Well, I never!
I have been left slightly flabbergasted this morning, since I have just had a lifelong belief, overturned by the course notes from my course S382, Astrophysics.
What do I mean?
Well, I had always assumed, nay, believed; that the reason why the sun and other stars, are so god-damn hot, was because of all that fusion energy going on within the star. I mean, just get too close to a 1 megaton H-bomb as it detonates, and you would feel the effects of why that assumption might seem correct.
However, If one explores the equations that govern luminosity, and work out, from the proton-proton chain fusion reactions that occur at the core of the sun; you would see that for each square meter of nuclear material that is available to 'burn', it only produces approximately 300W of power per cubic metre.
The course notes use the example of imagining three 100W light bulbs in a cupboard, as an equivalent amount of energy release per unit volume.
So then, why is the sun so damn hot?
Well, it turns out that the fusion energy only really prevents the sun from collapsing in on itself uncontrollably, due to the energy released from this reaction which counters the gravitational energy from the mass of all that material.
The Sun is hot, simply because, it is so massive, that it has a mindbending amount of gravitational potential energy. As all of this mass attracts itself and causes a contraction, the gravitational potential energy, is converted to kinetic energy. And, fast moving particles, are very hot.
So, the main source of heat for all stars is caused by this conversion of energy from gravitational to kinetic. The fusion reaction energy just seems to retard the gravitational collapse.
Well, it impressed me anyway....
"So, the main source of heat for all stars is caused by this conversion of energy from gravitational to kinetic."
ReplyDeleteI don't think this sounds quite right to me. It will be a proportion, but I would have said, the lesser proportion. Stars "light up" once fusion starts, otherwise they remain brown dwarfs that slowly cool over time.
Also, that proportion probably changes according to the mass of the star. The more massive stars probably have very much more power provided for by fusion than less massive ones.
Interesting question!
I have just looked this up. Gravitational collapse could sustain the sun at its present luminosity for 15 million years not the 10 billion year lifetime expected. Your books I think are giving you the wrong impression.
ReplyDeleteHmmm, I have had some more thoughts. I understand what it is you are trying to say. Perhaps it would have been better to say that the release of gravitational energy to kinetic was enough in the period of the formation of the sun to get the core up to the temperature for fusion to start but it is definitely fusion that now keeps the core at that temperature. There might not be very much energy released per cubic meter per second in the core but the core of the sun is so big that the total energy released per second from it is enough to power the energy we see emitted per second at its surface.
ReplyDelete