The Death of Stars - I.

! Be able to sketch the H-R diagram and include stars by size, spectral type, lifetime, color, mass, magnitude, temperature and luminosity, relative to our Sun

! Compare Red Dwarfs to our Sun in terms of their different masses and interiors

! Know the physics of the evolution of low mass stars from Main Sequence to white dwarf (fusion, gravity, collapse, expansion, temperature, composition, equilibrium)

! How do evolving stars move around the H-R diagram (where do they go when expanding, helium burning etc?)

! How does electron degeneracy pressure stabilize a dead star? What is a white dwarf? What is a planetary nebula?

Learning Objectives

Important bad drawingSo much information in the HR diagram...

The Evolution of Stars! A star’s evolution depends on its mass! We will look at the evolution of three

general types of stars!Red dwarf stars (less than 0.4 M⊙)!Low mass stars (0.4 to 8 M⊙)!High mass stars (more than 8 M⊙)

! We can track the evolution of a star on the H-R diagram!From Main Sequence to giant/supergiant

and to its final demise

Red Dwarf Stars! 0.08 M⊙ < Mass < 0.4 M⊙ ! Fully convective interior (no “radiative zone”)! Burn hydrogen slowly as convection constantly

circulates hydrogen to the core

! Live 100s of billions to trillions of years

! The Universe is only 13.8 billion years old, so none of these stars have died yet

Low mass star Helium-burning red giant

White dwarf and planetary nebula

Stellar Demise!

High mass star

Helium-burning red supergiant

Other supergiant phases

Core-collapse supernova

Main sequence

ProtostarRed giant

Helium flash

Horizontal branch

Asymptotic giant branch

Planetary nebula

White dwarf

Path of a Solar-Mass (1 M⊙) Star

Main sequence Core hydrogen burning

Tcore ~ 15 million K

Red giant Shell hydrogen

burning

Asymptotic branch giant Shell helium burning

Helium flash

Life of a Low Mass Star

Horizontal branch Core helium burning Tcore ~ 100 million K

Hydrostatic Equilibrium!The battle between

Gravity and Pressure!Pressure from fusion

pushes out and gravity pulls in – an equilibrium

!This is why a Main Sequence star isn’t shrinking even though it’s a big ball of gas

!A Main Sequence star’s life is all about this battle

The Red Giant Phase! When the hydrogen is gone

in the core, fusion stops! The core starts to contract

under its own gravity! This contracting heats

areas of the star near the core, and hydrogen fusion starts in a shell around the core

! The push from fusion in this shell expands the star! As the star expands, its surface moves farther from

the core and it cools – so it becomes a red giant

Main sequence

ProtostarRed giant

Helium flash

Horizontal branch

Asymptotic giant branch

Planetary nebula

White dwarf

Path of a Solar-Mass (1 M⊙) Star

The Horizontal Branch! When the core reaches 100 million K, it can

start to fuse helium into carbon ! this stabilizes the core

! The star shrinks slightlyand the surface heats up

! But helium doesn’t last very long as a fuel! Horizontal branch lifetime

is only about 10% that of a star’s Main Sequence lifetime

! Our Sun will burn helium for about a billion years

Main sequence

ProtostarRed giant

Helium flash

Horizontal branch

Asymptotic giant branch

Planetary nebula

White dwarf

Path of a Solar-Mass (1 M⊙) Star

! Fusion in the core stops – the helium has been converted to carbon and oxygen

! This is where carbon and oxygen in our universe come from!

! Every carbon atom in your body is the remnant of a dying star

! The star’s core collapses under its own gravity! A shell near the core starts to fuse helium and the star

expands and cools (again). The star is now called an asymptotic giant branch (AGB) star

When The Helium Runs Out…

Main sequence

ProtostarRed giant

Helium flash

Horizontal branch

Asymptotic giant branch

Planetary nebula

White dwarf

Path of a Solar-Mass (1 M⊙) Star

NGC 2440

T > 200,000 K

End Game! The outer layers of the

AGB star are cast off!Up to 80% of the

star’s original mass! The core remains, made

of carbon and oxygen from helium fusion!The core is very hot,

above 200,000 K! Ultraviolet radiation

from the core ionizes the cast off outer layers!The star becomes a planetary nebula

Planetary Nebulae

What About the Core?! Nuclear fusion has stopped, and gravity

begins to win the battle! The core contracts to the size of the Earth

! But it’s about 60% ofthe Sun’s mass

! Material in the core is compressed to a densityof about 1,000 kg/cm3

! The star ends its life as a white dwarf! It slowly cools off over

billions of years

Main sequence

ProtostarRed giant

Helium flash

Horizontal branch

Asymptotic giant branch

Planetary nebula

White dwarf

Path of a Solar-Mass (1 M⊙) Star

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Matter in the core of a normal star

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Electron-degenerate matter 1 ton per cubic cm

SQUEEZE

What stops a white dwarf from contracting further?

! A quantum effect: electron degeneracy pressure! The electrons are squashed together towards a

quantum limit that is classically unphysical!This creates pressure to counteract gravity!Which stops the contraction

A white dwarf – weighs about 0.6 M⊙

12,000 km

Relative Size of a White Dwarf

An utterly insignificant littleblue-green planet

White Dwarfs!

Next Time

The Death of Stars II