Transcript Chapter 14

Chapter 13
The Bizarre Stellar Graveyard
White Dwarfs...

...are stellar remnants for low-mass stars.

...are found in the centers of planetary nebula.

...have diameters about the same as the Earth’s.

...have masses less than the Chandrasekhar mass.
Sirius B is a white dwarf star
Sirius A And Sirius B In X-ray
Sirius A
Sirius B
Novas and Supernovas

Nova - a stellar explosion

Supernova - a stellar explosion that marks the end
of a star’s evolution

White Dwarf Supernova (Type I supernova)occur in binary systems in which one is a white
dwarf

Massive Star Supernova (Type II Supernova) occur when a massive star’s iron core collapses
Close Binary Systems and Mass Transfer
Nova
March 1935
Herculis
May 1935
Diagram of
nova process
A nova
occurs when
hydrogen
fusion
ignites on
the surface
of a white
dwarf star
system
Nova T Pyxidis
(HST)
Light Curve of typical Nova
Semidetached Binary System With White Dwarf Star
(may result in a white dwarf (type I ) supernova)
Type II Supernova

The star releases more energy in a just a few minutes
than it did during its entire lifetime.
» Example: SN 1987A

After the explosion of a massive star, a huge glowing
cloud of stellar debris - a supernova remnant - steadily
expands.
» Example: Crab Nebula

After a supernova the exposed core is seen as a neutron
star - or if the star is more than 3 solar masses the core
becomes a black hole.
On July 4, 1054 astronomers in China
witnessed a supernova within our own galaxy.
The remnant of this explosion is
The Crab Nebula
Supernova 1987a
Type I and Type II Supernova
Supernova Light Curves
Hydrogen and Helium Burning
4(1H )4He  energy 2neutrinos 2 positrons
3( He) C  energy
4
12
Carbon Burning and Helium Capture
C 12 C24Mg  energy
12
C 4He16O  energy
12
Still heavier elements are created in the
final stages of life of massive stars
28
Si  7( He) Ni  energy
4
56
Alpha Process –
Helium Capture
produces heavier
elements up to
Fe and Ni.
Elements beyond Fe and Ni involve neutron capture.
This forms unstable nuclei which then decay into
stable nuclei of other elements
57
Fe  n Fe
58
Fe  n Fe
58
59
Formation of Elements beyond Iron occurs very
rapidly as the star approaches supernova.
 The
supernova explosion then distributes
the newly formed matter throughout the
interstellar space (space between the stars).
 This
new matter goes into the formation
of interstellar debris.
 The
remnant core is a dense solid core of
neutrons – a neutron star!
Neutron Stars

...are stellar remnants for high-mass stars.
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...are found in the centers of some type II supernova
remnants.
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...have diameters of about 6 miles.
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...have masses greater than the Chandrasekhar
mass. (1.4M)
Relative Sizes
Earth
White Dwarf
Neutron Star
Pulsars


The first pulsar observed was originally thought to be signals
from extraterrestrials.
(LGM-Little Green Men was their first designation)
Period = 1.337301 seconds exact!
~ 20 seconds of Jocelyn Bell’s data- the first pulsar discovered

It was later shown to be unlikely that the
pulsar signal originated from
extraterrestrial intelligence after many
other pulsars were found all over the sky.
Pulsars
 The
pulsing star inside the Crab Nebula
was a pulsar.
 Pulsars
stars.
are rotating, magnetized neutron
The Crab Nebula
The Crab Pulsar
Period = 0.033 seconds = 33 milliseconds
Light House Model
– Beams of radiation emanate from the
magnetic poles.
– As the neutron star rotates, the beams
sweep around the sky.
– If the Earth happens to lie in the path of
the beams, we see a pulsar.
Rotating
Neutron Star
Light House model of neutron star emission
accounts for many properties of observed Pulsars
Artistic
rendering
of the
light
house
model
Rotation Rates of Pulsars
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The neutron stars that appear to us as pulsars
rotate about once every second or less.
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Before a star collapses to a neutron star it
probably rotates about once every 25 days.
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Why is there such a big change in rotation rate?
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Answer: Conservation of Angular Momentum
Neutron –Star Binaries
Mass Limits

Low mass stars
– Less than 8 M on Main Sequence
– Become White Dwarf (< 1.4 M)
» Electron Degeneracy Pressure

High Mass Stars
– Less than 100 M on Main Sequence
– Become Neutron Stars (1.4M < M < 3M)
» Neutron Degeneracy Pressure
Black Holes

...are stellar remnants for high-mass stars.
– i.e. remnant cores with masses greater than 3 solar
masses

…have a gravitational attraction that is so
strong that light cannot escape from it.
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…are found in some binary star systems and
there may be super-massive black holes in the
centers of some galaxies.
Supermassive Stars
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If the stellar core has more than three solar
masses after supernova, then no known
force can halt the collapse
Black Hole
Black holes were first predicted by the
General Theory of Relativity, which is
theory of gravity that corrects for some of
the short-falls of Newton’s Theory of
Gravity.
In general Relativity, space, time
and mass are all interconnected
Space-Time
No mass
Distortion
caused by
mass
Predictions of General Relativity
Advance of Mercury’s perihelion
 Bending of starlight

Advance of Mercury’s Perihelion
43” per century not due to perturbations
from other planets
Bending of Starlight
1.75”
Apparent position
of the star
Sun
Light from star bent by
the gravity of the Sun
Schwarzschild Black Hole
Event Horizon
Rs = 3(Mass)
+
Rs
Singularity
Mass
Rs
3 M
9 km
5
15
10
30
Near a Black Hole
What Can We Know?

Mass
– gravity
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Charge
– Electric Fields
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Rotation Rate
– Co-rotation
How Can We Find Them?
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Look for X-ray sources
– Must come from compact source
» White Dwarf
» Neutron Star
» Black Hole
– Differentiate by Mass
» WD - < 1.4 M
» NS - between 1.4 and 3 M
» BH - > 3 M
Cygnus X-1
End of Chapters
End of Section.
Nucleosynthesis
Evolutionary Time Scales for a 15 M Star
Fused
H
4
He
12
C
20
Ne +
16
O
28
Si +
56
Fe
Products
4
He
12
C
16
20
O, Ne,
24
Mg, 4He
16
O, 24Mg
28
Si, 32S
56
Fe
Neutrons
Time
7
Temperature
10 yrs.
Few X 106 yrs
1000 yrs.
4 X 106 K
1 X 108 K
6 X 108 K
Few yrs.
One year
Days
< 1 second
1 X 109 K
2 X 109 K
3 X 109 K
> 3 X 109 K
Energy Budget
H
He
C
Fusion Stages
Fe
Anazasi Pictographs
Supernova 1998S in
NGC 3877
Supernova Remnants
Tycho’s SNR - 1572
PSR 0628-28
LGM?
Several more found at widely different
places in the galaxy
 Power of a power equals total power
potential output of the Earth
 No Doppler shifts
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PULSARS
Light Time Argument
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An object which varies its light can be no
larger than the distance light can travel in
the shortest period of variation.
To Darken the Sun
Time Delay = Radius/c
500,000 km/300,000 km/s = 1.67 sec
Only candidates: White Dwarfs, Neutron Stars
Pulse Mechanisms

Binary Stars - How quickly can two stars orbit?
 Two WD about 1m
 Two NS about 1s.
 Neutron Stars in orbit should emit gravity waves which should
be detectable.

Oscillations - Depends only on density
 WD about ten seconds
 NS about .001s Little variation permitted.

Rotation - Until the object begins to break up.
 WD about 1s
 NS about .001s with large variation.
SS 433
Synchrotron Radiation
Radiation
Magnetic lines of force
Electron
Glitches