How Stars Ignite - The Fusion Reaction

[avito009]

Stars are formed from large clouds of gas. The gas, which might be the wisps of older stars which have exploded, or simply left from the Big Bang, is very cold and the cloud begins large but not at all dense.Gravity causes all the atoms of the gas to pull each other together, makes the cloud contract. As the cloud gets smaller, the atoms of the gas get closer together and begin to bump into each other more, which heats them up.

The energy of hydrogen atoms bumping into one another becomes so great that they will occasionally fuse together to form a helium atom.

So why does fusion reaction give out so much heat? Probably because heat is the increase in kinetic energy of the atoms. But what makes the atoms move at a higher speed at the first place? Is it Gravity?

 

[Chronos]

Yes, it's all about the gravity. The gas molecules acquire kinetic energy when compressed and bounce around like ping pong balls. Once they reach about 15 million kelvin, boom goes the dynamite. A star is born.

 

[avito009]

So it is actually the friction caused between the particles of gas that ignites the core of the star. Gravity causes gas to compress and causes particles to move at high speed. So Gravity causes friction between the gas particles, so the star ignites. Am I right?

 

[Simon Bridge]

Not so much "friction" as "collisions".
Temperature is the random kinetic energy of an object - the more the bits of the object bounce around, the higher the temperature.
Gravity makes all the bits of the gas cloud get closer together. The closer the bits are together, the more they will bounce off each other and have random motion.
Further, as they get closer together, they are exchanging gravitational potential energy for kinetic energy.

So the bits have more kinetic energy and it is more random, so the gas gets hotter.

When gasses get hot, they glow.

 

[mfb]

So why does fusion reaction give out so much heat? Probably because heat is the increase in kinetic energy of the atoms. But what makes the atoms move at a higher speed at the first place? Is it Gravity?

Fusion reactions release a lot of energy, usually as fast fusion products. This part has nothing to do with gravity.

 

[Astronuc]

So it is actually the friction caused between the particles of gas that ignites the core of the star. Gravity causes gas to compress and causes particles to move at high speed. So Gravity causes friction between the gas particles, so the star ignites. Am I right?

The gravitation causes a large gas cloud, mostly hydrogen, to heat up, or increase in temperature. The inflowing mass increases the temperature (kinetic energy) and momentum of the gas atoms. At some point, the gas is hot enough to become ionized (the gas becomes a plasma). When hot nuclei (protons in the case of hydrogen) achieve a certain energy, they begin to fuse, and the fusion reaction releases 'binding' energy in the form of positrons, which annihilate with electrons to produce gamma rays, or gamma rays, or in the case of t-t fusion, protons and alpha particles. As positrons, gamma rays, protons and alpha particles scatter, they also heat the plasma.

New stars use the pp-cycle as the principal fusion reaction. If some C is present, then a CNO-cycle can develop, otherwise it takes a while to form C.

http://cpt.phys.utk.edu/~th/Astro162/Lecture19.pdf
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/procyc.html
http://burro.astr.cwru.edu/Academics/Astr221/StarPhys/ppchain.html

 

[avito009]

Not so much "friction" as "collisions".

I think Simon the stars ignite due to compression of gas and that compression is similar to compression of air in a diesel engine. Compressed air in the combustion chamber heats up due to compression.

In the diesel engine, only air is introduced into the combustion chamber. The air is then compressed with a compression ratio typically between 15 and 22 resulting into a 40 bar (about 600 psi) pressure compared to 8 to 14 bar (about 200 psi) in the gasoline engine. This high compression heats the air to 550 °C (about 1000 °F). At about this moment (the exact moment is determined by the fuel injection timing of the fuel system), fuel is injected directly into the compressed air in the combustion chamber. The fuel injector ensures that the fuel is broken down into a mist, and that the fuel is distributed as evenly as possible. The heat of the compressed air vaporises the fuel. The start of vaporisation causes a delay period during ignition, and the characteristic diesel knocking sound as the vapour reaches ignition temperature and causes an abrupt increase in pressure above the piston. The rapid expansion of combustion gases then drives the piston downward, supplying power to the crankshaft.

 

[Drakkith]

I think Simon the stars ignite due to compression of gas and that compression is similar to compression of air in a diesel engine. Compressed air in the combustion chamber heats up due to compression.

Yes, it is exactly the same effect.

 

[Matterwave]

I think Simon the stars ignite due to compression of gas and that compression is similar to compression of air in a diesel engine. Compressed air in the combustion chamber heats up due to compression.

In the diesel engine, only air is introduced into the combustion chamber. The air is then compressed with a compression ratio typically between 15 and 22 resulting into a 40 bar (about 600 psi) pressure compared to 8 to 14 bar (about 200 psi) in the gasoline engine. This high compression heats the air to 550 °C (about 1000 °F). At about this moment (the exact moment is determined by the fuel injection timing of the fuel system), fuel is injected directly into the compressed air in the combustion chamber. The fuel injector ensures that the fuel is broken down into a mist, and that the fuel is distributed as evenly as possible. The heat of the compressed air vaporises the fuel. The start of vaporisation causes a delay period during ignition, and the characteristic diesel knocking sound as the vapour reaches ignition temperature and causes an abrupt increase in pressure above the piston. The rapid expansion of combustion gases then drives the piston downward, supplying power to the crankshaft.

You got it. The only difference with a star is that the core is about 15 million (to several billion in the larger stars) degrees hotter and instead of chemically igniting diesel fuel, the stars ignite pure hydrogen through nuclear fusion. :D

 

[Ken G]

What's more, I believe Simon Bridge was not saying that collisions are responsible for the heating, he said gravity did that (which is causing the compression). He was saying collisions are responsible for distributing the energy in a way that we call "temperature", and that in turn affects how fusion happens, because particles can collide at high enough energy to produce fusion. So it's not friction that causes fusion, it's collisions that does, but the reason the collisions are energetic is due to gravity. There is a common tendency to imagine that pressure somehow causes heating, but actually it's the opposite-- it's the heating that causes the pressure. Things don't heat up because they collide more and more as the density rises, they heat up because gravity gives them energy, and the increase in density is just another result of the contraction. Together, the rise in temperature and the rise in density cause the rise in pressure, but the rise in density does not cause the rise in temperature, they are both caused by the compression.

 

[joetommasi]

Like most stars, before ignition the sun was a cool object heating up because of gravitational compression/deuterium fusion. As hydrogen fusion reaction began producing energy, the sun must have expanded. Is there a book or any information on this specific event in the evolution of the sun/stars?

 

[Drakkith]

Like most stars, before ignition the sun was a cool object heating up because of gravitational compression/deuterium fusion. As hydrogen fusion reaction began producing energy, the sun must have expanded. Is there a book or any information on this specific event in the evolution of the sun/stars?

Prior to ignition, the Sun was a cloud of hydrogen, helium, and dust that was slowly collapsing. It continued to shrink until the core reached temperature high enough for fusion to begin, at which point the energy it lost through radiation began to be replaced by the energy from fusion and the Sun mostly stopped shrinking. The key here is that the Sun shines because it is hot, and it became hot through gravitational compression during its formation. Prior to reaching nuclear fusion temperatures, the energy radiated away from the Sun had no way to be replaced, so it just continued to lose energy and shrink until fusion could begin.

Just prior to the beginning of fusion, a star of less than 2 solar masses is classified as a T-Tauri star. These stars have a surface temperature similar to main sequence stars of the same mass, but are much more luminous thanks to their increased radius.

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

 

[BrandonP]

Is there a book or any information on this specific event in the evolution of the sun/stars?

Seriously? YES!

There are hundreds of textbooks on the subject. Any Astronomy 101 book would cover it in detail in at least 1 chapter

 

[Chronos]

See, for example, http://abyss.uoregon.edu/~js/ast122/lectures/lec13.html.