Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Aaron Thomas on 27/11/2010 18:30:04
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Aaron Thomas asked the Naked Scientists:
Dear Chris,
Thank you for answering my question in your Astronomy Podcast (http://www.thenakedscientists.com/HTML/podcasts/), I found the response to my question very useful and informative.
Professor Brian Cox seems to think and has stated that gravity is responsible for the commencement of the reaction at the centre of stars.  Also this extract: "In the nebular hypothesis, the majority of the mass of the dust cloud collects at the center. The intense gravitational forces present ultimately lead to nuclear fusion taking place. As most of the matter initially present in the nebula is hydrogen, the process of hydrogen burning takes place." (University of Winnipeg) Â
How can the above be correct when at the centre of a large mass such as the Sun, nebulae, there is essentially zero gravity in total contradiction to this theory! So I would hypothesise that nuclear fusion must take place at the point where gravity is the strongest, on the surface?
Finally, maybe if you were able to find your way to the centre of a large interstellar object you would not float at all in zero gravity but your atoms would all get pulled apart by the forces pulling on you in every direction. Â I imagine this would not be a very pleasant experience, but does it explain nuclear fusion?
I apologise if some of these questions seem rudimentary! Â
Aaron Thomas
What do you think?
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When a star first forms from a hydrogen cloud, gravity compresses it. If you compress a gas it becomes hotter, ie the atoms move faster, and eventually the electrons also get stripped away from the protons. When the star becomes hot enough the protons travel fast enough to overcome their mutual repulsion and so nuclear fusion can start.
So you need high temperatures for fusion, and gravity produces these high temperatures to start the process.
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It's not that there's NO gravity at the center of celestial objects. Every proton, neutron, and electron in the object (star, planet, asteroid, whatever) exerts a gravitational pull on every other proton, neutron, and electron in the object. The combination of all these tiny pulls adds up to a very large force because there are so many particles involved. This leads to intense pressure all the way through the object, but most especially at the center. That's where the pressure due to the gravitational pull is greatest, so that's also where the greatest heat is.
If we could hollow out the center of such an object and put a person in the hollowed out space, at the center, the gravitational forces on that person would be balanced in all directions. So it would appear to that person that they had no weight. Doing this would be extremely expensive and difficult, if not impossible due to the great pressures and temperatures involved.
Consider a rocket leaving the Earth's surface. As it moves away from the center of the Earth, the Earth's gravitational force becomes weaker at a rate proportional to the square of the distance. The surface of the Earth is about 4000 miles away from the center of the Earth. At a point 4000 miles above the surface (twice the distance), we'd feel about 25% of the gravitational force that we do at the surface.
Conversely, if we dug a deep hole, as we moved toward the center, we'd feel MORE gravity, up to a point. When there's about as much of the Earth's mass above you as below, the forces would balance and you'd have the sense of weighing less. All this would happen gradually, of course. You wouldn't all of sudden see your weight go from 150 to 75, just like as your rocket climbs out of Earth's gravity well, your weight changes smoothly with your distance from the center.
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Accretion of stellar matter accumulates, at a particular amount of matter the mass produces a gravitational force, as the mass increases so does the gravitational pull.
This will carry on until pressure of the gravitational forces at the center reach critical mass, producing a nuclear reaction.
At this point Gravity is great enough to counter act the outward energy of the nuclear chain reaction. A constant strugle between these forces are established until the fuel is used up.
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Conversely, if we dug a deep hole, as we moved toward the center, we'd feel MORE gravity, up to a point. When there's about as much of the Earth's mass above you as below, the forces would balance and you'd have the sense of weighing less. All this would happen gradually, of course. You wouldn't all of sudden see your weight go from 150 to 75, just like as your rocket climbs out of Earth's gravity well, your weight changes smoothly with your distance from the center.
To be totally correct the gravity never increases as you dig down. Within a sphere the gravitational attraction varies with 1/r from the surface - ie the further you go in the less gravity you feel. This is a direct consequence of the shell theorem; in a uniform sphere only the material closer to the centre has any gravitational effect on you.
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r from the center not 1/r
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r from the center not 1/r
yep. dunno why I said that.
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Conversely, if we dug a deep hole, as we moved toward the center, we'd feel MORE gravity, up to a point. When there's about as much of the Earth's mass above you as below, the forces would balance and you'd have the sense of weighing less. All this would happen gradually, of course. You wouldn't all of sudden see your weight go from 150 to 75, just like as your rocket climbs out of Earth's gravity well, your weight changes smoothly with your distance from the center.
To be totally correct the gravity never increases as you dig down. Within a sphere the gravitational attraction varies with 1/r from the surface - ie the further you go in the less gravity you feel. This is a direct consequence of the shell theorem; in a uniform sphere only the material closer to the centre has any gravitational effect on you.
Time slows , if I dig down ?
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Time slows , if I dig down ?
yes. according to general relativity
Δt (not clockrate) is proportional to gravitational potential
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Time slows , if I dig down ?
yes. according to general relativity
Δt (not clockrate) is proportional to gravitational potential
Won't the gravitational potential decrease as you dig down? (Assuming the Earth has uniform density, which it really doesn't.)
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the field strength decreases but the potential is the energy that would be released if you dropped an infinitesimal mass from infinity to that point.
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Exactly!! [:)]
This is now what is called "physics" that we are discussing here. [:)]
But surely clock rate must be proportional to the potential?
I mean, that it is absolutely so that a lower clock is slower than a upper clock.
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you reach zero clockrate before you reach r=0
and the field (and therefore the potential) is finite everywhere except r=0 for a singularity
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It would make very much sense if potential energy was zero at the event horizon of a black hole.
Below the event horizon potential energy may be negative or imaginary or undefined or something else, I don't care. Nothing goes there anyway, in finite time.
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potential is zero at infinity
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ok, then potential is mc² at event horizon
(if potential in some point is energy that is released when an object is dropped from infinity to the point)
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the field strength decreases but the potential is the energy that would be released if you dropped an infinitesimal mass from infinity to that point.
Ah! Thanks!