Homework Help: Why are stars "hot"?

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1. Dec 8, 2016

dykuma

1. The problem statement, all variables and given/known data
Why are stars hot?

2. Relevant equations
Non specifically, it's a conceptual question.

3. The attempt at a solution

I have always heard that stars are hot because (as a quick very basic example) stars are large collections of mass which collapse in on itself. Eventually the individual protons of this mass are able to tunnel past the Coulomb barrier and engage in fusion. Because of the mass defect, some mass is lost during fusion as energy, and this energy is what generates most of the heat of a star.

With that being said, I recall during a lecture in my astrophysics class that fusion can actually be thought of as a cooling process for the star. If it is not fusion that makes the star hot, then what is it? (this is not to say what I wrote above is wrong, but rather I am looking for a different way to view what produces the heat in a star.)

2. Dec 8, 2016

CWatters

Correct.

Correct. Where does the energy created by fusion go?

3. Dec 8, 2016

phinds

Fusion, as you would have confirmed with a small amount of research.

EDIT: Ah, CWaters beat me to it

4. Dec 8, 2016

dykuma

What exactly go you mean by, "where does the energy go?" If I remember correctly, the energy goes off in the form of positrons, neutrinos, and gamma rays. so specifically, light?

I recall that we set up a long list of equations for the sun to reach an equilibrium. basically we equated the force of gravity inward vs pressure outward. The pressure had multiple components, including photon pressure.

My initial assumption with this is that gravity initiates the fusion process by forcing the mass to be close together, and the fusion produces energy which forces the star to expand outward. If heat is generated by contraction, then expansion is cooling. However I have a hard time accepting that, because if it were not for the fusion in the first place, then no heat would have been generated anyway. It is kind of like blaming a car accident for just driving, if I can make a bad analogy.

5. Dec 8, 2016

phinds

He means that the answer to your original question "why are stars hot" can be found by thinking about where the energy of fusion goes. Do you think neutrinos make us feel the heat of the sun? Positrons? Think about what it means that we "feel heat from the sun".

6. Dec 8, 2016

CWatters

Exactly. Stars loose energy in the form of light and loosing energy is effectively a cooling process.

7. Dec 8, 2016

dykuma

(disregard this post, the question was answered before I posted this)

We feel heat from the sun due to the large amount of light that it emits from the fusion reactions in the core.

What I am trying to recall is something that the professor in the class said. It was an offhand comment that fusion can be viewed as a cooling process. I am trying to understand how fusion could be viewed in such a way.

Last edited: Dec 8, 2016
8. Dec 8, 2016

dykuma

Ah, so similar to how boiling is a cooling process?

Okay, so then if the light that is emitted by fusion reactions is a cooling process, then what can I say that makes the star hot? Is it also fusion, gravitational potential energy? Like I said in the previous comment, I have a problem with saying that gravity makes a star hot. I know that heat will be generated as a result of gravity (such as with accretion disks), however white dwarfs are effectively cooling stars that are no longer producing heat, even though they are super dense.

9. Dec 8, 2016

rootone

The fusion is responsible for releasing energy which eventually radiates away from the star.
As a result the total mass-energy of the star has been reduced.

10. Dec 8, 2016

CWatters

There are some good descriptions of how stars work on the web.

In short gravity increases the temperature and pressure until fusion occurs. Fusion releases energy heating the star and radiating light out into space

Regarding the cooling process. Consider an electric heater. The electricity makes the heater hot causing it to emit IR radiation. The emission of IR radiation can also be called a cooling process. It doesn't make the heater colder but it stops the heater getting ever hotter and hotter.