me a while to understand it to be honest.

Thanks to both for explaining this.

Edited by The Lone Ranger, 13 April 2012 - 11:08 PM.

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#9    sepulchrave

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Posted 13 April 2012 - 11:59 PM

View PostThe Lone Ranger, on 13 April 2012 - 09:59 PM, said:

So without a star there would be no planatary system?
I think planets come from accretion discs that are from the star.

But you could have a stable arrangement of planets without a star (obviously they would be cold and dark though), I am just not sure how you would get the planets to form.

View PostThe Lone Ranger, on 13 April 2012 - 09:59 PM, said:

but what is at that exact center, the star is fusing to counter balance against collapse. but what is it going against.
The gravitational attraction is being counteracted by pressure (an electromagnetic effect in stars, a spin-statistical effect in neutron stars) throughout the body of the star. As you progress towards the centre of the star the gravity decreases (there is less and less of the star below your feet), and the pressure increases (the material in the star reaches the limit of how much it can be squeezed).

View PostThe Lone Ranger, on 13 April 2012 - 09:59 PM, said:

what wil happens if a object interact at that center?
Nothing special. The centre has a lot of pressure and (undoubtably) a very high temperature, but apart from the lack of gravity, it isn't significantly different from any other location in the star's core.


#10    The Lone Ranger

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Posted 14 April 2012 - 12:42 PM

View Postsepulchrave, on 13 April 2012 - 11:59 PM, said:

I think planets come from accretion discs that are from the star.

But you could have a stable arrangement of planets without a star (obviously they would be cold and dark though), I am just not sure how you would get the planets to form.


The gravitational attraction is being counteracted by pressure (an electromagnetic effect in stars, a spin-statistical effect in neutron stars) throughout the body of the star. As you progress towards the centre of the star the gravity decreases (there is less and less of the star below your feet), and the pressure increases (the material in the star reaches the limit of how much it can be squeezed).


Nothing special. The centre has a lot of pressure and (undoubtably) a very high temperature, but apart from the lack of gravity, it isn't significantly different from any other location in the star's core.

Ok, thanks.
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Edited by The Lone Ranger, 14 April 2012 - 12:44 PM.

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#11    StarMountainKid

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Posted 15 April 2012 - 08:51 AM

So, the gravity well of a star or planet would look like a sort of "W".

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#12    sepulchrave

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Posted 15 April 2012 - 06:00 PM

View PostThe Lone Ranger, on 14 April 2012 - 12:42 PM, said:

Ok, thanks.Have you seen the TV series: The Universe?
No.

View PostStarMountainKid, on 15 April 2012 - 08:51 AM, said:

So, the gravity well of a star or planet would look like a sort of "W".
Yes. But since the middle part of the "W" only happens below the surface of the star/planet/etc. it usually isn't shown.





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