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Blackbody problem

by Sneil
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Sneil
#1
Oct4-05, 10:18 PM
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
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Andrew Mason
#2
Oct5-05, 02:17 AM
Sci Advisor
HW Helper
P: 6,654
Quote 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
san_1420
#3
Oct5-05, 03:51 AM
P: 10
Quote 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

Sneil
#4
Oct5-05, 08:11 AM
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
Astronuc
#5
Oct5-05, 08:54 AM
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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 = ([itex]\sigma T^4[/itex]) *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
Sneil
#6
Oct5-05, 09:05 AM
P: 18
thanks man, im a little lost as to how i incorporate the radius, but ill figure it out

thanks
Astronuc
#7
Oct5-05, 11:41 AM
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P: 21,808
Surface area of a sphere, i.e. the star, of radius r is

A = 4[itex]\pi r^2[/itex].


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