Question

Explain, compare, and contrast the processes by which main sequence stars generate light for low-mass stars (the proton-proton chain) and high-mass stars (the CNO cycle).

Answer 1

High mass stars form very rapidly, in less than 100,000 years a collapsing proto star can reach the Main Sequence. Massive stars at all stages of their lives live their lives in the very fast lane. The pressure on their cores is so great, far more powerful nuclear reactions power them than the simpler proton-proton chain that medium and low mass stars use to generate their energy. Massive stars instead use the CarbonNitrogenOxygen or CNO cycle to release their energy. Nuclei of carbon absorb protons in a multistage process where the carbon acts like a catalyst, and at the end of the process the oxygen nucleus ejects the newly formed helium nucleus leaving the carbon to repeat the cycle anew. Really massive star also can fuse hydrogen and helium at the same time. As a result, these stars are not only fiercely hot throughout, they're also incredibly luminous and can outshine the Sun 50,000, 100,000 or even more while fusing hydrogen for energy. So much energy streams out these stars actually tear themselves apart in the process. In fact, a very massive star often sheds half or more of it's substance before it explodes at the end of it's life. They are very large and have a surface temperature of 100,000 degrees Fahrenheit or even higher. But their lifetimes are also very short, and in as little as a million years, they run through all their stocks of hydrogen and turn to helium and heavier elements. Once they start to fuse carbon, in a few centuries a core of iron is formed, and iron is inert when it comes to nuclear fusion. Fusing or fissioning iron nuclei will consume energy, not release it and when iron fusion starts, the star will explode as a supernova, spewing vast amounts of heavier elements back into space. A tiny, ultra dense neutron star or a black hole will be left behind after the debris vanishes into the interstellar medium

Unlike the wastrel massive stars, a low mass star leads a boring life. It takes far longer for them to go from a collapsing proto star to a mature hydrogen fusing star. Interior temperatures and pressures are far lower than what exists in their super massive counterparts, and they mainly use the proton-proton chain as their source of energy. In three steps, two pairs of hydrogen atoms fuse and form two nuclei of heavy hydrogen, which then captures another proton to become nuclei of light helium. In turn, the light helium nuclei fuse to form a nucleus of normal helium, ejecting two excess protons apiece. A star like the sun will last for at least 10 billion years before the hydrogen in the core runs out and it expands into a red giant. Very low mass stars can last far longer, a trillion years in fact for a low mass red dwarf. They use their fuel so slowly, they will remain unchanged until the hydrogen runs out, which lead to them becoming a white dwarf. Sun like stars become red giants, fuse helium for a time, then become white dwarfs after ejecting their outer layers as planetary nebulae. They cannot fuse carbon and heavier elements, so they simply die away quietly. The white dwarfs remain hot for billions of years, but they cannot remain that way forever. Eventually, they cool and crystallize into what amounts to giant diamonds and become invisible, planet sized objects of stupendous density, but no where near the density of a neutron star or a black home.