ASTR 1200 Announcements I hope to have the exams returned next Tuesday, but no promises…. Class will be in the planetarium next Tuesday (October 14) Website http://casa.colorado.edu/~wcash/APS1200/APS1200.html

Full, Artistic H-R As mass of MS star increases, both R and T increase increasing size sAT4 T constant on any vertical line

Newly Formed Star M Spectral Type O +10 +5 -5 +15 B A F G K Protostar +15 B A F G K Sun Sirius a Cen B Prox Cen Procyon Rigel Capella Sirius B Main Sequence Giants White Dwarfs Protostar Large, Low T. Settles down to MS Then sits while burning H

MS Lifetime What determines amount of time a star stays on Main Sequence? Just like a kerosene heater: Amount of fuel and rate of burn. More Mass = More Fuel More Luminosity = Greater Burn Rate We can scale from the Sun: M = 1M L = 1L Sun lasts 1010 years M in solar masses L in solar luminosities

Some Lifetimes Mass Luminosity Lifetime in Billion Years Sun 1 1 10 Sirius 2 10 2 Prox Cen .4 .001 4000 Rigel 8 10,000 .008 Dinky little stars like Prox Cen will last trillions of years Huge stars like Rigel are gone in a few million There aren’t many large stars out there, because they don’t last. 10,000 O stars of the 100,000,000,000 Milky Way stars

Chemical Energy for Sun? Chemical Energy Generates 2eV per atom in forming molecule (burning) 2eV = 3x10-19 Joules Number of Atoms in Sun: Available Energy Time it can run:

Gravitational Energy? Available Gravity Energy: Time it can run: Sun can only run 30million years on gravity. It does this during formation Best understanding of Sun until Einstein.

Nuclear Energy for Sun? Nuclear Energy Generates 2MeV per atom in forming molecule (burning) 2MeV = 3x10-13 Joules Number of Atoms in Sun: Available Energy Time it can run: Sun can run 20 Billion years on nuclear energy Which is what it does.

What Happens When Hydrogen Starts to Deplete? Percentage of Helium in Core Rises Nuclear Reaction Rate Slows Energy Leaks Away and Pressure Drops Pressure from above squeezes core, making it smaller and hotter. Nuclear Reaction Rate Increases Star Gets Brighter! The less fuel a star has left, the faster it burns it….

Effect on Star M Spectral Type Core Shrinks and Gets Hotter Star gets brighter and bluer? -5 Rigel Giants Capella Sirius M Procyon Sun +5 Main Sequence a Cen B White Dwarfs +10 Sirius B Prox Cen +15 O B A F G K M Spectral Type Actually Goes Up and To Cooler Side Why??

Outer Envelope of Star Expands Force of photon flux from below lifts outer parts of star. Core gets smaller and hotter, but Surface gets larger and cooler.

Path on H-R Diagram Over Time a Star becomes a red giant.

What’s Happening in Core? H->He H->He He In the center, H is depleted, but He too cool to burn. “Shell Burning” describes source of nuclear power around dead center

He Compressed In Middle H->He Temperature and Density Rise in Center Star gets more luminous He

Electron Degeneracy Electron Degeneracy Helium Density Rises Center of Sun has density of 10g/cc (H2O = 1g/cc) When density in center of star reaches ~30,000g/cc a new phenomenon kicks in Electron Degeneracy A purely quantum mechanical phenomenon

A Metaphor for Degeneracy

Electron Degeneracy Pressure Because of electron degeneracy pressure, core stops shrinking. Gains Mass, Temperature Rises, Stays Same Size Has all the characteristics of a bomb. Burnable Material Confined Space What ignites it?

Helium Flash When degenerate He core reaches about 108K, the nuclei can burn by the Triple Alpha Reaction. (He+He+He->C) They start to burn and release energy. Pressure rises and temperature rise, but volume does not increase. P and T rise some more. Finally, P gets so great it lifts the degeneracy and thermal pressure equilibrium is re-established. This is the “He Flash” Could blow a star apart, but it doesn’t.

He Flash Star changes L and R in just 10,000 years!

After the Flash H H H->He H->He He->C Helium Burns in the Center Starts to Development Dead C Core

Red Giants

Nucleus Earth Orbit

Fate of The Earth We will be swallowed by the Sun In 5 billion years it will start to swell. T will rise on Earth Oceans will boil and then evaporate into space Sun will cover the sky. Giant ruddy ball. Then it will engulf us. Mountains will melt. Planet should survive. Will look like a polished bowling ball.

Red Supergiant Meanwhile, down in the core, the C is becoming degenerate. Luminosity is becoming so great it blows the H envelope into space. H H->He He->C C

Planetary Nebulae Gas blown out into space and illuminated by central star. The star is the degenerate C core. The Ring Nebula

Gorgeous Planetary Nebulae from Hubble Space Telescope Notice Rings. Star has “episodes”

Hourglass Shape Star throws material out in its ecliptic plane. That’s the equatorial plane of the star. Creates a dense disk around star.

Hourglass 2 Next Explosion is Constrained and Expands as Hour-Glass

What’s Left? Eventually all the envelope is blown away. The planetary nebula dissipates. All we have is the hot, degenerate core. Settles down to become a White Dwarf