Stellar Death
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Transcript Stellar Death
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