Hacker News Reader

Comment by gentleman11 | Hacker News Reader

I had assumed both were chain reactions due to stars. That’s interesting

Well, technically, that's right. Fusion is a chain reaction. (albeit, "chain" might be a stretch here. Let's say it is a continuous process in stars.) But it's driven by gravity, not neutrons. You just need to bring about 70 times the mass of Jupiter into one place and that's it. Child's play, essentially ;).

The enormous pressure inside stars is what sustains the reaction. Neutrons coming off a fusion reaction, even if moderated, can’t really “squeeze together” other particles in any sense.

Also, per unit volume, the sun produces about as much power as a compost pile.

Does the volume of the sun include its Corona in that last statement?

No, iirc that statement is for the part of the sun where fusion actually happens. Per volume, not a lot fusions happen in the plasma on the sun. The crux is that the volume of the compost heap grows order cubed to the radius, while the area through which the heat escapes grows quadratic. And the sun is like, a _really_ big compost heap.

In fusion on earth, we want to be considerably more efficient than the fusion process in the sun, as we don't have as much space to work with. ITER is already a pretty big machine.

That’s just the core. Go out to the photosphere, let alone the corona, and it’s about about a thousand times less, 0.27 W/m^3: https://www.wolframalpha.com/input/?i=%28solar+luminosity+%2...

This lead me to the following - what if you make a compost pile the size (mass) of Sun? Meaning, it won't be made of hydrogen, but rather some carbon-based molecules. I'm not sure about other atoms in these molecules, but I think carbon is stable enough not to initiate nuclear reactions. So probably fusion won't start. What then?

Carbon is capable of fusion, in a sufficiently compressed stellar mass. Everything up to Iron-56 can theoretically sustain fusion.

Gravity would compress it. Depending on the exact composition fusion could start, ie it would become a star. According to Wikipedia carbon fusion requires a mass of 8 suns to start but there's a bunch of hydrogen in compost so maybe that would fuse.

Maybe it could work via the CNO cycle: https://en.wikipedia.org/wiki/CNO_cycle ?

Fusion can be a chain ration. In stars its caused by intense uninterpreted gravitational pressure. In the H-Bomb its triggered by a smaller fission explosion. Fusion chain reactions just require so much energy its hard (if not impossible) to harness on a commercial level. One small break in the the chain reaction containment and it falls apart.

Reply on news.ycombinator.com