What regulates the nuclear fusion rate within stars?
Asked by
ETpro (
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September 15th, 2013
While Centauri Proxima is just 1/8th the mass of our Sun, Centauri Proxima is a main-sequence, red-dwarf star that will burn for trillions of years while our Sun will consume all its hydrogen, begin gravitational collapse and, with the rapidly rising temperatures that collapse triggers, transition into a red giant powered for a short time by fusion of increasingly heavy elements in just 8 billion years. Our sun’s red giant phase will last a relatively short time before it consumes all its fusionable elements, and without the pressure generated by fusion reactions it will collapse into a slowly cooling brown dwarf.
Some supermassive stars consume all their hydrogen, begin gravitational collapse and with their great mass, become so hot they explode as a supernova in relatively short timeframes; whereas others of similar size burn cooler and will last far longer. Stars aren’t like nuclear power stations, with control rods that can raised and lowered to manage their reaction rate. What then controls the rate at which a given star fuses its hydrogen atoms into helium during its main sequence phase?
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14 Answers
Nothing controls the reaction. It is a fusion reactor in meltdown. But just like the centralia Pennsylvania coal fire, it’s taking a while burning through all the fuel.
Wikipedia
The intermediate by products of solar fusion a so unstable, that they revert back to hydrogen far more often than form into helium. So I think that’s the rate limiting step.
The fusion is regulated by the laws of nature. It follows the path of least resistance.
It’s a function of the mass of the star, along with a few other factors.
When a star is fusing, it’s dense and hot. As it gets hotter, it expands, becoming less dense. As its density decreases, it cools. If it’s a star of low enough mass, it can expand enough to stop fusing. When it has cooled enough, it starts contracting again. The contraction heats it up, and soon it starts fusing again.
Our own Sun has an 11-year activity cycle from maximum to minimum emission. I don’t know if it actually stops fusing, but it has that cycle.
My spottily-informed answer is that the size (total mass) of the star plus its composition affect the rate of fusion. Heavier elements slow things down a bit so if a star is made of ejecta from a supernova (lots of elements heavier than hydrogen, including trans-uranic) it would have fusion at a different rate to that in a star made mostly of hydrogen.
@drhat77 Not a full answer, but a big high-five on the link regarding the fact all hydrogen is not fused into helium. I did not know that.
@YARNLADY Yes, well I didn’t really think it was controlled by Voodoo or Amon Ra. :-)
@rexacoracofalipitorius Could you point me to something where I can read about stars that intermittently stop fusion? I would guess our Sun isn’t such a star, as the radiation only fluctuates by 0.1% over its 11 year period.
@dabbler That makes good sense to me. Thanks.
@hiphiphopflipflapflop Thanks, but I had already gone through all the known classes of variable stars including the cepheid variables and not found any mention of ones where fusion stops and starts. It is helium ionization levels and resulting expansion and cooling of the helium that is thought to cause the cepheid variation, no?
Sorry, that’s literally just the term that popped into my head thinking about variability in stellar output. Whether stars exist that periodically “turn off” completely, and if so what they are called, I have no idea. And I didn’t follow up those links like you have.
@hiphiphopflipflapflop It seemed like a possibility in a star with low enough mass. If there were just enough mass to cause sufficient gravitational collapse to start fusion, and the fusion reaction then expanded the gasses out enough they cooled below the temperature required to maintain the reaction, then there could be such a class of stars. But the maths required to calculate whether this hypothesis could ever be true are well beyond my reach. I wish I were young and could do college over. :-)
@ETpro It was something I read in a Larry Niven story, and I assumed it was legit since he has a degree in astrophysics (from the 1970’s, granted, but that’s more than I’ve got.)
I’ve had a further look around and been unable to corroborate it, so I think Niven may have been blowing smoke (rings?).
CURSE YOU NIVEN!
Without reading any of the above answers.. I would say gravity.
@talljasperman Are you saying gravity just fluctuates randomly? I always thought it was a function of mass.
Gravity and the presence of metals.
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