Why Does The Sun Shine?
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drmellow- Greg Cohoon
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polished every day to shine that brightly
presumeably by someone using a fire proof
glove and cloth.
+Seraphim
A gigantic nuclear furnace
where hydrogen is built into helium
at a temperature of millions of degrees
If that isn't the answer, then I think the blame should go to the flying monkeys.
Q.
In any case, though it takes only a few minutes for a photon to go from the surface of the sun to the earth, it can take millions of years for a photon to go from the center of the sun, the location of fusion, to the surface (and even then, it's not really the "same" photon -- it gets reabsorbed and re-emitted countless times over in the very very dense plasma that is the sun.
But neutrinos, which are also produced in nuclear fusion, are barely affected by regular matter. I believe tau neutrinos may have some mass, but electron neutrinos, which are supposed to be produced in the H -> He fusion, are thought to be massless. So gravity is a non-issue. They're neutral, so E-M is a non-issue. They're affected by weak nuclear forces, and are supposedly pouring through us all the time -- and supposed to pass through the sun without being greatly absorbed.
There have been a few large-scale experiments to detect these neutrinos, and in all cases there have been =too=few=neutrinos=. Is the fusion in the center of the sun falling off? Are our nuclear theories wrong? (I don't think we've managed doing any hydrogen fusion on the earth) Do electron neutrinos have mass? Who knows...
it's very, very odd.
Or, the neutrinos may all be ganging together in the L3 Lagrange Point on the opposite side of the Sun. They're organizing the baryons and leptons in the Solar wind to sneak around the Sun when we're not looking and blast the Earth into pi-mesons. The Sun shines to hid their little plot from our view.
unless I'm behind on this?--it's been a while since I've read up.
peace,
--me
...no, I don't have a favorite pet theory as to where the missing neutrinos are going, unless AAA has some good deals published about neutrino vacation spots.
Seems to me that the article was talking about the character of a star changing as more and more heavy elements were formed. Sounds similar to what you mentioned. It's apparently not happening in the Sun, but may be happening elsewhere. I think that part of the speculation was about where the heavy elements came from in the first place.
Now I'll have to go look for articles on triple-&alpha fusion, 'cause I've never heard of it.
And, what's the CNO mess you're referring to?
I think I'm with you on the AAA theory.
Okay, here's the deal:
When a star lives the first part of its life, it's burning hydrogen into helium. If you care to look up the process, it's called the proton-proton chain, though I think it basically works like this:
1H + 1H -> 2H + e+ + νe
2H + 1H -> 3He
3He + 3He -> 4He + 2 1H
...You get a neutrino and a positron when you force a proton to turn into a neutron, which gives rise to the whole question of measuring neutrinos to see if our guesses on this proton-proton fusion mechanism is right--have to conserve various things in particle physics (e.g. charge: the positron carries away the +1; lepton number: the neutrino is +1 and the positron, an anti-electron, is -1, so that's still 0; and this has to happen at ridiculously high starting energies 'cause a neutron weighs more than a proton).
There's a delicate balance that goes on in this: gravity is trying to squnch the hydrogen closer together, but the energy being emitted by the fusion is enough to hold it apart. When the hydrogen starts running low, the energy output goes down 'cause there's all that 4He floating around in the way, so the equilibrium is disrupted, and the pressure at the center increases. And mind you, helium is very light, but it's heavier than hydrogen so it sinks to the center. When the pressure increases past a certain point, the helium ignites and fuses to carbon by the triple-alpha process (I've been using element names, but a 4-helium nuclide is just an alpha particle):
4He + 4He -> 8Be
8Be + 4He -> </sup>12</sup>C
...The beryllium fusing to carbon in particular generates enough energy to keep the star running for longer, but the energy output is different so the equilibrium is different and the star has changed shape and color.
This all can go on for a while, though triple-α doesn't produce quite as much energy as PP, so the star's life in this cycle is shorter, and you've got a ball of carbon building up in the middle. Eventually, the helium starts running low and the pressure increases again and a really massive star can fuse carbon for a while--the energy this takes to start this is pretty high, so a star like our sun will probably never burn carbon just because its own gravity isn't enough to build up the pressure to provide the energy to force that to happen. But even bigger stars can burn things past carbon--carbon to neon, neon to oxygen, oxygen to silicon, silicon to iron, and then you've hit the magic point where fusing nuclei no longer releases more energy than it takes. Each element releases less energy than the previous one and each one runs out quicker--I vaguely remember a typical sustainability of the iron generating stage of stellar fusion being on the order of minutes. After that, there's no more energy to be had and the outer layers collapse and you get a type IIa supernova and a black hole in the middle.
So that's the deal with heavy elements being produced up to iron in a very massive stellar core (I think right now we're talking about eight solar mass stars and bigger). You get layers of this stuff going on--a star that's busy burning carbon into neon has that lump of neon accumulating in the middle, but it has a layer of carbon outside it, a layer of helium fusing into carbon outside that, and a layer of hydrogen fusing into helium outside that--the older processes don't stop, they're just not enough to sustain equilibrium against the star's gravity anymore.
CNO, well, that's another ball of wax, which will be continued in the next post since I hit the character limit....
Basically, there's some small amount of heavy elements including carbon, nitrogen, and oxygen in the sun (it's not a first-generation star, so it didn't start with all hydrogen).
12C + 1H -> 13N
13N -> 13C + e- + `νe
13C + 1H -> 14N
14N + 1H -> 15O
15O -> 15N + e- + `νe
15N + 1H -> 12C + 4He
...so we've got a catalyst for turning hydrogen into helium here, 'cause you start with a bunch of carbon and input some protons and you end up with a bunch of carbon and some alpha particles.
I don't know what the timeline for this process is. I've read the first beta decay has a halflife of 10min, but I don't know about the second.
So there we are. Solar fusion and its generic stellar cousin. Yep, the character of a star does change when you get it to start fusing heavier elements. But no, the sun's not doing that right now--CNO notwithstanding, but there can't be that much carbon in the sun... or can there?
So just add this to the 1010 other bits of trivia.
Peace,
--me
...Triple-α... triple-A... we can explain those neutrinos away...
I had forgotten all about the idea of catalysis in nuclear reactions. I see how that would help things along. My old physics book shows the same CNO cycle (they called it the carbon cycle) as you cite, but they shows the beta decays as:
13N -> 13C + β+ + ν
and
15O -> 15N + β+ + ν
With this reaction, there's no `ν + ν -> γ annihilation to explain away the missing neutrinos (I'm just guessing that that's where you were headed) (Do neutrinos even do that?).
My old book doesn't suggest half lives for the β decays, either, but when you have a lifetime and working mass of cosmic proportions, what's the difference between a few minutes and a few days?
I also hadn't considered the Sun as stratified, but I guess that only makes sense: the bigger nuclei would tend to sink. But wouldn't that be overshadowed by currents and convections? I've never really studied this, but it seems to me that we see rather a lot of evidence of mixing (sunspots?) and other turbulence. The heat gradient has to stir something up. Though, if the heavier element reactions are less energetic, perhaps they stir less, too?
I don't know if I've learned any real physics here in the last few days, but I sure have learned how to type cool stuff in HTML.
I don't think ν + `ν is a likely reaction to occur, though--neutrinos barely interact with anything as it is.
You're right, though--I don't know if I was just typing faster than I was thinking or what, but those beta decays should have been e+ and not e- : nitrogen is one higher atomic number than carbon, so to decay from 137N to 136C something has to carry away a positive charge, so that'd be your positron. And to conserve lepton number, the positron, which is an antilepton (-1), has to be paired with a lepton (+1), which'd be ν.
I also hadn't considered the Sun as stratified, but I guess that only makes sense: the bigger nuclei would tend to sink. But wouldn't that be overshadowed by currents and convections? I've never really studied this, but it seems to me that we see rather a lot of evidence of mixing (sunspots?) and other turbulence.
Well, the actual fusion going on inside the sun is far, far below the convective zones and the surface turbulence. Basically, the model I was taught in school had fusion going on at the core of a star, then a radiative zone wherein the energetic photons just bounced around a lot on their way up, then a convective zone which is cool enough for the atoms to not all be completely ionized which makes it more or less opaque and forces the heat to be carried by convection, kind of like water boiling up from the bottom of a pot. After the convective zone comes the photosphere, which is where the opacity drops so the photons can escape--this is the much, much cooler 6000° area which gives the sun its apparent blackbody temperature. After that is all 'atmosphere'--the chromosphere which gives the sun its characteristic absorption lines, and then the corona.
So, basically, the fluid flow we see at the surface of the sun doesn't have to happen that deep inside. The sun's core and radiative zones aren't opaque enough to require that as a heat transfer mechanism. As a result, in much, much larger stars that are much farther along in their life cycle, we can have layers of progressively heavier elements forming.
My old book doesn't suggest half lives for the β decays, either, but when you have a lifetime and working mass of cosmic proportions, what's the difference between a few minutes and a few days?
Three orders of magnitude, my good fellow. And that makes a great deal of difference to me, 'cause I'm rather particular about the temperature outside being within a range of a few tens of degrees.
Peace,
--me
By the way, have you seen the stories on sonofusion?
I posted some silly comments about it in my journal. I looked at it from the point of view of acoustics, which I know a bit better than physics and heliology.
Upon re-reading my post, I think I'm barking up the wrong tree with generating bubbles through cavitation by shock wave. After all, in an adiabatic expansion/compression, the bubbles will just condense. But focusing a compression wave at bubbles that have already been generated makes at least a little sense.
SON-O-FUSION.
BRIDE-O-FUSION.
AND NOW IN SENSURROUND 3-D3-D!
...</old-horror-flick-mode>
...Yeah, I'd seen the assertion about sonofusion, if that's what they're calling it now. I don't know the physics of it well enough, but basically I'm a little skeptical when something seems too good to be true, especially when we've seen this snake oil bottle with a slightly different label before. ...'Cold fusion' seemed like it should work too--surely if you electrically impart enough kinetic energy you can get deuterons to fuse, right? Time'll tell, but I have doubts about it.
...Wish I knew more about shockwaves, but I don't (though I'd love a primer on the subject)... seems to me it would be difficult to use that as a way to deliver energy to any given nuclide though.
First, New Scientist is a tabloid, they'll publish anything, the more sensational, the better.
...heh. They're not exactly journal quality stuff, but you haven't seen tabloid 'til you've read the crap they print in Правда (or in English)...
Peace,
--me
My knowledge of acoustics is limited with respect to shock waves. I used to work in acoustics, but it was industrial, environmental, and architectural work, not stuff at the limits. The concept of the shock wave is simple, but very funny things happen with and around them.
Sound is a longitudinal compression wave, where you have moving regions of compression and rarefaction. A shock wave is one where the rarefaction phase reaches zero pressure. You can easily see a shock wave in the wake produced by a boat. A slowly moving boat produces waves that move ahead of it. As it starts to move faster, the angle of the bow waves becomes sharper. At the speed at which the waves stop receding from the bow, the shock wave forms.
My one technical experience with shock waves was when I was recording sound in a very wide, flat, area. I was measuring noise from hammer mills when the amplifier in my tape recorder went into saturation. I checked all of the electrical connections, and they were OK. This happened two or three times that day. I found out later that there was an explosives research facility not too far away. What I had likely observed was a single pulse shock wave, traveling parallel with the ground. I couldn't hear it, since it was a single pulse and there was nothing much to echo from. No reverberations. But the microphone picked it up quite well. Very weird. Back in the lab, I couldn't get much from the tape. At least I was able to edit it from the other data I was taking.
Правда, Truth, Veritas ... It's all what you make of it, if you're the media.
Мир
'Course, I thought I remembered hearing that they decided that νe had mass as well. Or at least, some wit/2 thought it was funny to quip they were Catholic for a while... he may have been right, I never did decide :)
Are they still saying massless? did I remember wrong?
peace,
--me
I'm not finding anything more specific than 'not measurable' or 'must be smaller than...' for νe, though I have a hard time believing that one neutrino is a massive particle and another flavor is massless, especially with talk about oscillations.
ah well. I'll read up more later, stuff to do today.
peace,
--me