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?
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

He core collapses, triggers expansion to
Red Giant

Fuses H to He in shell

Eventually fuses He to Carbon in Core

Creates dust grains in outer edges
Stellar Model of a Sun-Like Star
A red giant!
The Evolution of 1Mo Star

Not large enough to fuse Carbon to
heavier elements!
Central core eventually collapses
 Outer layers gradually “blow” off


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 “death shroud”

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?

Forming as a protostar!:

Thermal pressure < gravity! (collapsing!)

Pressure depends on temperature
What supports weight of 1Mo star?

Fusing H to He as
“main-sequence” star:

Radiation/Gas pressures =
gravity (stable!)

Pressure depends on
temperature
What supports weight of 1Mo star?

After Red Giant stage?
 No
longer fusing!
 Electrons
to the rescue!
 Degeneracy Pressure
(Pressure no longer depends on temperature)
Degeneracy Pressure
•
“Two particles cannot occupy same space with
same momentum (energy)”
•
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 Dwarfs
•
•
Degenerate matter obeys different laws of physics.
More massive star => smaller core becomes!
•
•
increased gravity makes star denser
greater density increases degeneracy pressure to
balance gravity
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.
What if end-state core is 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!
What if end-state core is larger?

Degeneracy applies to nucleus, too!
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
Even Larger 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!
SUPERNova!

Why so powerful?


Light layers fall onto heavier layers and recoil fast!
Collapse so electrons pushed into protons


=> form NEUTRONS & Neutrinos
Crush neutrinos together
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!
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, graduate student Jocelyn Bell and
her advisor Anthony Hewish accidentally
discovered a radio source in Vulpecula.
Sharp pulse recurred every 1.3 sec.
Determined it was 300 pc away.
They called it a “pulsar”, but what was it?
The Crab Pulsar
The mystery was solved when a 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 --- non-thermal
process where radiation is emitted by
charged particles moving close to the
speed of light around magnetic fields.
 Slow down over time
 Fastest signal oscillation in
Supernova Remnants
Neutron Star = Pulsar!!
Theory
1. Tiny
2. Rotating Fast
Observation
Small Pulse Width
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
Model of Pulsar as Rotating Neutron Star
“Lighthouse” Model of Pulsar
Pulsars are the lighthouses of Galaxy!
Pulsars as Celestial Beacons
Pulsars vs. Neutron Stars?

All pulsars are neutron stars, but all
neutron stars are not pulsars!!

Whether we see a pulsar depends on the
geometry.
if beam sweeps by Earth’s direction each
rotation, neutron star appears to be a pulsar
 if polar beam is always pointing toward or
always pointing away from Earth, we do not
see a pulsar

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