ASTR 1040 October 3 First MidTerm Exam on

ASTR 1040 – October 3 First Mid-Term Exam on Thursday (October 5) in class. Website http: //casa. colorado. edu/~wcash/APS 1040. html

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 Core Shrinks and Gets Hotter -5 Star gets brighter and bluer? Rigel ? M 0 Giants Capella Sirius Procyon Sun Main Sequence +5 a Cen B +10 +15 O Actually Goes Up and To Cooler Side Why? ? White Dwarfs Sirius B Prox Cen B A F G K Spectral Type M

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

Electron Degeneracy • Helium Density Rises • Center of Sun has density of 10 g/cc (H 2 O = 1 g/cc) • When density in center of star reaches ~30, 000 g/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 108 K, 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 He->C C Helium Burns in the Center Starts to Development Dead C Core

Red Giants

Earth Orbit Nucleus

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

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

Globular Clusters Very Old. G stars becoming giants. All the same age and composition Can actually see evolution off the MS

White Dwarfs • • Held up by electron degeneracy About the size of the Earth R~5000 km Mass Typically 0. 8 M Luminosity ~. 001 L Thin layer of “normal” H Degenerate Carbon

Some Famous White Dwarfs • Sirius B • 40 Eridani B • Procyon B All in binaries around nearby stars. Establishes the WD is close and small.

Earth vs White Dwarf

Earth vs. Sun

Mass Radius Relation As mass increases star gets smaller. Like ball of foam.

WD Density Water has a density of 1 g/cc Lead 11 g/cc Gold 19 g/cc 100, 000 times density of gold! NOT NORMAL MATTER!! 1 cubic centimeter masses one ton!

Surface Gravity This is 300, 000 gees If you weigh 150 lbs on Earth, you would weigh 45 million pounds on a White Dwarf! What would happen to you and your spaceship?

Escape Velocity Speed of light is 3 x 108 m/s, so escape velocity is. 02 c.

Gravitational Redshift Even light loses energy climbing out of this hole. a = 2 x 10 -4 At 5000Å have 1Å shift to red Looks like a 60 km/s Doppler Shift

Magnetic Field When a star shrinks from 109 m to 107 m So B increases from 1 Gauss to a Million Gauss A million Gauss can rip normal matter apart!

Chandrasekhar Limit A peculiarity of Degeneracy Pressure is that it has a maximum mass. Each electron added must find its own quantum state by having its own velocity. But what happens when the next electron has to go faster than light? The Chandrasekhar Limit for a White Dwarf is 1. 4 M No White Dwarf Can have more than 1. 4 M Otherwise it will groan and collapse under its own weight. We’ll come back to this later.

WDs are Common Every star with less than 5 M will end up as a White Dwarf Most stars with mass above 1. 3 M have reached end of MS life. White Dwarfs are VERY common ~ 10% of all stars Closest is only 2. 7 pc away. (Sirius B) Will become increasing common as universe ages.

Immortal Stars Regular stars need thermal pressure to balance gravity, and they need nuclear reactions to maintain the pressure, so the die when they run out of fuel. Not so White Dwarfs. They are as stable as a rock. Literally. A quadrillion years in the future all the stars will be gone, but the White Dwarfs will still be here. Their glow is fossil energy left from their youth as a regular star. Might die in 1031 years if protons prove to be unstable themselves. That’s 10, 000, 000, 000 years! Really don’t know if universe will still be here.