<p>The core of the Earth is hot because of its own density and radioactive materials. Basically, the more you compress an object, the hotter it gets. It does not undergo fusion, since only massive (stellar) objects do so. A significant amount of radioactive decay happens in the core, which is mainly the source of its heat, but no fusion.</p>

<p>Similarly, stars are hot because of their own densities, but at a much larger scale, and sans the nuclear decay. This is especially evident when stars are born as non-fusing protostars; they are prevented from getting smaller because as they do so, their core will get hotter. Hotter objects actually expand, so protostars are in a state of equilibrium; if they get any smaller, the temperatures will increase and they will become larger again.</p>

<p>However, protostars slowly get smaller and smaller over time. This is because they lose energy as they emit light. Light emission is, in fact, a source of energy loss. So the star will continue to collapse slowly, and once the core reaches a certain density, it will begin fusing hydrogen. This produces radiation pressure, which counters the force of gravity.</p>

<p>Again, let me clear up something: nucleosynthesis in stars does not cause heat, but rather it prevents the star's gravitational collapse. It's the star's density that causes such high temperatures. If a star runs out of "fuel", that spells out the last phases of its life — as far as I know, the end result reaches extremely high temperatures (supernovae, white dwarfs, etc.)</p>

CC BY-SA 3.0
2016-07-15T12:13:32.300Z
It is the radioactive decay that is the main source of heat in the core, not the compression.
James K
CC BY-SA 3.0
2016-07-15T12:15:48.440Z
@JamesKilfiger What source do you have for that?
Sir Cumference
CC BY-SA 3.0
2016-07-15T12:36:11.047Z
http://www.scientificamerican.com/article/why-is-the-earths-core-so/ During the formation of the Earth, "Descent of the dense iron-rich material [...] would produce heating on the order of 2,000 kelvins" However the core is now closer to 6000K, the difference is radioactivity. Without radioactivity, the core could have solidified within a few hundred million years (William Thomson)
James K
CC BY-SA 3.0
2016-07-15T12:51:35.813Z
thanks for that great article, @JamesKilfiger.  I had no idea.
Fattie
CC BY-SA 3.0
2016-07-15T14:46:39.547Z
@JamesKilfiger Ah, all right. I corrected myself.
Sir Cumference
CC BY-SA 3.0
2016-07-15T11:54:10.127Z
2016-07-15T14:46:21.310Z
2016-07-15T14:46:21.310Z
Sir Cumference
<p>A planet does not have any fuel. Only stars "burn" fuel either in their cores or concentric shells around their cores.</p>

CC BY-SA 3.0
2016-07-15T11:23:00.903Z
And what is the energy consuming that makes our core beeing hot? I mean it can't be a source for endless heat that is cousing planets cores to be hot, can it?!
Zaibis
CC BY-SA 3.0
2016-07-15T11:50:28.520Z
The heat in the core is from two or three sources, residual heat from the planet's formation, heat from the decay of radio active elements and for larger planets compression due to gravity. How long each of these lasts depends on the size and composition of the planet's core. Tidal interactions can also produce heat on planets with large natural satellites. As well as the amount of heat produced with respect to time these will all also effect what happens when the planet's core cools.
James Screech
CC BY-SA 3.0
2016-07-15T12:44:40.597Z
Then my question was wrong due to my presumptions. Should I rephrase ti into another question? Or is this the wrong site for it at all? Because google made me a hard time to get a hitn on it.
Zaibis
CC BY-SA 3.0
2016-07-15T12:49:17.767Z
Hi @Zaibis.  Yes, in the case of the Earth, our Earth ***started out hot***, and is still a bit hot, and is slowly cooling.  The Earth is not burning any fuel in the middle.  Lava and so on is just hot because the Earth started hot, and is still pretty hot.  (*However* note that J.F. above gives sources explaining that radioactivity actually makes a big contribution in Earth's case.) Yes, you would be better on the excellent Earth Sciences site.  Come back here with questions about astronomy!
Fattie
CC BY-SA 3.0
2016-07-15T12:50:16.503Z
@JoeBlow: I thought thats what astronomy is about, sorry for bothering you here.
Zaibis
CC BY-SA 3.0
2016-07-15T12:55:02.987Z
hi @Zaibis I am just new on this particular site myself.  You did not bother anyone in the slightest, at all.  It is a **great question** as I said above.  I learned a huge amount here from James comment and others.  It is **totally normal** that questions get migrated to other sites, you know?  Almost every question I ask gets migrated to a different site (I can't be bothered choosing the right site - I just let someone else sort it out :)  ).  This was a GREAT question and thanks for it!
Fattie
CC BY-SA 3.0
2016-10-08T08:57:03.370Z
@JamesScreech Your explanation in the comments here is great, can you edit them into your answer? Comments are a little volatile.
SE - stop firing the good guys
CC BY-SA 3.0
2016-07-15T10:00:25.207Z
2016-07-15T10:00:25.207Z
James Screech
<p>What would happen when a planet consumed all its fuel and its core extincts.
Would anything change on the planet?</p>

<p>Can this ever happen before its star did so?
If not, why?</p>

CC BY-SA 3.0
2016-07-15T12:10:09.533Z
This does not appear to be an astronomy question. It may be an Earth Science question. You should ask what happens when the outer core solidifies
James K
CC BY-SA 3.0
2016-07-15T12:47:10.813Z
This is an enthusiastic question, and an excellent question - which just happens to be on the wrong site.  Surely it should just be flagged for moving to another site, rather than downvoted.
Fattie
CC BY-SA 3.0
2016-07-15T13:55:38.753Z
It's a little unclear what you're asking.   Planetary cores cool slowly over time but radioactive elements and settling and sometimes tidal forces can warm them up as well.  Still it's generally a gradual cooling over many billions of years.   It's worth noting that too hot a core wouldn't solidify and might not generate a magnetic field, but that's the primary answer to your question is that as the core cools the magnetic field (if the planet already has one) will go away.   Earth science is definitely the place to ask this and versions of this have already been asked and answered there.
userLTK
<planet>
What would happen when the earth consumed all its fuel?
Zaibis
# What would happen when the earth consumed all its fuel?

(Asked by: username_0 on 2016-07-15)

What would happen when a planet consumed all its fuel and its core extincts.
Would anything change on the planet?

Can this ever happen before its star did so?
If not, why?

**Tags:** planet

> **Comments:**
> **username_1**: This does not appear to be an astronomy question. It may be an Earth Science question. You should ask what happens when the outer core solidifies  
> **username_2**: This is an enthusiastic question, and an excellent question - which just happens to be on the wrong site.  Surely it should just be flagged for moving to another site, rather than downvoted.  
> **username_3**: It's a little unclear what you're asking.   Planetary cores cool slowly over time but radioactive elements and settling and sometimes tidal forces can warm them up as well.  Still it's generally a gradual cooling over many billions of years.   It's worth noting that too hot a core wouldn't solidify and might not generate a magnetic field, but that's the primary answer to your question is that as the core cools the magnetic field (if the planet already has one) will go away.   Earth science is definitely the place to ask this and versions of this have already been asked and answered there.  

---

## 2 answers

### Answer by username_4 on 2016-07-15. Score: 2

The core of the Earth is hot because of its own density and radioactive materials. Basically, the more you compress an object, the hotter it gets. It does not undergo fusion, since only massive (stellar) objects do so. A significant amount of radioactive decay happens in the core, which is mainly the source of its heat, but no fusion.

Similarly, stars are hot because of their own densities, but at a much larger scale, and sans the nuclear decay. This is especially evident when stars are born as non-fusing protostars; they are prevented from getting smaller because as they do so, their core will get hotter. Hotter objects actually expand, so protostars are in a state of equilibrium; if they get any smaller, the temperatures will increase and they will become larger again.

However, protostars slowly get smaller and smaller over time. This is because they lose energy as they emit light. Light emission is, in fact, a source of energy loss. So the star will continue to collapse slowly, and once the core reaches a certain density, it will begin fusing hydrogen. This produces radiation pressure, which counters the force of gravity.

Again, let me clear up something: nucleosynthesis in stars does not cause heat, but rather it prevents the star's gravitational collapse. It's the star's density that causes such high temperatures. If a star runs out of "fuel", that spells out the last phases of its life — as far as I know, the end result reaches extremely high temperatures (supernovae, white dwarfs, etc.)

> **Comments:**
> **username_1**: It is the radioactive decay that is the main source of heat in the core, not the compression.  
> **username_4**: @JamesKilfiger What source do you have for that?  
> **username_1**: http://www.scientificamerican.com/article/why-is-the-earths-core-so/ During the formation of the Earth, "Descent of the dense iron-rich material [...] would produce heating on the order of 2,000 kelvins" However the core is now closer to 6000K, the difference is radioactivity. Without radioactivity, the core could have solidified within a few hundred million years (William Thomson)  
> **username_2**: thanks for that great article, @JamesKilfiger.  I had no idea.  
> **username_4**: @JamesKilfiger Ah, all right. I corrected myself.  

---

### Answer by username_5 on 2016-07-15. Score: 1

A planet does not have any fuel. Only stars "burn" fuel either in their cores or concentric shells around their cores.

> **Comments:**
> **username_0**: And what is the energy consuming that makes our core beeing hot? I mean it can't be a source for endless heat that is cousing planets cores to be hot, can it?!  
> **username_5**: The heat in the core is from two or three sources, residual heat from the planet's formation, heat from the decay of radio active elements and for larger planets compression due to gravity. How long each of these lasts depends on the size and composition of the planet's core. Tidal interactions can also produce heat on planets with large natural satellites. As well as the amount of heat produced with respect to time these will all also effect what happens when the planet's core cools.  
> **username_0**: Then my question was wrong due to my presumptions. Should I rephrase ti into another question? Or is this the wrong site for it at all? Because google made me a hard time to get a hitn on it.  
> **username_2**: Hi @username_0.  Yes, in the case of the Earth, our Earth ***started out hot***, and is still a bit hot, and is slowly cooling.  The Earth is not burning any fuel in the middle.  Lava and so on is just hot because the Earth started hot, and is still pretty hot.  (*However* note that J.F. above gives sources explaining that radioactivity actually makes a big contribution in Earth's case.) Yes, you would be better on the excellent Earth Sciences site.  Come back here with questions about astronomy!  
> **username_0**: @JoeBlow: I thought thats what astronomy is about, sorry for bothering you here.  
> **username_2**: hi @username_0 I am just new on this particular site myself.  You did not bother anyone in the slightest, at all.  It is a **great question** as I said above.  I learned a huge amount here from James comment and others.  It is **totally normal** that questions get migrated to other sites, you know?  Almost every question I ask gets migrated to a different site (I can't be bothered choosing the right site - I just let someone else sort it out :)  ).  This was a GREAT question and thanks for it!  
> **username_6**: @JamesScreech Your explanation in the comments here is great, can you edit them into your answer? Comments are a little volatile.  

---

