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Stellar Death
Astronomy 315
Professor Lee Carkner
Lecture 14
“I am glad we do not
have to try to kill the
stars. … Imagine if a
man each day should
have to try to kill the
sun? We were born
lucky”
--Earnest Hemingway,
The Old Man and the
Sea
Death Defined

The star can no longer support itself by
internal thermal pressure and so:


The details depend on mass
Very Low Mass
Red dwarfs (M < 0.4 Msun) burn their fuel
very slowly

Take a very long time (10’s of billions of
years) to use up all hydrogen

Red dwarfs will fade away as they run out of
fuel
Never become giants since they produce no
helium core
Solar Type
Stars with between 0.4 and 4 Msun go through
the following phases:


Hydrogen and helium shell burning (asymptotic
giant branch)

What happens next?
Evolution of 1 Solar Mass Star
Mass Loss
All stars lose mass
Mass loss is very low for main sequence stars

Giants have higher mass loss rates, due to:

Thermal pulses: changes in the core that cause
bursts of energy which can push the outer layers
away
Separation

Core gets denser, outer layers get less dense

If the core is hot enough, its radiation will
make the ejected outer layers glow
Planetary Nebulae
These glowing ejecta are known as planetary
nebulae

Have nothing to do with planets


Composition: low density gas producing
emission lines
IC3568 --HST
Mz3 -- HST
Ring Nebula -- HST
Structure of Planetary Nebulae
We would expect planetary nebulae to be
spherical

How does spherical star eject mater into a
non-spherical shape?

Blocked by companion stars or planets?

Different waves of ejecta interacting?
White Dwarf
The leftover core of the star becomes a
white dwarf

There is no fusion going on in a white
dwarf so it slowly cools

What supports a white dwarf?
Degeneracy
Electrons obey the laws of quantum physics
including the Pauli Exclusion Principle:

Due to its high pressure the core becomes
degenerate

Degenerate gas resists compression because
electrons cannot be forced any closer together
due to the Pauli exclusion principle
White Dwarf Properties
White dwarfs are very dense

Start out hot and then cool

White dwarfs obey the Chandrasekhar Limit
Must be less than 1.4 Msun, or they cannot be
supported by electron degeneracy pressure
Sirius A and B
High Mass Stars


Star will become a supergiant with a huge
radius (up to 5 AU) but most of its mass in a
small earth-sized core of layered elements
Evolutionary Paths
Core Collapse
In a short time (million years or less) the star
burns through all elements up to iron

There is no more thermal energy to support
the very dense core

Energy from the collapsing core rebounds to
produce a supernova

Supernova
A nova is a generic term for a sudden
brightening of a star
An exploding massive star is technically
known as a Type II supernova

Explosion is almost a billion times more
luminous than the sun

Leaves behind a supernova remnant
Supernova 1987a -- Before & After
Supernova 1987a -- Remnant
Anasazi Depiction of 1054 SN?
Crab Nebula -- Optical & X-ray
Post Main Sequence Paths
Stellar Corpses
After a supernova (or the planetary nebula phase)
the core of the star gets left behind
Low and medium mass stars leave white dwarfs

Higher mass stars produce neutron stars

Very high mass stars produce black holes

Next Time
Read Chapter 22.1-22.4