QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Astrophysics I:Astrophysics I:The Stellar LifecycleThe Stellar Lifecycle

Kathy Cooksey

How to Make a Star• Collapse interstellar clouds

•Visible and IR image of the hot protostars in Orion Nebula.

How to Make Systems

• Cloud around protostar spins faster

• Flattens to a disk– Think pizza dough

Protostars and Disks• Dust and gas

condense onto dust grains

• Small clumps grow bigger

• Bigger clumps have more mass and attract more matter

• Planetesimals become building blocks of planets Orion Nebula – Copyright O’Dell and Wong

Now what?• Mass of the star

determines rest of its life!

• More massive star more pressure in core

• More pressure more fusion

• More fusion:– More energy produced

– Hotter

– Shorter life span

Stellar Stellar EvolutionEvolution

The Life of a Star(like our Sun)

(which does not move in a circle as it evolves

Artist’s rendition)

The Main Sequence• Balance between:

– Force of gravity pulling in

– Pressure from the heat of fusion pushing out

• Stars on main sequence burn hydrogen in their core to produce heat

• Longest phase of a star’s life

What then?

• Gravity-pressure balance disturbed when hydrogen in core depleted

• Big change in structure and appearance of the star• “THE END” depends on star’s mass• Two cases:

– Low-mass (< 8 mass of Sun)

– High-mass (> 8 mass of Sun)

Red Giants

After hydrogen exhausted in core: • Core collapses, releasing energy to

outer layers• Outer layers expand• Increasing temperature and pressure

in core helium fuses

The End for Low Mass Stars

• Core is contracting and heating.– Surface is cooling and expanding.

• Will it finally become hot enough in core for Carbon to fuse?– For the Sun: No.

• Gravity keeps contracting the core: 1000 kg/cm3!• What stops it?

– Electron degeneracy pressure!

Electron Degeneracy

Pressure from motion of atoms

Electron Degeneracy

Pressure from electron shells

Where are we now?

• Core dead – nothing happening.• Shells – burning H and He, but soon stop too.• Outside atmosphere of star still cooling and

expanding.• …and expanding• …and expanding• Force of radiation from burning shells blows

atmosphere away.

M57 – Ring Nebula

White Dwarfs

• Leftover once atmosphere blows away

• Exposed electron degenerate carbon core

• Size of Earth• No more fusion• Glow by their heat alone• Eventually cool and fade

away black dwarf

High-Mass Stars• H and He burned in core• Core collapses hot enough

to fuse heavier elements (C, N, O …)

• Iron is most stable element and cannot be fused further– Instead of releasing energy,

it uses energy

End for High Mass Stars• Fuel runs out• Core collapses and rebounds• Supernova!• Matter thrown back into the

interstellar medium• Matter rushing outwards,

fuses with matterrushing inwards

• Every element heavier than iron is made in instant of supernova!

We are stardust!

About the core…

First Stop: Electron Degeneracy

Last Stop: Neutron Degeneracy

Otherwise…

M1 – Crab Nebula – copyright VLT

NGC 4526 – 6 Million parsecs away

Stellar Lifecycle Summary

Low-mass Stars

• Like Sun

• Long lived (measure in billion years)

• Fuse to mostly helium

• Planetary nebula and white dwarf end state

• Most common

High-mass Stars

• 8 more massive

• Short lived (measure in million years)

• More fusion (C, N, O …)

• Supernova and neutron star or black hole end

• Makes most important elements