ASTR 1120 – February 11

Websitehttp://casa.colorado.edu/~wcash/APS1120/APS1120.html

Review Sheet and Sample Exam posted

Recitations will be held in Duane G131, Wednesday Feb 19 at 5pm

Third Homework Due Today

. First Exam: Thursday, February 20

The H-R Diagram

Full, Artistic H-R

increasingsizeσAT4

T constanton any verticalline

As mass ofMS star increases,both R and Tincrease

Newly Formed Star

O

Spectral Type

+10

+5

-5

M

0

+15B MA F G K

Sun

Sirius

α Cen B

Prox Cen

Procyon

RigelCapella

Sirius B

Main Sequence

Giants

White Dwarfs

Protostar

Large,Low T.Settles downto MS

Then sits whileburning 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 FuelMore Luminosity = Greater Burn Rate

We can scale from the Sun: M = 1M L = 1L

Sun lasts 1010 years

LMMSLife 1010= M in solar masses

L in solar luminosities

Some Lifetimes

Mass Luminosity Lifetime in Billion Years

Sun 1 1 10Sirius 2 10 2Prox Cen .4 .001 4000Rigel 8 10,000 .008

Dinky little stars like Prox Cen will last trillions of yearsHuge 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

Nuclear Energy Generates 2MeV per atom in forming molecule (burning)2MeV = 3x10-13 Joules

Number of Atoms in Sun:

Nuclear Energy for Sun

5727

30

10106.1102

=== − kgxkgx

mMNp

JxxxE 445713 10310103 == −

yearssxWxJx

LEt 000,000,000,20107

104103 17

26

44

====

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 StarCore Shrinks and Gets Hotter

Star gets brighter and bluer?

OSpectral Type

+10

+5

-5

M

0

+15 B MA F G K

Sun

Sirius

α Cen B

Prox Cen

Procyon

RigelCapella

Sirius B

Main Sequence

Giants

White Dwarfs

?

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, butSurface gets larger and cooler.

Path on H-R Diagram

Over Time a Star becomesa red giant.

What’s Happening in Core?

H->He

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

HeTemperature and DensityRise in CenterStar gets more luminous

Electron Degeneracy

•Helium Density Rises

•Center of Sun has density of 10g/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 MaterialConfined Space

What ignites it?

Helium Flash

When degenerate He core reaches about 108K, the nucleican burn by the Triple Alpha Reaction. (He+He+He->C)

They start to burn and release energy. Pressure rises and temperaturerise, but volume does not increase. P and T rise some more.

Finally, P gets so great it lifts the degeneracy and thermal pressureequilibrium is re-established.

This is the “He Flash”

Could blow a star apart, but it doesn’t.

He Flash

Star changes L and Rin just 10,000 years!

After the Flash

H H

Helium Burns in the Center

H->He

He->C

H->HeHe->C

C

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 EarthOceans will boil and then evaporate into spaceSun 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->HeHe->C

C

Meanwhile, down in the core,the C is becoming degenerate.

Luminosity is becoming so great itblows the H envelope into space.

Planetary Nebulae

The Ring Nebula

Gas blown out into space andilluminated by central star.

The star is the degenerate C core.

Gorgeous Planetary Nebulaefrom 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 isConstrained and Expandsas 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

Globular Clusters

Very Old. G stars becoming giants.

All the same age and composition

Can actually see evolution off the MS