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Stellar End-States…
Now, my suspicion
is that the Universe
is not only queerer
than we suppose,
but queerer than we
can suppose.
J. B. S. Haldane (1892 – 1964)
from Possible Worlds, 1927
WHAT DO YOU THINK?
1.
Will the Sun someday cease to shine brightly?
2.
What is a nova? What is a supernova?
3.
Where does carbon, silicon, oxygen, iron,
uranium, & other heavy elements come from?
4.
What is a pulsar?
Essay Questions for the exam…

How will our Sun evolve as a star? What will its
final state be? Compare its predicted evolution
to that of higher-mass stars. How do they
end? How do we know?
Essay Questions for the exam…
(a) What is a pulsar? Where does it get its
energy? How do we know?

(b) Describe a black hole. How do astronomers
detect them if they give off no light?
Stellar Evolution

Building models of what
happens to stars

Low mass (0.08 to 0.4 Msun

Medium mass (0.4 to 4+ Msun

Higher Mass (5+ to >100 Msun
Low mass star evolution
(~8% to ~40% of our Sun’s Mass)
Slower fusion reaction rate
 Low Luminosity
 Cooler surface temp (K & M type red stars)
 Longer Lives


“Economy models!”
Medium Mass
40% to 400% of Sun’s Mass

Life like our Sun – about 10 Billion years
The Evolution of 1Mo Star

90% of Life as “Main
Sequence” star

Fuses Hydrogen to Helium
The Evolution of 1Mo Star

90% of Life as “Main
Sequence” star

Fuses Hydrogen to Helium

He collects in core & builds
up over time
The Evolution of 1Mo Star

90% of Life as “Main
Sequence” star

Fuses Hydrogen to Helium

He collects in core & builds
up over time

Gradually gets larger & larger
The Evolution of 1Mo Star

He core collapses, triggers expansion to
Red Giant

Fuses Helium to Carbon in Core

Fuses Hydrogen to Helium in shell

Creates dust grains in outer edges
Stellar Model of a Sun-Like Star
A red giant!
The Evolution of 1Mo Star

Not massive enough to fuse Carbon to
heavier elements like Iron

Fusion in core stops

Forms a planetary nebula “death shroud”

Core collapse finally stops as white dwarf
Planetary
Nebulae
“Death Shrouds”
of ejected gas
surrounding
collapsed white
dwarf corpse
(Not “planets”!)
Planetary
Nebulae
“Death Shrouds”
of ejected gas
surrounding
collapsed white
dwarf corpse
(Not planets!)
Model of Planetary Nebula
seen almost edge-on
The Evolution of 1Mo Star

Forms a planetary nebula

Core collapse finally stops as white dwarf

Stellar “corpse” is stable, tiny, hot…...

Supported by electron degeneracy pressure
Sirius &
White
Dwarf
Sirius &
White
Dwarf
In X- Rays
Note better
Resolution!
What supports weight of 1Mo star?
As it forms (a protostar)

Thermal pressure < gravity! (collapsing!)

Pressure depends on temperature
What supports weight of 1Mo star?
As it fuses H to He
(a main-sequence star)

Radiation/Gas pressures =
gravity (stable!)

Pressure depends on
temperature
What supports weight of 1Mo star?
As it fuses He to C
& H to He
(a red giant star)

Radiation/Gas pressures =
gravity (stable!)

Pressure depends on
temperature
What supports weight of 1Mo star?
After fusion stops
(beyond red-giant stage)
 Electrons
to the rescue!
 Degeneracy Pressure
(Pressure no longer depends on temperature)
Degeneracy Pressure
•
“Two particles cannot occupy
same space with same
momentum (energy)”
– Pauli exclusion principle
•
For very dense solids,
electrons cannot all be in
ground states
Degeneracy Pressure
•
Electrons become VERY
energetic--- velocities
approach speed of light.
•
Pressure holding up star
no longer depends on
temperature.
White Dwarfs

Very dense; 0.5 - 1.4 M packed into a
sphere the size of the Earth!
White Dwarf Stars

Stable! Gravitational pressure in =
electron degeneracy pressure out

Not fusing: Generates no new energy

Cooling off: Radiates heat into space,
getting fainter over time
White Dwarf Stars
•
Degenerate matter obeys different laws of physics
•
More massive star => smaller core becomes!
Limit on White Dwarf Mass

Predicted gravity will
overcome electron
degeneracy pressure
if white dwarf mass
greater than 1.4 M
Chandrasekhar Limit
Subrahmanyan Chandrasekhar
(1910-1995)
Subrahmanyan Chandrasekhar
Indeed, I would feel that an appreciation of
the arts in a conscious, disciplined way
might help one to do science better.
Nova!
Peak Brightness
2 months later
50,000 times
dimmer!
Nova!

If close binary star system:
Gravity pulls matter from nearby star
 Accretion disk around white dwarf
 Heats from Friction
 Infalling matter can possibly fuse!


White Dwarf suddenly, temporarily
gets much brighter….
Recurrent Novae
SUPERNova
(Type 1A)
If close binary star system
and
One star is a white dwarf
and
Companion transfers enough mass…


White Dwarf can exceed Chandresekhar
limit….
White Dwarf Supernovae
What if a star’s end-state core
is even larger?

Degeneracy applies to nuclear particles, too!

Collapses until neutron degeneracy pressure
holds up the corpse (neutron star)

If even neutron degeneracy can’t support the
weight of the core…
 Black
Hole!
High Mass Stellar Evolution

Much greater fusion rate

MUCH brighter stars

MUCH, MUCH shorter lives
High-Mass Stars – Ferraris!
Stars 10x larger than our Sun
 Fuse faster!
 Shine brighter!!
 Live very short lives…
But…
Make every element in your body after
Helium!

Even Larger Stars –Ferraris!
Evidence Supporting Theories

Periodic Table Abundances
 Multiples

Neutrinos from Supernova
 SN

of “4” match Helium fusion chain
1987a caught “early” in explosion
Cosmic Rays
The Periodic Table of Elements!
Periodic Table Abundances
Periodic Table Abundances
Atomic Masses
H=1
He = 4
C = 12
O = 16
N = 20
Mg = 24
Si = 28
S = 32
Fe = 56
Supermassive
stars lose
mass even
before they
blow up!
SUPERNova
(Type II) !
SUPERNova!

Why so powerful?

Lighter layers of the star have no support once
core stops fusing

Top layers fall onto heavier center & recoil fast

Collapse pushes electrons into protons

=> form NEUTRONS & Neutrinos
SUPERNova!

How long do they take to explode?

For a 20-Solar Mass star:

Formation:
Main Sequence:
Red giant fusing He:
Fusing Carbon:
Fusing Silicon to Iron:
Supernova Explosion:





100,000 years
10 Million years
1 Million years
1000 years
1 day
seconds!
A star explodes ….. 170,000 Light Years Away
A star explodes ….. 12 Million Light Years Away
A star explodes ….. 21 Million Light Years Away
A star explodes ….. 60 Million Light Years Away
A star explodes … 10 BILLIon Light Years Away
Neutron Stars
•
What is a neutron star? (THEORY)
•
What is a pulsar? (OBSERVATION)
•
What evidence do we have that they are
one in the same?
Neutron Star THEORY
•
Leftover cores from supernova explosions
•
Supported by neutron degeneracy pressure
•
Very TINY 1.5 M with a diameter of 10 to 20 km
Chandra X-ray image of the neutron star left
behind by a supernova observed in A.D. 386.
The remnant is known as G11.20.3.
Neutron Star THEORY
•
Very DENSE: (1012 g/cm3 ) & HOT
•
Very rapid Rotation: Period = 0.03 to 4 sec
•
VERY strong Magnetic fields: 1013 x Earth’s.
Chandra X-ray image of the neutron star left
behind by a supernova observed in A.D. 386.
The remnant is known as G11.20.3.
Neutron Star THEORY
Discovery of 1st Pulsar




In 1967, Jocelyn Bell & Anthony Hewish
accidentally discovered a pulsing radio source
Sharp pulse every 1.3 sec.
~1000 light years away
Called it a “pulsar”, but what was it?
The Crab Pulsar
The mystery was solved
when another pulsar was
discovered in the heart
of the Crab Nebula.
The Crab pulsar also
pulses in visual light.
Pulsar Observations
 Very
tiny pulse “width”
 Object
must be extremely small.
 Even white dwarf is too large!
 Very

regular pulse of energy
Occasional “Glitches” in signal
A
few seen in X-ray binary systems
 High
temperatures, large masses
Pulsar Observations
 Synchotron
emission --- radiation
emitted by charged particles moving
around strong magnetic fields
signal oscillation in “new”
Supernova Remnants
 Fastest
 Pulses
slow down over time
Process of Science!
Theory
1. Tiny
2. Rotating Fast
3. Strong Magnetic Field
4. Dense, Massive
5. Supernova Corpse
6. Energy From Rotation
Neutron Star = Pulsar!!
Theory
Observation
1. Tiny
Small Pulse Width
2. Rotating Fast
Regular Pulse up to
1000 times a second
3. Strong Magnetic Field
Synchrotron Radiation
4. Dense, Massive
X-ray Binary accretion
disks surround pulsar
5. Supernova Corpse
See in SN Remnants
6. Energy From Rotation
Slow Down over time
Pulsar Model
Model of Pulsar as Rotating Neutron Star
Pulsars vs. Neutron Stars?

All pulsars are neutron stars, but all
neutron stars are not pulsars!!

Seeing a pulsar depends on geometry.

if beam sweeps by Earth’s direction each
rotation, neutron star appears to be a pulsar
“Lighthouse” Model of Pulsar
Pulsars are the lighthouses of Galaxy!
Pulsars as Celestial Beacons
Neutron Stars as
Gamma Ray Bursters!

We sometimes see incredibly powerful,
and INCREDIBLY short bursts of gamma
ray radiation.

GRBs > 2 seconds ~ supernova and
collapse to a black hole

GRBs < 1 second ~ collision of TWO
merging neutron stars?
KEY “Key Terms” 
Chandrasekhar limit
cosmic ray
glitch
helium shell flash
helium shell fusion
lighthouse model
neutron degeneracy
pressure
neutron star
nova (plural novae)
nucleosynthesis
planetary nebula
pulsar
supernova
white dwarf
X-ray burster