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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The sun has like 330,000 Earths worth of mass crushing its core. This pressure dramatically lowers the temperature required to achieve fusion relative to the conditions the atoms in our experiments on Earth undergo.
The only difference I get is that the sun does not have the need to contain the reaction (and thus the high temperatures) since it’s itself the reaction, and also it has gravity like us but its is way stronger. Otherwise we need to contain that stuff since that temperatures can destroy anything, any material, so it a bit more complicated. We need to create sophisticated and strong magnetic fields since it’s our only way to contain that stuff at those temperatures.
The gravity of the sun is about 28 stronger than here on Earth. This makes it so that the particles you are trying to fuse are way closer to each other in the sun than here on Earth. Because the particles are closer together, you need less thermal energy (a lower temperature) to get them to fuse.
Enormous mass creates enormous pressure and heat, stars larger than the Sun do it even easier and as a result even though they have more hydrogen they last a shorter time than the Sun as they burn through the fuel quicker. https://youtu.be/vVE0B6g9F_0
Short answer: the sun is goddamned huge.
> 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?
Two reasons. First, the Sun’s core is under [incredible pressure](https://en.wikipedia.org/wiki/Solar_core), a million times higher than the [highest pressures we can achieve](https://gsecars.uchicago.edu/scientific-program/high-pressure-high-temperature-large-volume-press/) for a large volume of material in the lab. This mashes the atoms together real good and makes fusion more likely.
Second, even with the high pressure and temperature, fusion in the sun is really slow. Power output in the core is about [300 watts per cubic meter](https://en.wikipedia.org/wiki/Solar_core). This is far less than the heat density generated by the human body or a compost heap. You’d need a volume of superhot superdense plasma bigger than a football stadium just to power a small city. … and even then it wouldn’t work because all the heat would leak out the side walls of the football stadium.
So if the sun’s power density is so weak, how can it light up the whole solar system? See above: the sun is goddamned huge. Like, no matter how big you think the sun is, it’s bigger than that.
On Earth, if we want fusion to be a practical energy source, we need to fuse hydrogen atoms in a more compact way than the goddamned huge sun does. Which means we need higher power density, and hotter temperatures, than the sun.
Creating a fusion reaction isn’t that hard, it’s containing it that’s the problem. Modern Nuclear bombs main stage is all fusion reaction with a fission reaction to start it. The sun’s mass self contains the reaction in the core.
Gravity. The Earth has gravity, sure, but the sun has hundreds of thousands of times as much as Earth. That much force crushes the atoms at the core so powerfully that they occasionally fuse by force alone, causing a huge release of energy that makes fusion even more favorable, and — voila!
Something that people aren’t talking about is the time scale. It took millions of years for enough hydrogen to gather to for a gas giant big enough to begin the fusion process. So the process wasn’t exactly “easy”.
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.
My goddamn mind is blown. Stars are bad at fusion. Jeeeezus, THIS is the sort of shit my daughter should be learning in high school astronomy, not watching Interstellar like it’s a documentary.
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