Stellar Evolution #2 – Main Sequence



Last chapter focused on the first stages of the stellar lifetime, if you want to read this first please see here

When the core of a protostar becomes hot enough for nuclear fusion to begin, the main sequence of stellar life begins, and a fully fledged star is born. This is the longest stage of a star’s life, finishing only when its hydrogen stores are used up.

The main sequence of a star is also its most stable, the extreme heat of the star’s core balancing the immense structure and preventing any collapse.

pp Chains and the CNO Cycle

There are 4 main processes that dominate nuclear fusion in this stage. These are the ppI (for proton-proton), ppII, ppIII, and the CNO (carbon, nitrogen, oxygen) cycle. Which process dominate depends on the temperature of the star. Lower temperature stars are dominated by the ppI chain with the stars progressing through the chains as listed above with hotter core temperatures.

In the Sun it is the ppI that has a 85% dominance, and the remaining 15% is taken up by the ppII and ppII chains.

So what exactly are pp chains? Essentially they are chain reactions that involve the conversion of hydrogen nuclei into helium nuclei. For each of the pp chains the net effect is the production of a helium nucleus from four protons.

Moving into stars with hotter cores, there are a different set of reactions to the pp chains that become important. These are the CNO cycle which involve the nuclei of carbon, nitrogen and oxygen. As in the pp chains the net effect is a helium nucleus from four protons, however the carbon, nitrogen and oxygen act as  catalysts to help the reactions take place, therefore unlike hydrogen the concentration of CNO stays essentially the same (as catalysts are conserved in reactions).

Brown Dwarfs

As already pointed out, CNO cycles become important in high mass and temperature stars, but what of those that are too low in mass to sustain nuclear reactions? Well these become what are known as brown dwarfs, sort of a midway point between a gas giant and a star, though a ‘failed’ star may be a better term for them as they arent technically a planet.

Brown dwarfs are low luminosity, low temperature stars formed directly out of the interstellar medium. This fact places them different from gas giants which form out accretion discs.  Another difference is mass. The division from gas giant to brown dwarf is 0.013 solar masses (0.013 times the mass of the Sun), this is 13 Jupiter masses. This is the mass above which deuterium undergoes nuclear fusion providing an energy source.

Next chapter we will be looking at the end of a star’s life on the main sequence strip, and what fate lies before them.



IMAGES: public domain


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