Follow up on the recent post *”how can the sun just continuously burn and burn?*”
Can someone explain how the sun actually started its nuclear fission?
Oppenheimer did not go a dropped a nuke on it 😉
In: 1
(Fusion, not fission)
The sun’s original heat came from the gravitational potential energy of the infalling gas of the primordial cloud. That energy was converted to heat via the pressure/volume relation, PV = nRT. Once it got hot enough, fusion kept it hot. (remarkably little fusion, actually. The sun makes about 275 watts per cubic meter in its core region. About the same as the compost pile in your garden. There’s just a shitload of it.)
Jupiter does not do fusion, but is still leaking away the original heat from its collapse and consolidation.
Fusion requires heat and pressure to start. Fortunately, gravity can create both of these factors. The sun formed as more and more spinning gasses collapsed into it. This added more and more energy to the center while the gravity grew and grew. Eventually there was enough heat and pressure to start a hydrogen fusion reaction, which released even more energy.
One correction here: What happens in the sun is nuclear *fusion* (elements combining), not nuclear *fission* (elements splitting).
The short version: Gravity did it.
The still short, but slightly longer version:
It’s possible for nuclear fusion to happen just by the right light elements (like hydrogen) getting squeezed extremely hard. Under an absolutely incredible amount of pressure, the repulsive force between two atomic nuclei is overwhelmed, and the two nuclei combine into one, giving off more energy in the process than it took to push them together.
In a star, this pressure is provided by star’s material getting squeezed together by gravity, and the chain reaction starts when the energy each individual combination gives off makes other nuclei combine. This keeps going until eventually the star runs out of elements that can combine under those conditions because everything’s already been fused (since the repulsive force keeps increasing with each combination), at which point either the star dies quietly, or if it’s big enough it explodes in a nova or supernova (or even becomes a black hole).
The sun, and other stars, do not emit energy from nuclear fission, but nuclear fusion. This is where very simple atoms like Hydrogen collide into one another and fuse together. When they do that, they produce a tremendous amount of energy, including heat.
The fusion reaction is produced because there was an extremely large cloud of hydrogen and other elements in one region in space. Gravity pulled all of those elements together and condensed them into a (relatively) compact ball. The gravity pulled them together with enough force and pressure that a fusion reaction began – and has been going ever since. That ball of gas and its ongoing fusion reaction is Sol, our sun.
Not fission that is splitting atoms it is fusion squeezing atoms together. https://youtu.be/vVE0B6g9F_0
There is so much matter on the sun that the force of gravity causes fusion of hydrogen into helium. If this fails to start, it never becomes a star, but rather becomes what’s known as a brown dwarf.
This is an absolutely immense amount of matter. The absolute minimum is about 75x the mass of Jupiter.
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