Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Apr 21, 2009 #1
    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.
     
  2. jcsd
  3. Apr 21, 2009 #2
    Pretty sure Hydrostatic Equilibrium acts as a thermostat and that maintains a consistent fusion rate.

    Can someone verify please.
     
  4. Apr 21, 2009 #3

    Astronuc

    User Avatar
    Staff Emeritus
    Science Advisor

    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.
     
    Last edited: Apr 21, 2009
  5. Apr 22, 2009 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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)
     
  6. Apr 22, 2009 #5

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    The short answer is the pauli exclusion principle.
     
  7. Apr 23, 2009 #6
    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.
     
  8. Apr 23, 2009 #7

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Why do you say that?
     
  9. Apr 23, 2009 #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.
     
  10. Apr 23, 2009 #9

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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.
     
  11. Apr 23, 2009 #10

    Nabeshin

    User Avatar
    Science Advisor

    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.
     
  12. Apr 24, 2009 #11

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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.
     
  13. Apr 30, 2009 #12
    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?
     
  14. Apr 30, 2009 #13

    Ich

    User Avatar
    Science Advisor

    Last edited by a moderator: Apr 24, 2017
  15. Apr 30, 2009 #14

    Astronuc

    User Avatar
    Staff Emeritus
    Science Advisor

    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.
     
  16. Apr 30, 2009 #15
    How does the process begin...what conditions are required...is there a minimum mass or density?
     
  17. Apr 30, 2009 #16

    Astronuc

    User Avatar
    Staff Emeritus
    Science Advisor

    The start of the process

    http://www.ipac.caltech.edu/Outreach/Edu/sform.html [Broken]
    http://cass.ucsd.edu/public/tutorial/StevI.html
    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/
     
    Last edited by a moderator: May 4, 2017
  18. Jun 1, 2009 #17
    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.
     
    Last edited: Jun 1, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: What stops a star from fusing all its fuel at once?
Loading...