I How does a brown dwarf's temperature decrease over time?

[Thecla]

TL;DR Summary

Life history of a brown dwarf

A brown dwarf is not quite a star whose core temperatures are only a few million degrees and can't initiate fusion. After billions of years it has to cool down. Does the hydrogen just dissipate or does the hydrogen cool down to the temperature of the universe, a few degrees kelvin, and form solid hydrogen?

 

[Drakkith]

The star cools down, very slowly, until it reaches equilibrium with the rest of the universe (currently about 2.7 K). As for what phase it turns into, I admit I don't know for sure. Hydrogen freezes at about 14 K so I assume it solidifies.

 

[stefan r]

It should decay much faster than black holes of similar mass. There should be some spontaneous fusion too.

The brown dwarfs outer layer will eventually solidify. Much longer than billions of years. Deeper down should become metallic hydrogen. Phase transitions usually generate heat.

At low temperature the metals can precipitate and rain out of solution. The falling droplets release energy because they are denser and they are falling down a gravity well. That will generate heat and delay surface cooling.

The word "death" has always bothered me with regard to stars. In what sense is a star "alive".

 

[Drakkith]

The word "death" has always bothered me with regard to stars. In what sense is a star "alive".

About as alive as my laptop, which is in the process of dying on me apparently.

At low temperature the metals can precipitate and rain out of solution.

That's both terrifying and awesome to imagine. I wonder what sort of umbrella would protect me from metallic rain...

 

[256bits]

At low temperature the metals can precipitate and rain out of solution. The falling droplets release energy because they are denser and they are falling down a gravity well. That will generate heat and delay surface cooling.

Would that actually happen - the raining out that is.
The brown dwarf is pretty dense already throughout, with electron degeneracy pressure holding it up, rather than kinetic motion of the protons.

 

[snorkack]

Would that actually happen - the raining out that is.
The brown dwarf is pretty dense already throughout, with electron degeneracy pressure holding it up, rather than kinetic motion of the protons.

Electron degeneracy also holds up planets.
Three gas giants possesses internal heat - all except Uranus.

Would Uranus freeze at 1 bar level if it were in outer solar system?

 

[256bits]

Electron degeneracy also holds up planets.
Three gas giants possesses internal heat - all except Uranus.

Would Uranus freeze at 1 bar level if it were in outer solar system?

I have no idea about Uranus.

Planets are mainly supported from pressure produced by thermal motion of the atoms and molecules within.
Some degeneracy is present, but not to the extent of that within the dwarf stars, where the density is the deciding factor for its radius, regardless of temperature.
As a planet cools, motion of the mass within the interior does occur, as cooler material, being more dense will tend to flow towards the interior, producing mass displacement currents, or convection currents with regards to heat flow.

Some how the heat does flow from the interior of a dwarf star to the exterior, and this discussion has produced more questions on the actual mechanism.

 

[Thecla]

The only fuel for heating a brown dwarf is gravity and gravity is eternal. This is where I get confused: The brown drawf cools by radiating heat, it cools off and expands. Then the cycle is repeated , it will contract due to gravity and heat up again. Why wouldn't this go on forever since you are not running out of gravity?

 

[DaveC426913]

The only fuel for heating a brown dwarf is gravity and gravity is eternal. This is where I get confused: The brown drawf cools by radiating heat, it cools off and expands. Then the cycle is repeated , it will contract due to gravity and heat up again. Why wouldn't this go on forever since you are not running out of gravity?

Do you have a reference showing that it expands when it cools?

 

[Drakkith]

The only fuel for heating a brown dwarf is gravity and gravity is eternal. This is where I get confused: The brown drawf cools by radiating heat, it cools off and expands. Then the cycle is repeated , it will contract due to gravity and heat up again. Why wouldn't this go on forever since you are not running out of gravity?

I really don't think it expands as it cools.

 

[256bits]

Would Uranus freeze at 1 bar level if it were in outer solar system?

I think we both are on about different parts of a brown dwarf life cycle.

After some research I have found, that BD are convection driven, with H2 forming in the upper atmosphere.

 

[snorkack]

Planets are mainly supported from pressure produced by thermal motion of the atoms and molecules within.
Some degeneracy is present, but not to the extent of that within the dwarf stars, where the density is the deciding factor for its radius, regardless of temperature.

No.
A bucket of water is supported by degeneracy. Trivial proof: cool it to absolute zero, at which thermal motion of atom and molecules witin does not exist. The contents of bucket expand when their thermal motion is stopped.

Planets are likewise degenerate, in that thermal expansion of the material has minor effects on total volume. And so are brown dwarfs.

 

[Drakkith]

A bucket of water is supported by degeneracy. Trivial proof: cool it to absolute zero, at which thermal motion of atom and molecules witin does not exist. The contents of bucket expand when their thermal motion is stopped.

Planets are likewise degenerate, in that thermal expansion of the material has minor effects on total volume. And so are brown dwarfs.

I guess you could call this 'molecular degeneracy pressure' or 'atomic degeneracy pressure'?
I've honestly never heard anyone or anything describe atoms or molecules in terms of degeneracy pressure before.

 

[sophiecentaur]

gravity is eternal.

What would you be meaning by that statement? Gravitational Potential Energy transfers to thermal energy which can the lost by radiation. That's not "eternal".

 

[sophiecentaur]

The only fuel for heating a brown dwarf is gravity and gravity is eternal.

Hmm. Maybe you could say that but what counts here is Gravitational Potential Energy. That transfers partly to thermal energy, which radiates away and leaves less GPE. As the net energy goes down, so does the temperature.