Why doesn't the sun just explode all at once?

Is it because the heat necessary to keep the reaction going is sustained by gravity and thus holds it together. Is suppose supernova is when that’s no longer so?

It is hot enough only at the center of the sun. I guess the reaction rate is quite small, so only a little at a time (relatively speaking) fuses. First hydrogen fuses to deuterium. At first the level of D will be low (and some of it may fuse to tritium, but that is unstable) but eventually pairs of D will start to fuse to helium. This will continue for quite a while until there is only He. At this point, things get a bit murky, but essentially He starts to fuse to produce carbon. There are some problems with that since there is no stable nucleus of mass 8, but somehow it happens. I have read about it, but forgotten the details. The sun will not supernova. Stars supernova only when they are so massive (about 2 1/2 times the mass of the sun if I recall correctly) that at some point even neutron pressure cannot sustain them and they begin to collapse. The collapse takes about 1 second. This generates enormous heat (the release of all that graviational potential energy) and two things are thought to happen. First the core collapses into a black hole (and what is going on there is anybody’s guess) and second the outer layers get so hot that endothermic (heat absorbing) reactions create by fusion all the elements more massive than iron (which is at the bottom of fusion potential well) and they are blown away. Thus supernovas are the sources of all the elements in the universe heavier than iron! Any planet that formed in the early days of the universe would have no transferric elements at all. No gold, no lead, no uranium,… And the solar system was formed from the ashes of earlier supernovas.

The future history of the sun is something like this. Over the next five billion years it will grow gradually hotter and eventually larger (life as we know will be gone in about one billion years) to about the size of the earth’s orbit. It will undergo a number of ordinary nova explosions as it blows the last of the H from its surface. At that point it will be a red giant, hot on the inside and cool on the surface. I have forgotten what happens after that, but I think it ends up as a red dwarf.

Actually, it can, but it didn’t know it could, but now you’ve spilled the beans and well, we’d better start writing our wills. I hope you’re happy.

“It’s not just the heat, it’s the density.”

The Sun, while quite massive is not all that dense. The combination of heat and density necessary for fusion occurs only in the core. The heat generated keeps the rest so spread out, it doesn’t all go at once.

Actually, neutron pressure and black holes don’t have a lot to do with supernovas. The supernova phase begins when the heat is not sufficient (not enough good fuel) to maintain the low density. It collapses etc. Then you get a neutron star. Not very many supernova produce black holes.

Here is one slide of a series about the Sun.

Details of nucleosynthesis are murky indeed. We would like to think that the hydrogen to helium reaction was a good starting point, but IIRC from my stellar dynamics class, in second generation and beyond stars it is the CNO process that dominates fusion of Helium to Hydrogen. In any case, cross sections of both nuclear reactions are sufficiently small to ensure that even at the ridiculous pressures and temperatures experienced at a stellar core, the rate of reaction occurs on the order of millions to billions of years. This is perhaps another example of the anthropomorphic (apologies on the spelling, I’m sure) principle at work though (but let’s not get into that).


Just out of curiosity (not malice), do you have a site for that Hari? When you say “life as we know it,” do you mean no humans, no mammals, or no carbon-based life utilizing DNA as a form of reproduction? I mean, life originated ~1 billion years after the earth was formed, I would think it would be tenacious enough to hold on for a lot longer than that.

I see a few misconceptions in here.

First off, to the OP, the sun is held together by gravity. Gravity wants to shrink the sun into a black hole. However, as you start to compress stuff it heats up. Eventually there was enough pressure and heat at the sun’s core that a fusion reaction started. Like any good hydrogen bomb the fusion reaction wants to blow the sun apart. So, gravity pulling in and fusion reaction pushing out. The two settle down into a stable state where the two forces cancel each other out and there you go…a star is born.

FYI: The sun fuses around 700,000,000 tons of hydrogen per second. That sounds like a lot to me but maybe in the astronomical scheme of things that isn’t much…I don’t know.

As to the end of the sun’s life we have awhile to go yet. Probably around 3-4 billion years. Right now the sun is about 75% hydrogen and 25% helium by mass. Helium is harder to fuse than hydrogen is so it’s mostly just sitting there collecting in the core. As the sun runs out of hydrogen the fusion reaction will slow. Since the fusion reaction is what pushes outward the sun will start to shrink as the hydrogen fusing process slows. This will eventually lead to higher densities and temperatures in the core of the sun. Eventually the pressure and temperature will rise to the point where helium starts fusing into carbon. Fusing helium produces more energy than fusin hydrogen. This new, added outward pressure will cause the sun to expand…big time. The sun will grow sufficiently to envelope the earth (the earth will actually be orbiting inside the sun. At this point I think it is reasonable to assume all life on earth will perish. The sun’s surface will be far cooler than it is today but still…I think the earth’s atmosphere will be stripped off aside from starting to cook at several hundred degrees. Chances are life will be gone before that anyway as I believe the switch from hydrogen to helium fusing will cause an explosion that will expel a significant amount of stellar material that will likely waste life on earth. As I said, I believe this process will kick in around 3-4 billion years from now.

This process will continue. Helium will be used up, the core will collapse, heat up and start fusing carbon into ??? (I’m not sure). I do know, however, that this process stops with iron. Iron takes more energy to fuse than it releases so it won’t occur. All along this process the sun will continue to expand larger and larger. Eventually it’ll get so big that the outer layers will be able to float away from the sun forming a planetary nebula. At the end of the sun’s life our sun will form into a white dwarf which will be about the size of the earth. At this point the sun is done blowing off its outer layers and has settled down into its final state where it will slowly cool off. Eventually it will cool completely and be a black dwarf which, I think, is basically a huge hunk of cold iron. The black dwarf state is some 10 billion years away so the sun has a ways to go yet.

Of course, the final state of a sun is dependent on its initial mass. I just described what will happen to our sun. For a sun 1.5-3 times the mass of our sun the star will go supernova which, as you probably know, is a spectacular explosion blowing off the outer layers of the sun. For a neutron star there is more stuff left after the explosion than can be held away by fusion. The star continues to collapse till electrons are crammed into the nucleus of atoms (forming neutrons). At this point I think something called the Pauli Exclusion principle stops the collapse. What is left behind is called a neutron star which is a star roughly 10 miles in diameter and insanely dense (I’ve heard a teaspoon of neutron star material would weigh as much as Mt. Everest here on earth).

If the star is more than 3x the mass of our sun there is too much material leftover for even the Pauli Exclusion principle to stop the collpase and the star passes through the neutron star stage and turns into a black hole.

I will say that I have recently heard of an object called a Quark Star (or Strange Star) which might be a possible stable stage between a neutron star and a black hole. In this star atoms themselves have been ripped apart into a quark soup (quarks being the fundamental building blocks of matter). I think a quark star is hard to distinguish from a neutron star being only slightly smaller (around 8 miles or so in diameter). Still, some scientists think they have good theoretical evidence for such a creature so who knows?

Excellent summary, Whack-a-Mole! I just want to add that the rate you mention, 700,0000,000 tons per second, is about 3x10^-19 of the Sun’s mass. This means it will take about 3x10^17 seconds to use up the 10% of available Hydrogen in the Sun, or about 10 billion years. This is the expected lifetime of the Sun before the red giant phase begins. (The sun has already been around for about 5 billion years, so it’s half-dead.)

One thing to add to Whack-a-Mole’s answer. Hari Seldon mentions it, but it may not be clear to everyone.

Hydrogen is one proton and one electron.
Helium is two protons, two neutrons, and two electrons.

Sticking two hydrogens does not make a Helium. The neutrons are missing.

First you have to stick two protons together to get a Proton, Neutron, and anti-electron. The anti electron will quickly find a regular electron and go poof. This is the major but not sole preducer of neutrons.

A proton, nuetron, and electron makes the above mentioned deuteron. Deuteron + Deuteron makes Helium. This occurs very rapidly at the temp and pressure in the sun. Usually it is a Deuteron + Hydrogen to make light Helium (H3), and then later 2 H3’s join to make a regular Hydrogen (H4) and 2 spare protons.

It is the process of making the required neutrons that keeps the whole process running reasonable slowly. (At least that is how I understand it)

Oops, typo spotted.

2 H3’s join to make a regular Hydrogen (H4) and 2 spare protons.

Should read:
2 H3’s join to make a regular Helium (H4) and 2 spare protons.

Great answers guys thanks for the responses.!

I have a page about why massive stars go supernova [a href=“http://www.badastronomy.com/bitesize/sn87a_explosion.html”] on my website. That should clear things up a bit.

D’oh! Oh well, cut and paste,or click here.

I seem to recall that the probability that the collision of two protrons will result in the creation of a deuteron is vanishingly small. In almost all cases, they simply rebound from each other. A weird quantum effect has to happen to overcome the electrostatic repulsion. If this were not so, the sun would have burned out in a flash long ago.

The sun doesn’t explode all at once because it runs on fusion, not fission. You don’t get chain reactions in fusion.