What stops a star from fusing all its fuel at once?

  • #1

Main Question or Discussion Point

What's physically stopping all the fuel fusing at once or rather each stage of its fuel cycle fusing into the next quickly? i know theres a lot of it and gravity holds it together but what are these atoms doing while they wait to under go fusion exactly? They are after all under the same conditions as the rest, ignoring the outer stuff that may not be.
 

Answers and Replies

  • #2
Pretty sure Hydrostatic Equilibrium acts as a thermostat and that maintains a consistent fusion rate.

Can someone verify please.
 
  • #3
Astronuc
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Well, it really doesn't stop, but the rate of a particular reaction is affected by the production of heavier nuclei that do not readily fuse under the prevailing conditions.

Look at the conditions for pp or CNO cycles, and compare to alpha (He)-fusion. He (helium) accumulates in the core while displacing the hydrogen to the cooler outer regions, and this would cool the star (and slow the pp- or CNO-fusion cycles). If the core reaches sufficient temperature, alpha (He) fusion kicks in. The evolution depends on the mass of the star.

http://www.astronomy.ohio-state.edu/~ryden/ast162_4/notes15.html

Of course, if the core collapses to such a density that it forms the core of a neutron star, then fusion does not occur. And then there are black holes.
 
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  • #4
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The "throttle" on nuclear processes in stars is the weak force. When you fuse 4 protons into a helium nucleus, you need to convert two protons into neutrons. This reaction is trillions of times slower than anything else happening in the star. If this were not the case, stars would last for years or less rather than billions. (More accurately, they would form when they were smaller)
 
  • #5
Chronos
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The short answer is the pauli exclusion principle.
 
  • #6
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I read somewhere that a given volume of the Sun gives out less heat than the same volume of garden compost. My garden compost hasn't done any fusion recently so I'd say we are in with a chance.
 
  • #7
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The short answer is the pauli exclusion principle.
Why do you say that?
 
  • #8
So is fusion actually a slow process then or just within the sun?

I don't get what these unfused atoms are doing while they wait, surely if the conditions are right they should fuse as quickly as any other, i thought fusion itself happens fast so it seems strange to me why it all doesn't just happen within a matter of days or years.
 
  • #9
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So is fusion actually a slow process then or just within the sun?
As hot as the sun's core is, it's quite cold by nuclear standards. It's at 15 million kelvins. To have the weak force be strong enough that you could flip neutrons and protons back and forth requires a temperature more like a trillion kelvins.
 
  • #10
Nabeshin
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So is fusion actually a slow process then or just within the sun?

I don't get what these unfused atoms are doing while they wait, surely if the conditions are right they should fuse as quickly as any other, i thought fusion itself happens fast so it seems strange to me why it all doesn't just happen within a matter of days or years.
Without even getting into the mechanics of the p-p chain just think that in order to fuse elements, they have to collide. Protons are damn small, and the sun is damn big. Even though the temperature is sufficient to fuse nuclei when they meet, the big question is IF they meet. It's a surprisingly rare event.
 
  • #11
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Protons are damn small, and the sun is damn big. Even though the temperature is sufficient to fuse nuclei when they meet, the big question is IF they meet. It's a surprisingly rare event.
That doesn't seem right to me. The density in the sun's core is ~100 that of water, which means you have 6E25 protons per cubic centimeter - i.e. each proton occupies a space roughly 25 angstroms - or about 2.5 million proton diameters - on a side. That means a proton's mean free path for collisions with other protons is (2.5 million)^2 proton diameters, or about a centimeter.

Speed of a proton in a stellar core must be around 300,000 m/s, so each proton must have tens of millions of collisions per second with other protons.
 
  • #12
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i ALWAYS wondered this since the first moment i was told the sun is a fusion reactor. how is this reaction moderated? where is the fuel kept? where are the spent byproducts of the reaction? It cant be one big homogeneous fusion ball- you'd expect the reaction to explode the sun if that were the case. Rather, i'd guess fusion is only occurring at the surface--as you travel towards the center of the sun, gravity might be less and so there would be less force compressing the fuel->no fusion? If I could launch a supercondicting magnet into the sun to 'drill a hole' in its surface, could we peek inside?
 
  • #13
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  • #14
Astronuc
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i ALWAYS wondered this since the first moment i was told the sun is a fusion reactor. how is this reaction moderated? where is the fuel kept? where are the spent byproducts of the reaction? It cant be one big homogeneous fusion ball- you'd expect the reaction to explode the sun if that were the case. Rather, i'd guess fusion is only occurring at the surface--as you travel towards the center of the sun, gravity might be less and so there would be less force compressing the fuel->no fusion? If I could launch a supercondicting magnet into the sun to 'drill a hole' in its surface, could we peek inside?
pp-chain and CNO-cycle

Most of the energy production occurs in the core where the particle density and temperature are great enough to enourage the fusion reaction.

http://hyperphysics.phy-astr.gsu.edu/hbase/astro/procyc.html
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/solarpp.html

http://csep10.phys.utk.edu/astr162/lect/energy/ppchain.html
http://csep10.phys.utk.edu/astr162/lect/energy/cno.html
http://csep10.phys.utk.edu/astr162/lect/energy/cno-pp.html

The reaction rate of the pp-chain is relatively slow.

In some cases, the star's energy production does become explosive - e.g. nova or supernova.
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/snovcn.html
http://csep10.phys.utk.edu/astr162/lect/death/death.html
http://csep10.phys.utk.edu/astr162/lect/supernovae/supernovae.html

It would not be practical bore a hole into the sun or any star. Anything that humans make is tiny compared to the sun and most other stars. In fact the earth is puny compared to the sun and other stars.

As one goes deeper into a planet or star, the force of gravity decreases, but the pressure increases due to the overlying mass which is pulled toward the center of gravity.
 
  • #15
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Well, it really doesn't stop, but the rate of a particular reaction is affected by the production of heavier nuclei that do not readily fuse under the prevailing conditions.

Look at the conditions for pp or CNO cycles, and compare to alpha (He)-fusion. He (helium) accumulates in the core while displacing the hydrogen to the cooler outer regions, and this would cool the star (and slow the pp- or CNO-fusion cycles). If the core reaches sufficient temperature, alpha (He) fusion kicks in. The evolution depends on the mass of the star.

http://www.astronomy.ohio-state.edu/~ryden/ast162_4/notes15.html

Of course, if the core collapses to such a density that it forms the core of a neutron star, then fusion does not occur. And then there are black holes.
How does the process begin...what conditions are required...is there a minimum mass or density?
 
  • #16
Astronuc
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The start of the process

http://www.ipac.caltech.edu/Outreach/Edu/sform.html [Broken]
Many of the most interesting infrared objects are associated with star formation. Stars form from collapsing clouds of gas and dust. As the cloud collapses, its density and temperature increase. The temperature and density are highest at the center of the cloud, where a new star will eventually form. The object that is formed at the center of the collapsing cloud and which will become a star is called a protostar. Since a protostar is embedded in a cloud of gas and dust, it is difficult to detect in visible light. Any visible light that it does emit is absorbed by the material surrounding it. Only during the later stages, when a protostar is hot enough for its radiation to blow away most of the material surrounding it, can it be seen in visible light. Until then, a protostar can be detected only in the infrared. The light from the protostar is absorbed by the dust surrounding it, causing the dust to warm up and radiate in the infrared. Infrared studies of star forming regions will give us important information about how stars are born and thus on how our own Sun and Solar System were formed.

. . . .
http://cass.ucsd.edu/public/tutorial/StevI.html
The actual process of star formation remains shrouded in mystery because stars form in dense, cold molecular clouds whose dust obscures newly formed stars from our view. For reasons which are not fully understood, but which may have to do with collisions of molecular clouds, or shockwaves passing through molecular clouds as the clouds pass through spiral structure in galaxies, or magnetic-gravitational instabilities (or, perhaps all of the above) the dense core of a molecular cloud begins to condense under its self-gravity, fragmenting into stellar mass clouds which continue to condense forming protostars. As the cloud condenses, gravitational potential energy is released - half of this released gravitational energy goes into heating the cloud, half is radiated away as thermal radiation. Because gravity is stronger near the center of the cloud (remember Fg ~ 1/distance2) the center condenses more quickly, more energy is released in the center of the cloud, and the center becomes hotter than the outer regions. As a means of tracking the stellar life-cycle we follow its path on the Hertzsprung-Russell Diagram.
http://cosmology.berkeley.edu/Education/ISTATPage/HighSchool/stellarE/Stellar.html

http://www.yale.edu/ynhti/curriculum/units/2005/4/05.04.01.x.html

There is a certain critical mass necessary to form a star, or perhaps more accurately, a particular type of star, between brown dwarfs and stars like the sun.
http://www.universetoday.com/2008/12/03/brown-dwarfs-form-like-stars/
 
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  • #17
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fusion heats up the sun causing it to expand which reduces the pressure in the core which slows down the rate of fusion. its a simple feedback process (not, of course, that there arent many other complicating factors)

as the star ages its core becomes denser (because its made of denser elements) so the fusion rate increases.

why the fusion rate is so sensitive to the presure is something that I dont know.
 
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