Introduction to Astronomy

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

Star Formation Fusion of Hydrogen into Helium 4 1H (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) Surface temp (K) Luminosity (suns) Lifetime (years)

25 15 3 1.5 1.0 0.75 0.50 35,000 30,000 11,000 7,000

6,000 5,000 4,000 80,000 10,000 60 5 1 0.5 0.03 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 stars 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! Red giants are big! Mars Fusion of helium into carbon, oxygen

4 4 He 12 C He 4

He 4 He 16 O 3 He nuclei must merge quickly, since 8Be 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 whats left of the stars 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 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).