The Sun's Nuclear Core

[kirkster501]

I was watching a program about star eveolution last night. I wondered how big or what radii would the fusion core of the sun be? Tens of thousands of km? A few hundred? Apparently 450 million tons of hydrogen are being converted to helium every second. So at the pressures involved - tens of millions of atmospheres that can't be much volume? And how does "new" hydrogen get down to the "fusion zone" ? And why doesn't the whole shebang undergo fusion in a few years? Mass and gravity perhaps? What controls the rate?

Just musing. Ignore my inane ramblings by all means!

Steve

[Zakalwe]

About 1/4 of the Solar radius is made up of the core. You could work out the volume using this formula V = ⁴⁄₃πr³

I'm not sure if hydrogen gets into the core..whats in there is being fusioned into helium There's just that there's an awful lot of it in there.

These links will tell you more:

http://en.wikipedia.org/wiki/Solar_core

http://solarscience.msfc.nasa.gov/interior.shtml

[Earl]

if only i could burn calories like the sun burns hydrogen !!!

[JulianO]

The problem is that the initial steps happen very very slowly. That is two hydrogen nuclei coming close enough to fuse.

Both are positively charged, so naturally repel each other, they have to come close enough for the weak interaction to step in and convert one proton into a neutron (+ some lose change).

This is a very hard thing to do, even at the temperatures and pressures in the sun. In fact if you treat it classically, the sun is not hot enough to fuse in this way, the electrical repulsion is too strong to allow the protons to approach close enough. However quantum tunneling lets it happen now and again by chance. Perhaps once in a few billion years for a given proton. Once you have H->D step complete, the rest of the reactions to make He happen very quickly in comparison.

So this is why stars last so long. There is a certain amount of convection which cycles new hydrogen into the core, but it's limited. This is why they tend to go into the red giant phase, as they run out of H in the core, and start to burn it in a shell around the inert He core.

[Knight of Clear Skies]

And why doesn't the whole shebang undergo fusion in a few years? Mass and gravity perhaps? What controls the rate?

Stars are conveniently equipped with a natural thermostat. Gravity compresses the core, but the energy produced (radiation pressure) acts against this, regulating the rate of reaction.

[Qualia]

I've also read that solar fusion is quite a slow process. It's something like 1 proton in every 100,000,000 moves fast enough to even be able to fuse with another proton and of this only 1 collision in every 10²² actually results in fusion. Your average solar proton will spend about 14,000 million years bouncing around the core before it meets a partner, but due to the enormous size of the sun's core there are enough protons to provide some 600 million tonnes of hydrogen fusion every second which in turn is such a small proportion of the sun's mass that after 4,500 million years as a main sequence star, only 4% of the sun's hydrogen has been converted into helium. Nature is mind boggling.

[Knight of Clear Skies]

I've also read that solar fusion is quite a slow process. It's something like 1 proton in every 100,000,000 moves fast enough to even be able to fuse with another proton and of this only 1 collision in every 10²² actually results in fusion.

Yes, it is staggering. It also demonstrates just how weak gravity is compared to the other forces. It takes the gravitational force of about 75 Jupiter masses to overcome the repulsion between two positively charged protons and allow them to fuse.

So this is why stars last so long. There is a certain amount of convection which cycles new hydrogen into the core, but it's limited. This is why they tend to go into the red giant phase, as they run out of H in the core, and start to burn it in a shell around the inert He core.

The exceptions to this are low-mass red dwarf stars which are fully convective, exchanging material between the core and outer layers. As a result they can burn a very high percentage of their fuel and potentially shine for trillions of years, compared to the expected 10 billion year lifetime of our own Sun.

[ollypenrice]

if only i could burn calories like the sun burns hydrogen !!!

We like you, Earl, and at 2 million tons per second you'd soon be gone!!!

Olly

[Knight of Clear Skies]

if only i could burn calories like the sun burns hydrogen !!!

Fun fact: every night you lose weight just breathing. You inhale oxygen (O₂) and exhale carbon dioxide (CO₂), losing carbon one atom at a time; it adds up to about 100g of carbon overnight, the mass of a slab of Cadbury's. (You also lose weight by breathing out water vapour and through sweating, but the amount will vary depending on temperature and humidity.) So even doing nothing you lose weight millions of times more rapidly than the Sun, proportionally speaking.

Sleeping Beauty would have wasted away to nothing.

[kirkster501]

Fun fact: every night you lose weight just breathing. You inhale oxygen (O₂) and exhale carbon dioxide (CO₂), losing carbon one atom at a time; it adds up to about 100g of carbon overnight, the mass of a slab of Cadbury's. (You also lose weight by breathing out water vapour and through sweating, but the amount will vary depending on temperature and humidity.) So even doing nothing you lose weight millions of times more rapidly than the Sun, proportionally speaking.

Sleeping Beauty would have wasted away to nothing.

I am always about 1kg lighter in the morning