Chronos said:The short answer is the pauli exclusion principle.
only_theory said:So is fusion actually a slow process then or just within the sun?
only_theory said:So is fusion actually a slow process then or just within the sun?
I don't get what these unfused atoms are doing while they wait, surely if the conditions are right they should fuse as quickly as any other, i thought fusion itself happens fast so it seems strange to me why it all doesn't just happen within a matter of days or years.
Nabeshin said:Protons are damn small, and the sun is damn big. Even though the temperature is sufficient to fuse nuclei when they meet, the big question is IF they meet. It's a surprisingly rare event.
pp-chain and CNO-cyclebwana said:i ALWAYS wondered this since the first moment i was told the sun is a fusion reactor. how is this reaction moderated? where is the fuel kept? where are the spent byproducts of the reaction? It can't be one big homogeneous fusion ball- you'd expect the reaction to explode the sun if that were the case. Rather, i'd guess fusion is only occurring at the surface--as you travel towards the center of the sun, gravity might be less and so there would be less force compressing the fuel->no fusion? If I could launch a supercondicting magnet into the sun to 'drill a hole' in its surface, could we peek inside?
Astronuc said:Well, it really doesn't stop, but the rate of a particular reaction is affected by the production of heavier nuclei that do not readily fuse under the prevailing conditions.
Look at the conditions for pp or CNO cycles, and compare to alpha (He)-fusion. He (helium) accumulates in the core while displacing the hydrogen to the cooler outer regions, and this would cool the star (and slow the pp- or CNO-fusion cycles). If the core reaches sufficient temperature, alpha (He) fusion kicks in. The evolution depends on the mass of the star.
http://www.astronomy.ohio-state.edu/~ryden/ast162_4/notes15.html
Of course, if the core collapses to such a density that it forms the core of a neutron star, then fusion does not occur. And then there are black holes.
Many of the most interesting infrared objects are associated with star formation. Stars form from collapsing clouds of gas and dust. As the cloud collapses, its density and temperature increase. The temperature and density are highest at the center of the cloud, where a new star will eventually form. The object that is formed at the center of the collapsing cloud and which will become a star is called a protostar. Since a protostar is embedded in a cloud of gas and dust, it is difficult to detect in visible light. Any visible light that it does emit is absorbed by the material surrounding it. Only during the later stages, when a protostar is hot enough for its radiation to blow away most of the material surrounding it, can it be seen in visible light. Until then, a protostar can be detected only in the infrared. The light from the protostar is absorbed by the dust surrounding it, causing the dust to warm up and radiate in the infrared. Infrared studies of star forming regions will give us important information about how stars are born and thus on how our own Sun and Solar System were formed.
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The actual process of star formation remains shrouded in mystery because stars form in dense, cold molecular clouds whose dust obscures newly formed stars from our view. For reasons which are not fully understood, but which may have to do with collisions of molecular clouds, or shockwaves passing through molecular clouds as the clouds pass through spiral structure in galaxies, or magnetic-gravitational instabilities (or, perhaps all of the above) the dense core of a molecular cloud begins to condense under its self-gravity, fragmenting into stellar mass clouds which continue to condense forming protostars. As the cloud condenses, gravitational potential energy is released - half of this released gravitational energy goes into heating the cloud, half is radiated away as thermal radiation. Because gravity is stronger near the center of the cloud (remember Fg ~ 1/distance2) the center condenses more quickly, more energy is released in the center of the cloud, and the center becomes hotter than the outer regions. As a means of tracking the stellar life-cycle we follow its path on the Hertzsprung-Russell Diagram.
Nuclear fusion is the process by which two or more atomic nuclei combine to form a heavier nucleus. This process releases a large amount of energy in the form of radiation, which is what makes stars shine. In the core of a star, hydrogen atoms fuse together to form helium, releasing energy in the process.
Stars have a delicate balance between the force of gravity pulling the star inward and the energy produced by nuclear fusion pushing the star outward. When a star runs out of fuel, it can no longer produce enough energy to counteract the force of gravity, causing it to collapse. Therefore, stars must fuse their fuel at a steady rate to maintain stability.
The high temperatures and pressures in a star's core are needed for nuclear fusion to occur. However, these conditions are not uniform throughout the entire star. The outer layers of the star are not hot or dense enough for fusion to take place, so the fuel can only be fused in the core where the conditions are just right.
The amount of time it takes for a star to use up all its fuel depends on its mass. The more massive a star is, the faster it burns through its fuel. For example, a star like our sun will take billions of years to use up all its hydrogen fuel, while a more massive star may only last a few million years.
When a star runs out of fuel, it no longer has the energy to counteract the force of gravity and begins to collapse. If the star is massive enough, it can explode in a supernova, scattering its elements into space. Smaller stars may become white dwarfs or neutron stars, while larger stars may collapse into black holes.