# Can a solid planet become a star?

by fbs7
Tags: planet, solid, star
P: 365
 Quote by fbs7 Hmm... I've been thinking about these points, and indeed they are troubling questions for my thought experiment. If I understand your explanations right, the problem with the assumption that "a sphere of gas submitted by a large enough pressure will necessarily have a large enough temperature for fusion" is that density of that gas, right? That is, you can have a cloud of gas with mass equal to say 10 Suns, but its center may still not ignite if the density is small enough. But then, wouldn't the pressure at the center be very low too? I mean, how can you have high pressure but low density? If the pressure is high, won't the density be high too? It's because I have trouble in imagining a sphere of gas, say hydrogen, that is at high pressure but low temperature. The mental image that I have is say butane - even if the thing is at room temperature, you compress it just a little bit and it becomes liquid. Even more I imagine it would become solid. So I reckon the same thing happens with hydrogen. Therefore, I couldn't imagine a way to get a cold, gaseous planet to remain gaseous and cold, and yet be big enough to become a star. The core of cold gaseous planet should solidify at low temperature and high pressure, and then behave the same as if that core was made of say silicon to begin with. Therefore, the only way I could see that a sun-mass sphere of gas to remain a gas is by having high temperatures - that's why my assumption that a sphere of gas submitted at high enough pressure will necessarily be hot and will become a star (that is, if it was cold, it wouldn't be a gas a high pressure).
I think we are both getting a bit confused about what we are actually talking about here. The point that I am trying to make is that you are ascribing properties to gases and solids, pressure, heat dissipation etc. based on your everyday experience of materials on Earth and this is leading to some invalid conclusions when dealing with masses and therefore pressures and temperatures many orders of magniture greater. And you need to drop the idea that the temperature of a body of gas is sustained by pv=nRT - it isn't.

Oh something else I didn't notice in your orignal post - about planets enclosing a star, have you heard of the (science fiction) concept of a Dyson sphere?
P: 284
 Quote by MrAnchovy But at the centre of a star the pressure is very high which allows fusion to take place at much lower temperatures than would otherwise be the case, which is what I mean by "equally important". But then fusion won't start.
If high pressure allows fusion at low temperatures then it allows fusion in liquids and in solids. Crystal order would not stop nuclei from tunnelling into other nuclei, like it does not stop electrons from tunnelling around metal conduction band.
 Quote by MrAnchovy And you need to drop the idea that the temperature of a body of gas is sustained by pv=nRT - it isn't.
Temperature isnīt. Pressure is. That is what defines gas.

 Quote by fbs7 I mean, how can you have high pressure but low density?
By having high temperature.

At low temperatures and pressures, you have
V=n/ρ, independent on P and T
At intermediate temperatures and densities, you have
pv=nRT
At high temperatures, you have
P=cT^4, independent on n

 Quote by fbs7 It's because I have trouble in imagining a sphere of gas, say hydrogen, that is at high pressure but low temperature. The mental image that I have is say butane - even if the thing is at room temperature, you compress it just a little bit and it becomes liquid. Even more I imagine it would become solid. So I reckon the same thing happens with hydrogen. Therefore, I couldn't imagine a way to get a cold, gaseous planet to remain gaseous and cold, and yet be big enough to become a star. The core of cold gaseous planet should solidify at low temperature and high pressure, and then behave the same as if that core was made of say silicon to begin with.
Correct. All substances save helium (both isotopes) solidify at low temperature and pressure. Even helium freezes around 30 atmospheres. And the freezing point will increase with increase of pressure.

But solids are actually better heat insulators than gases. It is just that they are not heated much by compression.

Deep sea is icy cold. Below +2 degrees.

Ice will not be heated by compression at all - to the contrary, compression will cause ice to melt and cool. Cold fresh water will contract on heating - and for that very reason, will cool on compression.

Warm fresh water, and salty water, do warm on compression - slightly. As stated, deep sea is still icy cold. On compression to 1000 atmospheres, it warms by merely 1,2 degrees!

On the same compression, 1 atmosphere to 1000 atmospheres, air should heat by over 1000 degrees!

This means that liquids and solids are easier to ignite. Thermal runaway is not slowed down by heat losses.

A gas will expand on heating - and therefore cool by expansion. This limits the warming by any added heat.

A liquid as mentioned expands only slightly and therefore the heat loss on expansion is negligible. The added heat is spent on heat capacity alone - so the liquid heats up more and is more liable to thermal runaway.

A solid, like liquid, expands only slightly and is cooled by heat capacity alone. But a solid, unlike a liquid, is solid. Therefore even if the solid does expands, it is rigidly in place and cannot convect upwards like gas or liquid can. Heat can only be transferred by conduction, which is extremely inefficient. Water at 10 km depth is icy cold, just 1,2 degrees warmer than surface. Rock at 10 km depth is everywhere hot - over 150 degrees everywhere.
P: 127
 Quote by snorkack If high pressure allows fusion at low temperatures then it allows fusion in liquids and in solids. Crystal order would not stop nuclei from tunnelling into other nuclei, like it does not stop electrons from tunnelling around metal conduction band. Temperature isnīt. Pressure is. That is what defines gas. By having high temperature. At low temperatures and pressures, you have V=n/ρ, independent on P and T At intermediate temperatures and densities, you have pv=nRT At high temperatures, you have P=cT^4, independent on n

Wow, that's amazing. Learning a new thing every day!

Actually learned two: that materials can fuse at high pressure and low temperature, and pressure of gas at high temperature does not depend on density.

Quite surprising information - it's incredible to have fusion at low temperatures. Whammy wow!

That completely changes my thought experiment, uh? If I accumulate enough cold carbon, and do it so slowly that it remains cold, eventually it will start fusion at the center at cold temperatures and huge pressures, will explode the outer cold junk and leave a small neutron star behind, uh? What an extraordinary process!
P: 284
 Quote by fbs7 pressure of gas at high temperature does not depend on density.
Yes, because the pressure of light is independent on the amount of matter. At high temperature and low density, the pressure is mainly that of light.
 Quote by fbs7 That completely changes my thought experiment, uh? If I accumulate enough cold carbon, and do it so slowly that it remains cold, eventually it will start fusion at the center at cold temperatures and huge pressures, will explode the outer cold junk and leave a small neutron star behind, uh?
Probably not. Type Ia supernovae are thought to come completely unbound and leave nothing behind.

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