# When a star exhausted it's nuclear fuel

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What powers a star?

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Drakkith
Staff Emeritus
Gravitational contraction is the sole remaining source of energy for stars that have exhausted their fuel. Once a star reaches a point where it can no longer contract, such as in white dwarfs, it simply cools off until it reaches ambient temperature of around 2 kelvin. Note that no stars have had long enough to cool off to 2 kelvin. I believe the oldest and coolest white dwarf ever found was somewhere above 3,000 k.

Gravitational contraction is the sole remaining source of energy for stars that have exhausted their fuel. Once a star reaches a point where it can no longer contract, such as in white dwarfs, it simply cools off until it reaches ambient temperature of around 2 kelvin. Note that no stars have had long enough to cool off to 2 kelvin. I believe the oldest and coolest white dwarf ever found was somewhere above 3,000 k.

You didn't answer my question what fuels after nuclear fuel nuclear property keeps it staple but what keeps it going

Drakkith
Staff Emeritus
You didn't answer my question what fuels after nuclear fuel nuclear property keeps it staple but what keeps it going
I'm not sure I understand what you're asking. Once a star has burned all of its fuel, there is nothing left except gravitational potential energy, which is quickly expended by contraction. Is that a better answer?

Nuclear fuel is the last stand for a star to hold it's mass against gravity

Drakkith
Staff Emeritus
Nuclear fuel is the last stand for a star to hold it's mass against gravity

What powers it after that

Is it just the mass of the star left that that leaves this massive amount of gravity

Drakkith
Staff Emeritus
What powers it after that
Gravitational contraction, like I already said. If that doesn't make sense, please elaborate on what exactly doesn't make sense so I can address it.

DaveC426913
Gold Member
Gravitational contraction generates a lot of heat.

Drakkith
Staff Emeritus
Is it just the mass of the star left that that leaves this massive amount of gravity
You're bouncing around different concepts. A star emits light because it is hot. It reaches such a high temperature because, during its initial formation, gravitational contraction of its parent gas cloud heats up the cloud to millions of kelvin in the core. This in turn ignites fusion, turning the cloud into a star and providing the energy needed to keep the star in its present main-sequence state for billions of years. Once this fuel runs out, the star continues to collapse. The collapse process itself generates heat, which is radiated away from the star as light and other forms of EM radiation. Once all of the available gravitational potential energy of the star has been exhausted, meaning that it has collapses to the point that it cannot collapse any further, it has run out of energy sources and will gradually cool as its energy is carried away into space by EM radiation.

The gravity of the star is always due to its mass, and only a tiny fraction of its mass is converted into energy and radiated away.

All stars produce light (and other kinds of energy) through nuclear reactions, using the energy stored in the tiny nucleus at the center of atoms. These reactions make the star so hot that it glows—it's like an enormous ball of fire, giving out light and heat.

You're bouncing around different concepts. A star emits light because it is hot. It reaches such a high temperature because, during its initial formation, gravitational contraction of its parent gas cloud heats up the cloud to millions of kelvin in the core. This in turn ignites fusion, turning the cloud into a star and providing the energy needed to keep the star in its present main-sequence state for billions of years. Once this fuel runs out, the star continues to collapse. The collapse process itself generates heat, which is radiated away from the star as light and other forms of EM radiation. Once all of the available gravitational potential energy of the star has been exhausted, meaning that it has collapses to the point that it cannot collapse any further, it has run out of energy sources and will gradually cool as its energy is carried away into space by EM radiation.

The gravity of the star is always due to its mass, and only a tiny fraction of its mass is converted into energy and radiated away.
Photon energy is the energy carried by a single photon. The amount of energy is directly proportional to the photon's electromagnetic frequency and inversely proportional to the wavelength. The higher the photon's frequency, the higher its energy. Equivalently, the longer the photon's wavelength, the lower its energy

DaveC426913
Gold Member
All stars produce light (and other kinds of energy) through nuclear reactions, using the energy stored in the tiny nucleus at the center of atoms. These reactions make the star so hot that it glows—it's like an enormous ball of fire, giving out light and heat.
Yes. Until they run out of fuel.
Then they stop using nuclear reactions as the source of light and heat.
But light and heat are still generated from gravitational collapse.

DaveC426913
Gold Member
Photon energy is the energy carried by a single photon. The amount of energy is directly proportional to the photon's electromagnetic frequency and inversely proportional to the wavelength. The higher the photon's frequency, the higher its energy. Equivalently, the longer the photon's wavelength, the lower its energy
Relevance?

Drakkith
Staff Emeritus
All stars produce light (and other kinds of energy) through nuclear reactions, using the energy stored in the tiny nucleus at the center of atoms. These reactions make the star so hot that it glows—it's like an enormous ball of fire, giving out light and heat.
Stars produce light and EM radiation because they are hot! That is the direct cause of the light and radiation. The reason they get hot in the first place the result of the way stars are formed. The reason they are still hot after millions or billions of years is because of fusion. All of the EM radiation generated directly from fusion reactions (i.e. gamma rays) is absorbed in the core of the star. None of it makes it to the photosphere to be emitted into space.

Photon energy is the energy carried by a single photon. The amount of energy is directly proportional to the photon's electromagnetic frequency and inversely proportional to the wavelength. The higher the photon's frequency, the higher its energy. Equivalently, the longer the photon's wavelength, the lower its energy
Please don't post random facts that are only tangentially relevant to the topic.

Relevance?
But it's not the same and not as strong as the star generating the heat or energy

Drakkith
Staff Emeritus
But it's not the same and not as strong as the star generating the heat or energy

DaveC426913
Gold Member
But it's not the same and not as strong as the star generating the heat or energy
What is 'it'?

PeterDonis
Mentor
2019 Award
What powers a star?
Nuclear fusion. But apparently you already knew that:

All stars produce light (and other kinds of energy) through nuclear reactions, using the energy stored in the tiny nucleus at the center of atoms. These reactions make the star so hot that it glows—it's like an enormous ball of fire, giving out light and heat.

davenn