- Slides: 15
Announcements • Homework 10 due Monday: Make your own H-R diagram!
Red Giants and White Dwarfs 3 November 2006
Today: • Life cycles of stars • Aging stars: red giants • “Planetary” nebulae • Spent stars: white dwarfs
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.
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 cool (counterintuitive!)… • We now have a very large, cool, luminous star: a “red giant”!
Red giants are big! Mars
Fusion of helium into carbon, oxygen 4 He 12 C 4 He 4 He 16 O • 3 He nuclei must merge quickly, since 8 Be is unstable • Requires very high temperatures (100 million K) due to greater electrostatic repulsion • Produces less energy per kg than hydrogen fusion • Can continue in core of a star for about 20% of mainsequence lifetime
Final stages in the life of a low-mass star • Core runs out of helium, again collapses and heats up • Helium burning continues (quickly) in a thin, hot shell surrounding the core; hydrogen burning continues in a larger shell • Instabilities cause inner temperature to fluctuate, which causes outer layers of star to swell, pulsate • Pulsations eject outer layers into space, gradually expanding into a “planetary nebula” • Eventually, energy production stops and a very dense “dead” star is left behind: a “white dwarf”
“Planetary” Nebulae Slowly expanding shells of gas, ejected by pulsating stars, still heated by what’s left of the star’s core
More Planetary Nebulae
White Dwarf Stars • “Dead” cores of former stars, no longer burning nuclear fuel, radiating away leftover heat • Made mostly of carbon and oxygen nuclei, in a diamond crystal structure (“like a diamond in the sky”) • Crushed to incredible density by their own gravity: the mass of the sun but the size of the earth! (Higher-mass white dwarfs are smaller!) • Sirius B and Procyon B are nearby examples
H-R Diagram Patterns Luminosity = (constant) x (surface area) x (temperature)4 For a given size, hotter implies brighter. A bright, cool star must be unusually large (“red giant”). A faint, hot star must be unusually small (“white dwarf”).