What causes the outward pressure that balances the inward pull of gravity in a star?
This is straight out of my Astronomy textbook from the last semester: "… imagine a slab of material in the solar interior. In equilibrium, the slab on average will move neither up nor down. (In, fact there are upward and downward motions of material inside the Sun, but these motions average out.) Equilibrium is maintained by a balance of among three forces that act on this slab:
1 The downward pressure of the layers of solar material about the slab.
2 The upward pressure of the hot gasses beneath the slab
3 The slab's weight - that is, the downward gravitational pull it feels from the rest of the sun. The pressure from below must balance both the slab's weight and the pressure from above. Hence, the pressure below the slab must be greater that that above the slab. In other words, pressure has to increase with increasing depth. For the same reason, pressure increases as you dive deeper into the ocean or as you move toward lower altitudes in our atmosphere." So I guess the answer you are looking for is "the inward pull of gravity is balanced out by the pressure of hot rising gasses."
Nuclear fusion occurs in stars due to the shear density of a star. They are so dense that the pressure in the core ionizes hydrogen, stripping them into bare atoms. The inward gravitational pull of the dense core causes the atoms to be smashed together, fusing into helium. The energy from the fusion provides enough outward pressure to counteract the core's own gravity.
Essentially everything falls towards the centre of gravity unless there is something preventing it from doing so. Standing on the Earth we are prevented from falling towards the centre of gravity by the rigid surface which exactly balances out our weight. Velocity also prevents an object from falling towards the centre of gravity. Throwing a ball causes it to travel a distance before its trajectory causes it to hit the ground. (The friction of the…
The force of gravity from the Sun's mass causes the Sun to contract; the power and heat generated by the hydrogen fusion reaction at the Sun's core causes the Sun to expand. These two forces are in balance; otherwise, the Sun would be compressed to the point where the heat of fusion DID equal the compressive force of gravity.
Glaciers form from the accumulation of snow when it exceeds the melting rate. The pressure of overlying snow causes it to change to solid ice. Eventually, gravity will cause it to move downward or outward from its own weight. Also, melting glaciers can cause glacial lakes to become too full of water and cause them to breach their natural boundaries, flooding the villages below.
Gas is subject to gravity in the same way as everything else. The Earth retains its atmosphere because of gravity. Gases have a general tendency to expand to fill all of the space available to them, but the gases that make the Earth's atmosphere cannot expand for ever into space because gravity causes them to stick to the Earth. The balance between expansion and gravity means that atmospheric pressure is higher where gravity is higher…
"Gravity is a centripetal force, it exerts its tremendous force inward, toward the center of gravity, always trying to form the Earth into a perfect sphere while the angular momentum is an outward tangential inertia (centrifugal force) that causes the oblate shape of the Earth." ~http://novan.com/earth.htm
In all bodies, gravitation must be balanced by some kind of outward pressure to prevent collapse. In most stars, the energy being liberated from nuclear fusion in the core is sufficient; every photon that is emitted will interact with countless atoms on its way to the surface, bumping each of those atoms outward in the process (yes this is a simplification but it's close enough). When the fuel is expended, there is no more outflow…