Announcements • Wednesday night star parties begin this week, 8:45 pm, weather permitting. Attend one for 4 points extra credit! (Staff signature required.) • 10:00 office hour cut short today • Majors Fest at Union Building Gallery 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 1H (protons) 4He 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 Hottest stars are actually somewhat larger Should be white, not green! Reds are greatly exaggerated! Main Sequence Lifetimes (predicted) Mass (suns) 25 15 3 1.5 1.0 0.75 0.50 Surface temp (K) 35,000 30,000 11,000 7,000 6,000 5,000 4,000 Luminosity (suns) 80,000 10,000 60 5 1 0.5 0.03 Lifetime (years) 3 million 15 million 500 million 3 billion 10 billion 15 billion 200 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”!