I’ve read maybe it has something to do with gravity…? But we have gravity here on earth too.
And while the sun’s internal temperature is around 15M degrees celsius, apparently for us to replicate the fusion effect here on Earth, we need to superheat atoms to 100M degrees celsius. Why the difference?
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Fusion is not easy in the sun, it is quite rare compared to human-made fusion reactors.
The power per cubic meter of the solar core is around 276W/m^3. A human at rest produces around 100W of heat but our volume is less than 0.1m^3 so the human power density per unit of volume is around 100W/m^3. This means the core of the sun generates around a quarter of the heart per unit of volume compared to you.
A pile of compost generates around the amount of heat per cubic meter as the sun does, we do not use them for electrical power generation because that is not a lot of energy compared to the volume.
What makes this even worse for the sun is that the density in the core is around 150x the density of water, and humans’ density is around the same as water. This means that compare to mass you generate around 4 x 150=600 tims more heat compared to the mass
Large nuclear fission reactors are at around 2,000,000 W thermal energy, 2000,000/276= 7246 cubic meters, which is a cube with sides of 19 meters.
Liquid hydrogen has 1/14 the density of water so to get to the solar density you need to compress it so the density increase 2100 times. The pressure in the sun to accomplish that is 256 billion times atmospheric pressure.
So good luck trying to keep a 19 cubic meter cube of hydrogen at a pressure of 256 billion times the atmosphere and 15 million kelvin warm.
A lot higher temperature increases the power density a lot and make it a lot more practical to do
The amount of fusion fuel that is in fusion reactors is in a few gram range, more would be harder to keep in place. To get high energy output of that small amount of fuel we need a lot higher temperature because the power density both compared to mass and volume needs to be many times that of the sun.
The sun output a lot of energy because it is huge not because it is efficient. The sun lives for billions of years so the rate it uses up fuel is very slow.
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