How Stars Evolve

How Stars Evolve

• Pressure and temperature– Normal gases

– Degenerate gases

• The fate of the Sun– Red giant phase

– Horizontal branch

– Asymptotic branch

– Planetary nebula

– White dwarf

Normal gas• Pressure is the force exerted by atoms in a gas• Temperature is how fast atoms in a gas move

• Hotter atoms move faster higher pressure

• Cooler atoms move slower lower pressure

Pressure balances gravity, keeps stars from collapsing

Degenerate gas

• Very high density• Motion of atoms is not due to kinetic

energy, but instead due to quantum mechanical motions

• Pressure no longer depends on temperature

• This type of gas is sometimes found in the cores of stars

Fermi exclusion principle

• No two electrons can occupy the same quantum state

• Quantum state = energy level + spin

• Electron spin = up or down

Electron orbits

Only two electrons (one up, one down) can go into each energy level

Electron energy levels

• Only two electrons (one up, one down) can go into each energy level.

• In a degenerate gas, all low energy levels are filled.

• Electrons have energy, and therefore are in motion and exert pressure even if temperature is zero.

Which of the following is a key difference between the pressure in a normal gas and in a degenerate gas?

1. Degenerate pressure exists whether matter is present or not.

2. In a degenerate gas pressure varies rapidly with time.

3. In a degenerate gas, pressure does not depend on temperature.

4. In a degenerate gas, pressure does not depend on density.

The Fate of the Sun

• How will the Sun evolve over time?

• What will be its eventual fate?

Sun’s Structure

• Core– Where nuclear fusion

occurs

• Envelope– Supplies gravity to keep

core hot and dense

Main Sequence Evolution

• Core starts with same fraction of hydrogen as whole star

• Fusion changes H He

• Core gradually shrinks and Sun gets hotter and more luminous

Gradual change in size of Sun

Now 40% brighter, 6% larger, 5% hotter

Main Sequence Evolution

• Fusion changes H He

• Core depletes of H

• Eventually there is not enough H to maintain energy generation in the core

• Core starts to collapse

Red Giant Phase

• He core– No nuclear fusion– Gravitational contraction

produces energy

• H layer– Nuclear fusion

• Envelope– Expands because of

increased energy production– Cools because of increased

surface area

Sun’s Red Giant Phase

HR diagram

Giant phase is when core has been fully converted to Helium

A star moves into the giant phase when:

1. It eats three magic beans

2. The core becomes helium and fusion in the core stops.

3. Fusion begins in the core

4. The core becomes helium and all fusion in the star stops.

Broken Thermostat• As the core contracts,

H begins fusing to He in a shell around the core

• Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting

Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion

Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon

Helium fusion

Helium Flash• He core

– Eventually the core gets hot enough to fuse Helium into Carbon.

– This causes the temperature to increase rapidly to 300 million K and there’s a sudden flash when a large part of the Helium gets burned all at once.

– We don’t see this flash because it’s buried inside the Sun.

• H layer• Envelope

Movement on HR diagram

Movement on HR diagram

Helium Flash

• He core– Eventually the core gets

hot enough to fuse Helium into Carbon.

– The Helium in the core is so dense that it becomes a degenerate gas.

• H layer• Envelope

Red Giant after Helium Ignition• He burning core

– Fusion burns He into C, O

• He rich core– No fusion

• H burning shell– Fusion burns H into He

• Envelope– Expands because of

increased energy production

Sun moves onto horizontal branch

Sun burns He into Carbon and Oxygen

Sun becomes hotter and smaller

What happens next?

What happens when the star’s core runs out of helium?

– The star explodes– Carbon fusion begins– The core starts cooling off– Helium fuses in a shell around the core

Helium burning in the core stops

H burning is continuous

He burning happens in “thermal pulses”

Core is degenerate

Sun moves onto

Asymptotic Giant

Branch (AGB)

Sun looses mass via winds

• Creates a “planetary nebula”

• Leaves behind core of carbon and oxygen surrounded by thin shell of hydrogen

• Hydrogen continues to burn

Planetary nebula

Planetary nebula

Planetary nebula

Hourglass nebula

When on the horizontal branch, a solar-mass star

1. Burns H in its core.

2. Burns He in its core.

3. Burns C and O in its core.

4. Burns He in a shell around the core.

White dwarf

• Star burns up rest of hydrogen

• Nothing remains but degenerate core of Oxygen and Carbon

• “White dwarf” cools but does not contract because core is degenerate

• No energy from fusion, no energy from gravitational contraction

• White dwarf slowly fades away…

Evolution on HR diagram

Time line for Sun’s evolution

In which order will a single star of one solar mass progress through the various stages of stellar evolution?

1. Planetary nebula, main-sequence star, white dwarf, black hole

2. Proto-star, main-sequence star, planetary nebula, white dwarf

3. Proto-star, red giant, supernova, planetary nebula

4. Proto-star, red giant, supernova, black hole

Death of stars

• Final evolution of the Sun

• Determining the age of a star cluster

• Evolution of high mass stars

• Where were the elements in your body made?

Higher mass protostars contract fasterHotter

Higher mass stars spend less time on the main sequence

Determining the age of a star cluster

• Imagine we have a cluster of stars that were all formed at the same time, but have a variety of different masses

• Using what we know about stellar evolution is there a way to determine the age of the star cluster?

Turn-off point of cluster reveals age

The HR diagram for a cluster of stars shows stars with spectral types A through K on the main sequence and stars of type O and B on the (super) giant branch. What is the approximate age of the cluster?

1. 1 Myr

2. 10 Myr

3. 100 Myr

4. 1 Gyr

Higher mass stars do not have helium flash

Nuclear burning continues past

Helium

1. Hydrogen burning: 10 Myr2. Helium burning: 1 Myr3. Carbon burning: 1000 years4. Neon burning: ~10 years5. Oxygen burning: ~1 year6. Silicon burning: ~1 day Finally builds up an inert Iron core

Multiple Shell Burning• Advanced nuclear

burning proceeds in a series of nested shells

Why does fusion stop at Iron?

Fusion versus Fission

Advanced reactions in stars make elements like Si, S, Ca, Fe

Supernova Explosion• Core degeneracy

pressure goes away because electrons combine with protons, making neutrons and neutrinos

• Neutrons collapse to the center, forming a neutron star

Core collapse

• Iron core is degenerate• Core grows until it is too heavy to support itself• Core collapses, density increases, normal iron

nuclei are converted into neutrons with the emission of neutrinos

• Core collapse stops, neutron star is formed• Rest of the star collapses in on the core, but

bounces off the new neutron star (also pushed outwards by the neutrinos)

If I drop a ball, will it bounce higher than it began?

Do 8B10.50 - Supernova Core Bounce

Supernova explosion

Crab nebula

Cas A

In 1987 a nearby supernova gave us a close-up look at the death of a

massive star

Neutrinos from SN1987A

Where do the elements in your body come from?

• Solar mass star produce elements up to Carbon and Oxygen – these are ejected into planetary nebula and then recycled into new stars and planets

• Supernova produce all of the heavier elements– Elements up to Iron can be produced by fusion– Elements heavier than Iron are produced by the

neutrons and neutrinos interacting with nuclei in the supernova explosion

Energy and neutrons released in supernova explosion enable elements heavier than iron to form, including Au and U

Review Questions

• What are the evolutionary stages of a one solar mass star?

• How does the evolution of a high mass star differ from that of a low mass star?

• How can the age of a cluster of stars, all formed at the same time, be determined?

• Why does fusion stop at Iron?

• How are heavy elements produced?