# How much larger would a star have to be to cause thermonuclear reactions if it was made out of mostly rock like Earth, instead of gases?

I assume it would have to be considerably larger since rock isn't as flammable as gases are? But it's also more dense to begin with..

Do we know of any suns that are made like this?

Is it even possible to have a thermonuclear reaction with solid rock?

• A body that large would rather collapse into a neutron star (or black hole) instead of undergoing fusion. – Florin Andrei Jul 1 '15 at 20:41
• @FlorinAndrei that's not strictly true. Rock, mostly Silicon and Oxygen would undergo some kind of type 1 supernova explosion as Rob Jeffries describes and in the process, blowing off a fair amount of it's matter. Fusion needs to reach the Iron stage before a Neutron Star of Black Hole could form. – userLTK Nov 29 '15 at 15:27

## 2 Answers

It really depends what you mean by "rock". At the temperatures and pressures at the cores of stars (and at which nuclear fusion reactions are possible), "rocks" as I suspect you are thinking of, do not exist.

Thermonuclear reactions do not occur because the gas is "flammable", they occur because the kinetic energies of the nuclei in the gas (at these temperatures, atoms are fully ionised) are sufficient to get them close enough together for nuclear fusion to take place.

"Rocks" are made of atoms of silicon and oxygen (for example) in the form if silicates. But these are dissociated at fairly low temperatures compared to the centres of stars. Oxygen and Silicon thermonuclear fusion ignition requires temperatures in excess of $10^9$ K, and these temperatures are only reached late in the lives of stars of mass $>8 M_{\odot}$.

It is possible to have stars with solid cores. This is thought to be the fate of white dwarf stars as they cool. Most white dwarfs are made of a mixture of carbon and oxygen and this "crystallises" once the core of the white dwarf cools below about a few million degrees.

Ordinarily, the cores of white dwarfs are inert as far as nuclear reactions are concerned, because the temperatures are too cool. However, if the white dwarf is massive enough (or mass is added to it), then the central densities climb, and for white dwarfs of mass $\sim 1.38M_{\odot}$ it is thought that the densities become high enough ($\sim 10^{13}$ kg/m$^3$) to start nuclear fusion of carbon via the zero point energy oscillations in the crystalline material (so-called pyconuclear reactions). Such reactions in degenerate matter are highly explosive and might result in the detonation of the whole star in a type Ia supernova.

If you put a large enough mass of earth-like rock together, you might get lithium fusion. Stars fuse what little lithium they have at masses just under the mass needed to become a red dwarf ("stars" that only burn lithium and not hydrogen are large mass brown dwarf stars). But the lithium concentration in a "rock" star (hur, hur. Rock star) is certainly much higher than in a regular brown dwarf. https://en.wikipedia.org/wiki/Lithium_burning