Reply
 
Thread Tools Display Modes
  #1  
Old 08-02-2005, 07:54 PM
Whack-a-Mole's Avatar
Whack-a-Mole is offline
Guest
 
Join Date: Apr 2000
Location: Chicago, IL USA
Posts: 20,896

What keeps a star from fusing all its hydrogen at once?


Title pretty much says it all.

Clearly what I am asking does not happen but why not? I would think once everything in the core was smushed together enough and hot enough every hydrogen atom would jump on its nearest neighbor and fuse and it would all be over in a matter of moments.
  #2  
Old 08-02-2005, 08:07 PM
Bryan Ekers's Avatar
Bryan Ekers is offline
Guest
 
Join Date: Nov 2000
Location: Montreal, QC
Posts: 58,850
Well, simplified, when Atom A and Atom B are made to fuse, they produce so much energy that nearby Atoms C, D, E and F get pushed away, to fuse another day.
  #3  
Old 08-02-2005, 08:07 PM
Cerowyn is offline
Guest
 
Join Date: Oct 2000
Location: In a hole
Posts: 3,437
Stellar fusion requires huge pressures, which only occurs when enough hydrogen accretes in one place that the gravitational force creates sufficient compression. As soon as the fusion begins, it ejects energetic particles which produce pressure on the surrounding solar material. Stable fusion results when the gravitational pressure inwards is balanced by the fusion force outwards. This prevents all of the fuel from being used at once, and yet sustains sufficient pressure to keep the hydrogen (or later, other fuels) fusing.
  #4  
Old 08-02-2005, 08:08 PM
Cerowyn is offline
Guest
 
Join Date: Oct 2000
Location: In a hole
Posts: 3,437
Ugh.

"Energetic particles" should be "energy."
  #5  
Old 08-02-2005, 08:14 PM
Bryan Ekers's Avatar
Bryan Ekers is offline
Guest
 
Join Date: Nov 2000
Location: Montreal, QC
Posts: 58,850
My version is cuter.
  #6  
Old 08-02-2005, 08:19 PM
Whack-a-Mole's Avatar
Whack-a-Mole is offline
Guest
 
Join Date: Apr 2000
Location: Chicago, IL USA
Posts: 20,896
Why do H-bombs manage to go boom then? Wouldn't the first few fusing atoms push the other atoms away and make it fizzle? Obviously it doesn't but by the explanations above that seems like what should happen. (not trying to be a pain...just curious)
  #7  
Old 08-02-2005, 08:30 PM
Exapno Mapcase is offline
Charter Member
 
Join Date: Mar 2002
Location: NY but not NYC
Posts: 31,250
Oversimplified, an a-bomb is used to create the instantaneous pressure needed to the make the hydrogen fuse. With all the hydrogen forced together into that tiny space it all fuses at once.

Suns are so huge that they can't go off all at once. As Cerowyn said, the outward pressures balance off the gravity. It's only when this outward pressure is turned off, in an aged sun that has fused everything that can be fused, that the sudden gravitational collapse triggers a supernova. And we don't want that happening next to us.
  #8  
Old 08-02-2005, 08:36 PM
Lumpy's Avatar
Lumpy is offline
Charter Member
 
Join Date: Aug 1999
Location: Minneapolis, Minnesota US
Posts: 16,382
Even at the tremendous pressures and temperatures found in the core of a star, the actual rate of fusion is surprisingly low; slow enough that a star lasts for millions or billions of years. There just isn't the potential for a runaway <kablooey> reaction. Mainstream stars are by definition in equilibrium between collapsing and blowing apart.

Hydrogen bombs work because they use special heavy isotopes of hydrogen that fuse MUCH more easily than your everyday protium hydrogen. And their design is that the fission triggers not only heat the heavy hydrogen isotopes but also strongly compress them.
  #9  
Old 08-02-2005, 08:58 PM
mks57 is offline
Guest
 
Join Date: Dec 2003
Location: Seabrook, Maryland
Posts: 3,045
A nice description of how a hydrogen bomb works can be found here.
  #10  
Old 08-02-2005, 10:45 PM
Cerowyn is offline
Guest
 
Join Date: Oct 2000
Location: In a hole
Posts: 3,437
Quote:
Originally Posted by Lumpy
Even at the tremendous pressures and temperatures found in the core of a star, the actual rate of fusion is surprisingly low; slow enough that a star lasts for millions or billions of years.
"Slow" being a relative term; our sun is still consuming 700,000,000 tons of hydrogen per second.
  #11  
Old 08-03-2005, 01:39 AM
Soylent Gene is offline
Guest
 
Join Date: Apr 2005
Location: Seattle, WA
Posts: 384
Quote:
Originally Posted by Exapno Mapcase
Oversimplified, an a-bomb is used to create the instantaneous pressure needed to the make the hydrogen fuse. With all the hydrogen forced together into that tiny space it all fuses at once.:
isn't that an H-bomb?
  #12  
Old 08-03-2005, 02:26 AM
Larry Mudd is offline
Guest
 
Join Date: Aug 2001
Location: Ass end of Alberta
Posts: 17,897
An H-bomb actually includes an A-bomb as a first-stage of a three-stage process.

If I remember right, the fission of the uranium core of a triggering "A-bomb" creates the tremendous heat necessary for the fusion of hydrogen fuel. The hydrogen fusion in turn triggers the fission of an outer uranium jacket.

fission --> Fusion --> FISSION! (Bim bam boom!)
  #13  
Old 08-03-2005, 02:27 AM
matt is offline
Member
 
Join Date: Aug 1999
Location: A Brit in 'Stralia
Posts: 1,964
This is from memory, so the details may well be incorrect, but for what it's worth...

Hydrogen cannot fuse to itself to make helium 2 - the binding energy isn't enough to hold helium 2 together. If two hydrogen nuclei fuse, the most likely thing to happen is that they fission apart again.

Occasionally however, when the two hydrogen nuclei fuse, one of them will turn into a neutron by kicking out a positron and a neutrino. This yields a deuterium nucleus.

Then a whole bunch of reactions can occur - deuterium can fuse with itself, or with hydrogen, to make tritium or helium 3, which can themselves fuse with other nuclei. But the rate-limiting reaction is hydrogen fusion to form deuterium, which is comparitively slow.

H-bombs don't actually fuse hydrogen - they fuse deuterium, which can indeed all go off at once, more or less. The deuterium is contained in the compound lithium deuteride, which is rather more practical than having liquid deuterium contained inside the bomb. IIRC, the lithium can also participate in the fusion reaction because some of it is converted into tritium by the neutron flux.
  #14  
Old 08-03-2005, 02:39 AM
matt is offline
Member
 
Join Date: Aug 1999
Location: A Brit in 'Stralia
Posts: 1,964
Okay, found a cite:

http://www.shef.ac.uk/physics/people...13_fusion.html

"The reaction rate of the PP chain is set by the rate of the slowest step, which is the fusion of two protons to produce deuterium. This is because it is necessary for one of the protons to undergo an inverse decay:

p --> n + e+ + neutrino

This reaction occurs via the weak nuclear force and the average proton in the Sun will undergo such a reaction approximately once in the lifetime of the Sun, i.e. once every 1010 years. The subsequent reactions occur much more quickly, with the second step of the PP chain taking approximately 6 seconds and the third step approximately 106 years in the Sun."
  #15  
Old 08-03-2005, 09:01 AM
Bytegeist is offline
Guest
 
Join Date: Jul 2003
Location: Maryland, US
Posts: 2,377
Quote:
Originally Posted by Cerowyn
"Slow" being a relative term; our sun is still consuming 700,000,000 tons of hydrogen per second.
This is the point I was going to raise. An H-bomb uses only about 10 kg of mass, so it's not surprising it gets consumed "all at once". The same thing is happening in the Sun, only the Sun has a lot more hydrogen to go through.
  #16  
Old 08-03-2005, 09:37 AM
chrisk is offline
Guest
 
Join Date: Nov 2003
Location: Southern ontario
Posts: 6,588
As far as the H-bomb comparison, does it really 'fuse out' or is it more accurate to say that an H-bomb fuses however much it fuses in one relatively small instant and 'that's all it get'?

To elaborate... in an H-bomb, as in the sun, the energy of fusion will tend to create a spectacular increase of pressure that would tend to push unfused hydrogen nuclei away from each other, preventing them from fusing.

In an H-bomb, as in a sun, there is also a counterforce which is attempting to push the hydrogen nuclei close to each other, maintaining critical mass and promoting fusion. In a sun, that counterforce is gravity. In an H-bomb, it is a shock wave from nuclear fission explosions around the edge of the edge of the bomb.

The difference, as I see it, is that gravity is a constant force, while the fission explosions were one-shots. Gravity keeps exerting pressure on the core of a sun, often resulting in a relatively even rate of fusion, as energy produced from fusion reactions spreads up through the body of the star and radiates out into space.

With an A-bomb, you get the shock wave pushing your fusionable nuclei close to each other, and some proportion of them fuse, eventually counteracting that inward shockwave (or the shock wave eventually pushes through itself,) resulting in a big ka-boom. I'd imagine that there was probably a lot of fusionable material left from the A-bomb core, but enough of it fused to get the job done. Is this correct??
  #17  
Old 08-04-2005, 04:20 AM
matt is offline
Member
 
Join Date: Aug 1999
Location: A Brit in 'Stralia
Posts: 1,964
Quote:
Originally Posted by chrisk
As far as the H-bomb comparison, does it really 'fuse out' or is it more accurate to say that an H-bomb fuses however much it fuses in one relatively small instant and 'that's all it get'?
Fusion is still a statistical process - it depends on the extremely high temperature deuterium nuclei smacking into each other, which takes time. In an H-bomb, the deuterium is temporarily explosively confined at enormous temperature (inertially confined) using the energy from an A-bomb trigger. How many fusions occur before you lose the confinement is a matter of design. I've read that typically you'd fuse 1-10% of the material, depending how good the design is.

The shockwave from the atom bomb trigger does not itself result in fusion - surrounding an A-bomb with lithium deuteride gets you almost zero fusion (the "supernova" design.) Instead some complicated (and classified) arrangement of material results in energy from the fission trigger compressing and confining the lithium deuteride.

It's quite hard to get your head around, but everything has to happen in a miniscule amount of time. Firstly, blocks of conventional explosive are detonated, converting them into "blocks" of high pressure gas. These kablooie outwards and inwards a few inches, compressing the fission pit. At that instant, the fission pit turns into a ball of plasma emitting gamma and x-rays, neutrons and fission fragments. These overtake the still-expanding conventional explosives and hit the secondary, compressing and containing the lithium deuteride before the conventional explosive shockwave has got that far.
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 04:01 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@straightdope.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Copyright 2018 STM Reader, LLC.

 
Copyright © 2017