Stellar Evolution

The Lives and Deaths of StarsThe Lives and Deaths of Stars

Protostars. These objects radiate energy away in the form of light. That energy comes from gravity – released by contraction.

On the Main Sequence, the contraction stops because gravity and internal pressuregravity and internal pressure exactly balance each other.

Nuclear reactions occur at exactly the right rate to b l ibalance gravity.

A stars’ mass determines its luminosityA stars mass determines its luminosity

Nuclear reactions slowly convert Nuclear reactions slowly convert H H toto He in the core.He in the core.

Newly formed stars are typically:y yp y~91% Hydrogen (H)~9% Helium (He)

In the Sun’s core, the conversion of In the Sun’s core, the conversion of H to He ill take 10 billion earsH to He ill take 10 billion earsH to He will take ~10 billion years H to He will take ~10 billion years (its Main Sequence lifetime).(its Main Sequence lifetime).

Composition of a SunComposition of a Sun--like starlike star

What happens when the core Hydrogen isthe core Hydrogen is used up?

Nuclear reactions stop.

Core pressure decreasesCore pressure decreases.

Core contracts and gets hotter h ti l i l- heating overlaying layers.

4H → 1He burning moves to a hot “shell.”

Post-Main Sequence evolution

•4H → 1He reactions occur faster than before.

•The star gets brighter (more luminous)!

•The hot shell causes the outer layers to expand and cool!

Th ff h M i•The star moves off the Main Sequence, …up the “Red Giant” branch.

A i th d i tAscension up the red giant branch takes ~100 million years

What happens in the core as it continues to contract and

t h tt ?get hotter?

Remember why Hydrogen burning requires 107 K?

(Protons repel each other.)

Helium nuclei (2 protons) repel ( p ) peach other even more…

Helium begins to fuse into Carbon at >108 K.

This reaction is called “triple alpha” = 3He → C.This reaction is called triple alpha 3He → C.

Post-Main Sequence evolutionThe Helium Flash:

After the core reaches 108 K, Helium “ignites” to make Carbon.

The onset of this burning causes the temperature to rise sharply in a runaway explosion - Helium Flash (stage 9)explosion Helium Flash (stage 9)

Eventually the core expands, density drops and equilibrium is re-established

Core structure is now readjusted during Helium core burning and total luminosity is actually decreasedis actually decreased.

During core Helium burning, the star is on the Horizontal Branch.

Relative sizes of main-sequence, red giant, and horizontal branch starshorizontal branch stars.

Stage 9Stage 7

Stage 9Stage 10

Stage 10 - Helium-to-Carbon burning occurs stably in core with Hydrogen-to-

Carbonoccurs stably in core, with Hydrogen toHelium burning in shell…

until the core Helium runs out

Helium

…until the core Helium runs out…. in just 20-50 million years…

Carbon

liHelium

Carbon

Helium

The increased shell burning causes the outer layers to yexpand and cool (again).

The star moves up the asymptotic giant branch(in only ~10,000 years!).(in only 10,000 years!).

becoming a red supergiant g p g(stage 11)

•During this phase the•During this phase the Carbon core contracts and heats up.p

•Helium is burning to Carbon in a shell around theCarbon in a shell around the Carbon core while H-to-He burning occurs in an outer gshell.

What do you suppose happens next in the core?

For a Sun like star: Solar mass stars cannot squeeze andFor a Sun-like star: nothing…. Why?

Solar mass stars cannot squeeze and heat the core enough to ignite Carbon.

So what does happen? The Carbon core continues to contract and heat.

Shell He burning grows more intense.

He flashes occur in the shell.

Surface layers pulsate and are finallySurface layers pulsate and are finally ejected (slowly, at ~10s of km/s).

The hot, tiny core (White Dwarf) is revealed.

And a Planetary Nebula appears! (expanding emission line nebula heated by intense radiation from theheated by intense radiation from the hot white dwarf)White Dwarf

Planetary Nebulae have nothing to do with planets.

Th i li di iThey emit line radiation (hot gas) but are muchsmaller than the emission nebulae (HII regions) wenebulae (HII regions) we discussed in Chapter 11.

They are important sources of heavy elements (C, N and O), ( , ),which will go into the next generation of stars.

White dwarfs have about ½ the Sun’s mass (the rest was (expelled).

They are about the size of theThey are about the size of the Earth! (~0.01 solar radii)

D it 6600 lb / 3 !!Density: ~6600 lbs/cm3 !!

Very low luminosities, y ,(L = 4πR2 σT4)

What eventually happens to a white dwarf?

l d f iIt gets cooler and fainter (at the same radius).

•White Dwarf fades away.•Planetary Nebula dissipates

This is the End:

Planetary Nebula dissipates into interstellar space.•End of story for stars like ythe Sun.

Or is it?

Some white dwarfs end up in•A main-sequence or giant star in bi t ith WDSome white dwarfs end up in

explosively active situations.

N

binary system with a WD.

•Material gets pulled off the bigger star b i i l i f Nova by gravitational attraction – forms an accretion disk around WD.

•This material builds up on the WD surface, getting hotter and denser.

•Reaches 10 million K and Hydrogen burning ignites on the white dwarf surface!surface!

•Burning is fast and furious - Nova

•After material burns off, accretion resumes and the process repeats.

Summary:

Gravity squeezes and heats the

What happens to higher mass stars?

Gravity squeezes and heats the core enough to ignite Carbon.

Then Oxygen, then Neon…as each fuel gets exhausted in the

it b i tcore, its burning moves to a shell.

Concentric fusion shells form an “onion skin” structure.

Formation of an Iron core is the last stage…

Why is Iron formation the end of the line?

Creating elements heavier than Iron requires energy!

With no more sourcesWith no more sources of energy, and Fe fusion taking energy from the gas press refrom the gas, pressure support in the star’s core is lost…

The core quicklyll d icollapses under its own

weight….

Protons and electrons are crushed together in the collapsing core, making ne tronsmaking neutrons.

Eventually, the neutrons are so close y,together they “touch” neutron degeneracy pressure (which is verystiff).stiff).

The densities reach 100 billion kg/cm3

(at those densities the whole Earth would fit in our football stadium!!)

The collapsing neutron core then bounces!

An explosive shock wave propagates outward,

lli ll t lexpelling all outer layers.

Supernova

Two types of Supernovae:Two types of Supernovae:

Type II – we just discussed – massive star explosion

Type I – The long term effects of the Nova situation

•WD eventually “builds up” material that was not completely ejected during nova explosions

•Added mass causes gravity to squeeze the WD allowing Carbon to finally start burning.

•Carbon burns all over the WD at once (not just in core) and the star explodes in a carbon-detonation

(T I)supernova (Type I)

Comparison of Type I and Type II SupernovaeComparison of Type I and Type II Supernovae

This supernova was recorded by Chinese

astronomers in 1054 ADastronomers in 1054 AD.

Supernova ejecta like this spread pheavy elements throughout the GalaxyGalaxy.

Crab Nebula

Observing Stellar Evolution in Star Clusters