Stellar Evolution

Where do gold earrings come from?

Interstellar Clouds

• Where new stars form

Orion Nebula – copyright Robert Gendler

Concept Test

• These interstellar clouds appear red because they are:a. hot and things that are hot glow red.b. ionized hydrogen which appears red

because the brightest emission line is red.c. cold and things that are cold appear red.d. full of red stars.e. Composed of dust between the observer

and the region which blocks the blue light, but lets the red light through.

Cold Dark Clouds

• If dust clouds block light, then inside thick dust clouds there should be no light at all.

• Without light, there is little energy.

• With little energy, gas inside is very, very cold.

Star Formation• Remember:

• Cold interstellar clouds:No heat = no velocity = no outward pressure.Gravity wins.

• Gas begins to contract.

HOTTER COOLER

How to Make a Star

1

2

3

Star “turns on” when density and pressureat the core enough to fuse H into He..

The Main Sequence• A star is a

delicate balance between the force of gravity pulling in, and pressure pushing out.

• Stars on the main sequence fuse hydrogen in their core to produce thermal pressure.

• Longest phase of a star’s life.

The Main Sequence• Stars

characterized by what holds them up.

• 90% held up by heat of Hydrogen fusion

4H He + Energy

What then?• When the hydrogen in the core is almost

consumed the balance between gravity thermal pressure pushing out and gravity pushing in is disturbed.

• The structure and appearance of the star changes dramatically.

• What happens then, depends on the star’s mass.

• Two cases:– Low-mass (< 8 x mass of Sun)– High-mass (> 8 x mass of Sun)

Helium Ash

• Heavier elements, sink to the “bottom.”• After 10 billion years, core is “choked” with helium

“ash”.• H He continues in shell around non-burning core.

The Red Giant Branch

• Without fusion pressure in core:– Helium core collapses (no

counter to gravity)– Density in core increases.

• 3He C + Energy in core

• 4H He + Energy in shell

• Extra energy results in extra pressure. Star expands.

• The star gets bigger while its outside gets cooler.

The Onion Sun

• Red Giant Stars• Layers of:

– Non-fusing H– Fusing H– Fusing He– Non-fusing C

“ash”

…And the Solar System?

• In a few million years: “a last perfect day”:– Sun becomes slightly brighter– Ocean’s begin to evaporate– “Hot House” Earth

• In a few billion years:– Sun swells up– Engulfs the inner Solar System– Certain death for terrestrial planets– Possible “spring” on the Jovian ocean-

moons!

Death

• 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

NGC3242 – HST – Bruce Balick

M57 – Ring Nebula

M27 – Dumbbell Nebula – copyright VLT, ESO

Cat’s Eye

Eskimo Nebula

Hourglass Nebula

NGC2440 – HST – Bruce Balick

White Dwarf• Mass of Sun• Radius of Earth• Hot as Sun’s core• A million times denser than

lead• Slowly cool off

High-Mass Stars

• Think back to the first carbon core.

• How they get from main sequence to the carbon core stage is a little different.

• Now however, there is enough mass that it becomes hot enough to fuse carbon?

• Hot enough to eventually fuse lots of elements.

The Iron Core

• 4H He + Energy• 3He C + Energy• C + He O + Energy• The ash of one reaction, becomes the

fuel of the next.• Fusion takes place in the core as long

as the end result also yields energy.• This energy causes pressure which

counters gravity.• But Iron doesn’t fuse.

Core-Collapse

• Iron core – no outward pressure.• Gravity wins!• Star collapses rapidly!• Electron degeneracy can’t stop it.• Atomic structure can’t stop it.• Electrons and protons crushed together

to produce neutrons.• Neutrons pushed together by force of

gravity.

Supernova

Supernova

Supernova

Supernova

Supernova

• The result of the catastrophic collapse is the rebound and explosion of the core.

• From start of collapse to now: 1 second!

• Matter thrown back into the interstellar medium.

• Matter rushing outwards, fuses with matter rushing inwards.

• Every element after Fe is made in the instant of a supernova!

M1 – Crab Nebula – copyright VLT

Veil Nebula – Lua Gregory (English ’05)

NGC 4526 – 6 Million parsecs away

Discussion Question

• For Wednesday:• What is our cosmic connection? How

does it change (or does it) your opinion of the world around you?

• Consider the statement, “The Universe makes me feel so small.” In what way is this statement true? In what way is this statement false. Defend or refute the statement.