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Lecture 7

PCS 181 Lecture Notes - Lecture 7: Strong Interaction, Neutrino, Alpha Particle


Department
Physics
Course Code
PCS 181
Professor
Margaret Buckby
Lecture
7

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75% Hydrogen, 24% Helium, 1% everything else.
The Main Sequence Stage:
TCore = 10 x 106 K =
Hydrogen fusion nuclear reactions in the core.
A star spends its life in a delicate balance between outward radiation pressure and inward
gravitational pull.
~90% ~95% of stars are Main Sequence…
Including our sun.
Hydrogen Fusion Reactions= Proton-Proton Chain:
1H + 1H = 2H + e+ + V (deuterium, position, neutrino).
2H =1H=3He + V (helium, istotope, radiation burst).
3He = 3He = 4He +1H +1H (helium, hydrogen).
*NOTE: FILL IN THE MISSING BLANKS FOR THE ABOVE EQUATIONS ON
TEST # 2!
Strong force nuclear force 2 particles with in 10-15 m together. Dominant nuclear
reactions that take place in 90-95% stars
Note that four Hydrogen nuclei fuse to form one Helium nucleus and …
A small amount of mass is released as energy.
(4.3 x 10-12 J).
Not much energy produced…
But…a typical star has 1038 such reactions each second!
Therefore, the energy produced each second is
(4.3 x 10-12J) (1 x 1038) = ~ 4 x1026 w.
This is the Luminosity (energy per second) observed for our sun (a typical star).
Luminosity = L = 4 (3.14) R2 (SIGMA SIGN) T4 = energy/s.
Life on the Main Sequence:
A “Main Sequence” star fuses Hydrogen nuclei into Helium nuclei deep in its core via p-
p chain(=proton-proton chain).
Core temperature must be at least 10 x 106 K = 10 million K. REMEMBER THIS FOR
TEST
4 H = 1He
~1038 such reactions occur each second.
Massive stars: greater core densities and greater core pressures.
Faster rate of Hydrogen fusion=
Shorter Main Sequence lifetime of only a few Million years.
Average, normal stars (like our sun) will fuse Hydrogen over lifetime of several billion
years.
90% - 95% of stars are Main Sequence stars. ASK THIS ON TEST
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