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 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 311,00060500 million 1.57,00053 billion 1.06,000110 billion 0.755,0000.515 billion 0.504,0000.03200 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”!
Fusion of helium into carbon, oxygen 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 main- sequence lifetime 16 O 4 He 12 C 4 He
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
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 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”).