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Nuclear Reactions in Stars (part 2) 1 November 2006

Today : What makes the stars shine? How long do they last? Then what happens?

The Sun’s Interior Thermonuclear energy zone Radiative zone Convective zone

The Sun’s Interior Nuclear reactions only take place in the innermost 30% of the sun’s radius. The central density is 150 times that of water; the central temperature is 15 million kelvin.

In summary… The sun is a mass of incandescent gas, A giant nuclear furnace, Where hydrogen is built into helium At a temperature of millions of degrees. -- Zim and Baker, A Golden Guide to Stars, 1951; borrowed by They Might Be Giants, 1993

Can we test any of this theory? Yes! Look for the neutrinos…

Fusion of Hydrogen into Helium 4 1 H (protons) 4 He This reaction powers all main-sequence stars. The more massive the star, the more pressure at its center and therefore the faster the reaction occurs.

Masses of Stars

Sizes of Main-Sequence Stars Should be white, not green! Hottest stars are actually somewhat larger Reds are greatly exaggerated!

Main Sequence Lifetimes (predicted) Mass (suns) Surface temp (K) Luminosity (suns) Lifetime (years) 2535,00080,0003 million 1530,00010,00015 million 311, million 1.57,00053 billion 1.06, billion 0.755, billion 0.504, billion

What happens when the core of a star runs out of hydrogen? With no energy source, the core of the star resumes its collapse… As it collapses, gravitational energy is again converted to thermal energy… This heat allows fusion to occur in a shell of material surrounding the core… Due to the higher central temperature, the star’s luminosity is greater than before… This increased energy production causes the outer part of the star to expand and cool (counterintuitive!)… We now have a very large, cool, luminous star: a “red giant”!