The Bizarre Stellar Graveyard

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The Bizarre Stellar Graveyard
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Sirius A And Sirius B In X-ray
Sirius A
Sirius B
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White Dwarfs...

• ...are stellar remnants for low-mass stars.
• ...are found in the centers of planetary nebula.
• ...have diameters about the same as the Earths.
• contain extremely dense matter - a pair of dice
  made of white-dwarf matter would weight about 5
  tons on Earth.
• are built of a degenerate matter.

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Degenerate matter
It is a matter which has so high density that the
dominant contribution to its pressure arises from
the Pauli exclusion principle (1925). The Pauli
principle forbids two particles from having
identical quantum states.The pressure maintained
by a body of degenerate matter is called the
degeneracy pressure.
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White dwarfs degenerate obejcts

• High gravity produces a high pressure and the
  matter degenerates, i.e. turns to a plasma
  whose electrons are degenerated.
• White dwarfs are stable since their
• gravity is compensated by electron
• degeneracy pressure.
• At 1.4 MSun the electrons pressure cannot resist
  to the crush of gravity. This is the
  Chandrasekhar limit to a white dwarfs mass.

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White dwarfs in Close Binary Systems

• White dwarfs in close binary systems can gain
  matter from a companion through an accretion
  disk.
• Particles in the disk obey Keplers laws. Inner
  parts move faster than outer parts creating
  friction and heating.
• Particle orbits become smaller and smaller until
  they fall onto the WD.

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Novae
Diagram of nova process

• Dwarf nova sudden increase in brightness by
  factor 10 due to enhanced accretion (disk
  instability).
• Nova Thermonuclear flash of H-shell burning.
  L105LSun.
• Nova recur each 102-104 years.

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Nova Herculis
March 1935
May 1935
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Nova T Pyxidis (HST)
A nova occurs when hydrogen fusion ignites on the
surface of a white dwarf star system
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Novas and Supernovas

• Nova - a stellar explosion, happens periodically
  in close binary systems that contain a white
  dwarf
• 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 stars iron core collapses

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White Dwarf Supernovae (Type I)

• Accreting white dwarfs can gain net mass that
  reaches the 1.4MSun limit (Chandrasekhar limit).
• When gravity overcomes electron degeneracy, the
  white dwarf collapses until the temperature
  reaches the threshold of Carbon fusion.
• Carbon ignites throughout, making the white dwarf
  explode into a supernova (Type I).
• The supernova shines with L1010LSun.
• No remnant results from a Type I supernova.

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Semidetached Binary System With White Dwarf Star
(may result in a white dwarf
(type I ) supernova)
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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.

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On July 4, 1054 astronomers in China witnessed a
supernova within our own galaxy.
The remnant of this explosion is The Crab
Nebula
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Supernova 1987a
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Supernova 1998S inNGC 3877
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The Iron Wall

• Iron is the only one element from which it is not
  possible to generate any kind of nuclear energy.
• Average mass per nuclear particle from hydrogen
  to iron decreases and then increases for atomic
  masses greater than iron.

Iron has the lowest mass per nuclear particle of
all nuclei (cannot convert mass to energy) It
cannot release energy by either fusion or
fission. It is the most stable element in the
universe.
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Heavier elements are created in the final stages
of life of massive stars
Formation of elements beyond iron occurs very
rapidly as the star approaches supernova.
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• 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.

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Supernova Remnants
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Neutron Stars

• ...are stellar remnants for high-mass stars.
• ...are found in the centers of
• some type II supernova remnants.
• ...have diameters of
• about 6 miles.
• ...have masses greater than the Chandrasekhar
  mass (1.4M?) but less than 3 M?
• The pressure inside a neutron star is so high
  that electrons get pressed in protons and form
  neutrons

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Relative Sizes
Neutron Star
Earth
White Dwarf
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Neutron Stars

• A giant ball of neutrons.
• Mass at least 1.4 x mass of the Sun.
• Diameter 20 km!
• Density 1018 kg/m3
• A thimble weighs as much as a mountain

• Day 1 0.001 seconds!
• Magnetic fields as strong as the Sun, but in the
  space of a city.

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How were Neutron Stars discovered?

• In 1967 Jocelyn Bell observed a strange radio
  pulse that had a regular period of 1.3373011
  seconds.
• This graph is a radio light curve a plot of
  intensity (of radio emission) versus time (in
  seconds).

Jocelyn Bell
Anthony Hewish

• In 1974, Jocelyn Bells teacher, Anthony Hewish,
  won a Nobel Peace prize for
• his role in the discovery of pulsars.

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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 Bells data- the first
pulsar discovered
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Pulsars

• 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.

• The pulsing star inside the Crab Nebula was a
  pulsar.
• Pulsars are rotating, magnetized neutron stars.

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The Crab Pulsar
Period 0.033 seconds 33 milliseconds

• It is shown that the period of pulsation is the
  period of rotation of the object and so small
  period of rotation is the fact, which proved
  that pulsars are neutron stars.

Show that an object that has a period of rotation
33 millisecond is a neutron star (calculate its
size and mass using the Newton Gravity law)
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Pulsars

• While every pulsar is a neutron star, the
  opposite isnt true.
• Interstellar Lighthouses since they produce
  periodic bursts of radiation.
• Perfect clocks the period of pulsation is very
  stable

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Rotating Neutron Star
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Synchrotron Radiation
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Light House model of neutron star emission
accounts for many properties of observed Pulsars
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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.

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Rotation Rates of Pulsars

• The neutron stars that appear to us as pulsars
  rotate about once every second or less.
• Before a star collapses to a neutron star it
  probably rotates about once every 25 days.
• Why is there such a big change in rotation rate?
• Answer Conservation of Angular Momentum

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Mass Limits

• Low mass stars
• Less than 8 M? on Main Sequence
• Become White Dwarf (
• Electron Degeneracy Pressure
• High Mass Stars
• Less than 100 M? on Main Sequence
• Become Neutron Stars (1.4M?
• Neutron Degeneracy Pressure

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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.
• are found in some binary star systems and there
  may be super-massive black holes in the centers
  of some galaxies.

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General Relativity Theory

• Matter distorts space like weights on a taut
  rubber sheet.
• The greater the mass, the greater the distortion
  of space.

Einsteins theory of gravitation produces the
same results as Newtons theory when the masses
of the bodies are small, but one must use
Einsteins theory if the masses are large (for
example, considering black holes, entire Galaxy,
the Universe)
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The Sun
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Bending of Starlight
1.75
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Predictions
Observations
Einsteins predictions were confirmed when the
positions of stars near the sun were observed to
be shifted during a 1919 solar eclipse.
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Low Gravity
Very small amount of bending
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Stronger Gravity
Light at an angle is bent noticeably
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Exit Cone and Photon Sphere
Photon Sphere
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Comparing the Denting of Space
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Black Holes in General Relativity
If an object is massive and small, gravity
overwhelms pressure and the object collapses.
Gravity is so strong that nothing, not even
light, can escape.
For an observer Radius 2GM/c2
2 miles for a solar mass NOT a solid
surface In reality All Mass at the Center

(point-like object)
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The Event Horizon Radius
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Black Holes

• All of the fireworks and drama
• originate close to the black holes
• Far away from a black hole, gravity
• is no different than for any
• other object of the same mass
• If a black hole were to replace the sun, the
  orbits of planets, asteroids, moons, etc., would
  be unchanged.

A Cosmic Vacuum Cleaner?
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The tidal force produced by a black hole destroys
a star
The only way to see a black hole is to see the
material of other stars it sucks.
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Question
Black holes can be part of a binary system,
forming an accretion disk around the black hole,
which heats up to a few million K. In which
wavelength band will this disk preferentially
radiate?
a) Infrared
b) Red
c) Blue
d) X-rays
e) Radiowaves
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Seeing Black Holes

• Cant see black hole itself, but can see matter
  falling into a hole.
• Gravitational forces stretch and rip matter
  heats up.
• Very hot objects emit in X-rays (interior of Sun)
• Cygnus X-1.

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Cygnus X-1
Accretion disk
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How accretion disks form tidal disruption of
stars
Black hole
Star
View from high above, along orbits axis.
Simulation by P. Quinn and G. Sussman, 1985
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Structure of an accretion disk
Jet
Not drawn to scale!
Innermost stable orbit
Outgoing X and g rays, heating disk and
accelerating jets
Ingoing matter, being accreted
Horizon
Accretion disk (cross-section view)
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Origin of the jets charged particles that move
in a strong magnetic field
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The Search for Black Holes

• Cygnus X-1
• 10-15 solar masses well above limit of 3 for
  neutron stars
• Few dozen in binaries

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SS 433
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