# Blackbody problem

by Sneil
Tags: blackbody
 P: 18 Hi, im having trouble with the last question in my assignment. It's on blackbody radiation and total power per area and whatnot. those questions i was able to do fine, however when i came to this problem i was a little lost.. here it goes, i'll type the whole thing out.. "The most important energy source in so called main sequence stars is fusion of four protons into He nuclei. A star has a radius r How many he nuclei does the star produce per second if He production is the dominant energy source? Use the Q value for the conversion 4H ->He to estimate the amount of raditation energy generated by hydrogen burning, and assume that the radiation energy output per second corresponds to the energy generated by hydrogen burning in a second." any help would be greatly appreciated. thanks -Neil
HW Helper
P: 6,654
 Quote by Sneil "The most important energy source in so called main sequence stars is fusion of four protons into He nuclei. A star has a radius r How many he nuclei does the star produce per second if He production is the dominant energy source? Use the Q value for the conversion 4H ->He to estimate the amount of raditation energy generated by hydrogen burning, and assume that the radiation energy output per second corresponds to the energy generated by hydrogen burning in a second."
Where did you get this problem? First of all, how do 4 protons fuse to become one He nucleus? Shouldn't it be 2 deuterons fuse to become a He nucleus?

You appear to be missing an essential piece of information: the surface temperature of the star. Without that, it is not possible to answer the question.

AM
P: 10
 Quote by Andrew Mason Where did you get this problem? First of all, how do 4 protons fuse to become one He nucleus? Shouldn't it be 2 deuterons fuse to become a He nucleus? You appear to be missing an essential piece of information: the surface temperature of the star. Without that, it is not possible to answer the question. AM
Sure that's possible 4 H fuse to form 4He2 .
through Carbon Nitrigen Oxygen cycle
Well I can't remeber whole thing but you can find it in any text book.
Also a minimum temperature is required at core for this process ro take place
But surface temperature would be much lesser

 P: 18 Blackbody problem sorry, i calculated surface temp to be 4990k the question says turns to He (of atomic mass # 4) i was just hoping for a formula i would be able to work with as i don't kow where to begin, but ill give the radius too if you want all the info. r=7.02*10^8 m anyone able to help this early in the mourning? thanks for the help, it is greatly apreciated -Neil
 Admin P: 21,808 To be in equilibrium, the energy production in the star must equal the energy radiated, so Egen = Erad. Egen = Number of reactions * Qreaction. and one should have a formula for blackbody radiation, the Stefan-Boltzmann law E = ($\sigma T^4$) *A. For He can be produced from 4 protons indirectly by the PP or CNO cycles. http://csep10.phys.utk.edu/astr162/l...y/ppchain.html http://csep10.phys.utk.edu/astr162/lect/energy/cno.html or more generally http://csep10.phys.utk.edu/astr162/lect/index.html
 P: 18 thanks man, im a little lost as to how i incorporate the radius, but ill figure it out thanks
 Admin P: 21,808 Surface area of a sphere, i.e. the star, of radius r is A = 4$\pi r^2$.

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