I’m sure that the answer to this question was given to me at some point during my school years, but I can’t remember. My 10 year old son asked me this question today and I didn’t have a convincing answer for him. His point was that the sun is basically a huge ball of gas and it appears that just the surface is burning. What keeps the entire fuel source from just exploding in one big bang? (he he he… Big Bang, see what I did there? :D)
I guess the second question is: Is the sun only burning fuel on it’s surface or is the core of the sun burning as well? I’m not sure if I’ve asked this question in the best way, but I’m tired and I promised him an answer in the morning.
The sun is in equilibrium. It’s balanced between gravitational collapse and the heat generated by fusion.
As the sun grows older it will fuse helium instead of hydrogen like it does now. This will make the sun “burn” hotter which will cause it to expand. More heat fighting the same gravitational mass means the radius will expand.
I understand it’s a fusion reaction and that requires a LOT of pressure. So I suspect there isn’t enough pressure on the surface of the sun. Which doesn’t necessarily mean it takes place in the core. But there are a lot smarter people than me here who might know more about it.
In fact, only the core is “burning”; or rather, fusing.
And as I understand it, it doesn’t go off all at once because it is in equilibrium, kept from collapsing further by the force released by its own fusion. So the core never gets squeezed down enough for that to happen.
I was just watching Extremely Loud & Incredibly Close, and in it the boy says that if the sun did go supernova it would take 8 minutes for people on Earth to realize it, what with the speed of light and all that.
Cool. However, the Sun is entirely the wrong type of star to go supernova - in several billion years it will become a red giant, then collapse peacefully down to a white dwarf as the fusion runs down and it spends billions more years just radiating away trapped heat.
Something most people don’t realize is that the gravitational force at the core isn’t sufficient on its own to sustain fusion. The only reason that the sun can support the fusion reaction is due to quantum tunneling.
Wikipedia: The rate of nuclear fusion depends strongly on density, so the fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level. Solar core - Wikipedia
Self-correcting: gotcha. But why does the rate at which hydrogen fuses to helium depend upon density?
More: About 3.6×10^38 protons (hydrogen nuclei) are converted into helium nuclei every second, releasing mass and energy at the mass-energy equivalence rate of 4.3 million tonnes per second, 380 yottawatts (3.8×1026 watts), equivalent to 9.1 ×1010 megatons of TNT per second. I’ve been exposed to astronomy since a young age, but I still shake my head at the scale of these effects. That’s a huge amount of conversion, yet the sun still has a sufficient stock of hydrogen atoms to last billions of years.
To fuse they have to hit each other just right. If they’re closer together you get more collisions and more fusions. (Assuming no change in temperature.)
There’s one curious detail that partly explains why suns do not fuse their cores quickly.
Beginning with Helium (He[sub]4[/sub]), the key products of fusion have weights that are multiples of four: C[sub]12[/sub], O[sub]16[/sub], Ne[sub]20[/sub], etc. Most of these isotopes are very stable. But Be[sub]8[/sub] is, for some reason, extremely unstable, with a half-life that is a tiny fraction of a femtosecond. (A femtosecond is one millionth of a nanosecond.) If Be[sub]8[/sub] were stabler, the Sun’s helium would fuse quickly (much more quickly than the 4 H --> He) and a solar system would not enjoy the luxury of billions of years of leisurely slow hydrogen fusion.
In the “Goldilocks Universe” concept, the instability of Beryllium-8 (Be[sub]8[/sub]) may be one of the key facts that leads to complexity and intelligent life!
“The power production density of the core overall is similar to the metabolic production density of a reptile. The peak power production in the Sun’s center, per volume, has been compared to the volumetric heat generated in an active compost heap. The tremendous power output of the Sun is due not to its high power per volume, but rather to its gigantic size.”
Another amusing factoid illustrating how big these things are:
If you turn off a light bulb, it pretty much turns completely off all at once.
If you were to turn of the sun, so that the entire surface of the sun stopped radiating light all at the same time, you’d still see a circle of darkness growng and spreading from the center of the sun’s surface (from your viewing angle) towards the limbs, taking more than two seconds.
That’s because the sun is more than four light-seconds in diameter. The point on the sun’s surface closest to you is more than two light-seconds closer to you than the farthest point on the sun’s surface that you can see: the limbs (edges). That means that the darkness from the center point of the sun’s surface gets to you two seconds sooner than the darkness from the edges.