4 ms·

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

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.

only_as_i_fall 5y ago
Does the volume of the sun include its Corona in that last statement?
- 317070 5y ago
  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.
- ben_w 5y ago
  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+%2F+volume+of+sun%29+in+watts+per+cubic+meter https://www.wolframalpha.com/input/?i=%28solar+luminosity+%2...
innot 5y ago
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?
- shadofx 5y ago
  Carbon is capable of fusion, in a sufficiently compressed stellar mass. Everything up to Iron-56 can theoretically sustain fusion.
- thinkcontext 5y ago
  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.
- jabl 5y ago
  Maybe it could work via the CNO cycle: https://en.wikipedia.org/wiki/CNO_cycle https://en.wikipedia.org/wiki/CNO_cycle ?

only_as_i_fall 5y ago

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

317070 5y ago
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.
ben_w 5y ago
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+%2F+volume+of+sun%29+in+watts+per+cubic+meter https://www.wolframalpha.com/input/?i=%28solar+luminosity+%2...

innot 5y ago

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?

shadofx 5y ago
Carbon is capable of fusion, in a sufficiently compressed stellar mass. Everything up to Iron-56 can theoretically sustain fusion.
thinkcontext 5y ago
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.
jabl 5y ago
Maybe it could work via the CNO cycle: https://en.wikipedia.org/wiki/CNO_cycle https://en.wikipedia.org/wiki/CNO_cycle ?