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James J Marie, Astronomy, 2005

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Title: James J Marie, Astronomy, 2005


1
Stellar Exotica
James J Marie, Astronomy, 2005
2
White Dwarfs
  • A white dwarf is the inert core left over after
    a low mass star has exhausted
  • all of its nuclear fuel.

James J Marie, Astronomy, 2005
3
Planetary Nebula
  • White Dwarfs are often
  • seen at the center of
  • planetary nebula.
  • Shown here is a young
  • planetary nebula
  • PKS285-02 ?
  • The complex geometry
  • of this nebula may
  • have been produced
  • by high speed,
  • collimated outflows
  • of gas during the late
  • phase of this stars
  • evolution.

James J Marie, Astronomy, 2005
4
Composition of White Dwarfs
  • A white dwarf is made of
  • elements from a stars
  • final nuclear burning stage.
  • Very low mass stars end
  • up as helium white dwarfs.
  • Stars of 1 solar mass
  • eventually leave behind
  • white dwarfs made of
  • carbon.
  • Intermediate mass stars
  • may leave behind cores
  • containing a mixture of
  • carbon, oxygen and
  • possibly a few heavier
  • elements.

James J Marie, Astronomy, 2005
5
Size of a White Dwarf
  • A one solar mass star evolves into a white
    dwarf about the size of the
  • Earth.
  • The density of a white dwarf is extraordinary
    (a million times greater than
  • the Sun)!

James J Marie, Astronomy, 2005
6
Degenerate Matter
  • A white dwarf is exotic because general
    relativity and quantum physics are
  • needed describe its internal state.
  • The matter in a white dwarf is degenerate.
  • This means that the electrons in the atoms that
    make up the white
  • dwarf are squeezed so tightly together that
    they move nearly at the
  • speed of light.
  • The momentum of the electrons produce pressure
    within the white
  • dwarf called degeneracy pressure.
  • The degeneracy pressure prevents the white
    dwarf from collapsing
  • in upon itself under its own weight.

James J Marie, Astronomy, 2005
7
White Dwarf Matter
  • A piece of white dwarf matter the size of a
    sugar cube would weigh 5 tons!

James J Marie, Astronomy, 2005
8
White Dwarf Masses
  • Oddly, more massive white dwarfs are smaller in
    size than less massive
  • white dwarfs.
  • This is because gravity compresses the more
    massive white dwarfs to a
  • smaller size.

James J Marie, Astronomy, 2005
9
Chandrasekhar Limit
  • An Indian astrophysicist, S. Chandrasekhar
    discovered that white dwarfs have
  • a maximum mass limit.
  • The mass of a white dwarf cannot exceed 1.4
    solar masses.
  • When the mass reaches 1.4 solar mass, the
    electrons within the white dwarf
  • are forced to move at almost the speed of
    light.
  • Since nothing can move faster than the speed of
    light, the degeneracy pressure
  • created by the electrons cannot prevent a
    collapse if the mass exceeds 1.4
  • solar masses.

For masses greater than 1.4 solar masses,
destruction is inevitable.
James J Marie, Astronomy, 2005
10
White Dwarf Binary Systems
  • Many white dwarfs are part of a binary system.

? Hot spot
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11
Close Binary With a White Dwarf
  • If the companion of the white dwarf is a main
    sequence star, the strong
  • gravitational field of the white dwarf can
    pull matter from the companion
  • onto the white dwarf.

James J Marie, Astronomy, 2005
12
Accretion Disk
  • As the matter falls toward the white dwarf, it
    swirls into an accretion disk.
  • Friction within the disk causes the gas in the
    disk to become very hot.
  • The gases are hot enough to glow.

James J Marie, Astronomy, 2005
13
Renewed Life
  • Accretion in a binary system
  • adds mass to the white dwarf.
  • A thin shell of hydrogen gas
  • can build on the surface of
  • the white dwarf.
  • The pressure and temperature
  • at the bottom of the shell
  • increases as the mass continues
  • to fall onto the white dwarf.
  • When the temperature reaches
  • 10 million K, hydrogen fusion
  • is ignited.
  • This produces a thermonuclear
  • flash and the white dwarf
  • blossoms into a brilliant nova.

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14
Nova
  • For few glorious weeks the
  • nova shines as bright as
  • 100,000 Suns!
  • Heat and pressure from
  • nova ejects material into
  • interstellar space creating
  • a nova remnant.
  • The nova process can repeat
  • many times!
  • A nova remnant from a
  • white dwarf in a binary
  • system called T Pyxidis ?
  • The bright spot at the center
  • is the binary star system.

James J Marie, Astronomy, 2005
15
White Dwarf Supernovae
  • If conditions are just right, a white dwarf may
    accumulate mass despite the
  • reoccurring nova.
  • Eventually, the mass may reach the
    Chandrasekhar limit of 1.4 solar masses.
  • The temperature rises high enough to trigger
    carbon fusion inside of the white
  • dwarf.
  • Because the white dwarf is made of degenerate
    matter, the entire star
  • catastrophically explodes!
  • This carbon bomb is a Type I Supernova.
  • Type I supernovae are even brighter and more
    powerful than a supernova
  • triggered by the collapse of a massive star
    (Type II supernova)!
  • Both types of supernovae can reach a peak
    luminosity of 10 billion Suns!

James J Marie, Astronomy, 2005
16
Standard Candle
  • Because Type I supernova occur at the
    Chandrasekhar limit, their absolute
  • luminosities are all identical.
  • Furthermore, their absolute luminosity is
    known.
  • Therefore, Type I supernova provide a way of
    measuring distances in
  • the cosmos.

apparent brightness
Luminosity distance formula
  • A candle of a given brightness
  • appears dimmer as you move
  • farther away from it.
  • Therefore, the apparent dimness of
  • the candle is a measure of distance.

James J Marie, Astronomy, 2005
17
Neutron Stars
  • A neutron star is one of natures most bizarre
    objects.
  • Neutron stars are composed mainly of degenerate
    neutrons held together
  • by a fantastically strong gravitational field.
  • The force of gravity at the surface of a
    neutron star is a 200 billion times
  • stronger than the force of gravity at the
    surface of the Earth!
  • If you were to drop an object onto a neutron
    star from space, it would hit the
  • surface at half the speed of light!
  • The density of a neutron star is so high that a
    teaspoon of neutron star
  • matter would weigh a billion tons on Earth!

James J Marie, Astronomy, 2005
18
Neutron Stars
  • A neutron star is created
  • during the collapse of the
  • iron core of a massive star
  • in a supernova.
  • The surface of a neutron star
  • would be a very unpleasant
  • place to visit in addition to
  • being vaporized by the
  • intense heat, you would be
  • squashed to a dimension
  • smaller than an atom!
  • If you were to approach a neutron star from an
    initially far distance, the
  • incredibly strong magnetic field would
    scramble the atoms inside of
  • your body long before you reached the surface!
  • Neutron stars may have an atmosphere only a few
    centimeters thick and
  • mountain ranges poking up only a few
    centimeters. A neutron star is

James J Marie, Astronomy, 2005
19
Nature of Neutron Stars
  • Typically, neutron stars have a
  • mass of 1.4 times the Sun and
  • and a diameter of only 15 miles!
  • Some neutron stars may have a
  • mass as high as 3 solar masses.
  • Neutron stars rotate very rapidly
  • it takes much less than a second
  • for one complete rotation!
  • The magnetic field of a neutron
  • star is 1 trillion times stronger
  • than the magnetic field of the
  • Earth!
  • The magnetic fields direct intense
  • beams of radiation from the
  • magnetic poles.

James J Marie, Astronomy, 2005
20
Inside a Neutron Star
  • The crust is probably
  • a crystalline form of
  • iron.
  • The interior is probably
  • a strange quantum
  • superfluid of neutrons.
  • The core may be a sea
  • of pions, kaons or
  • quarks (no one really
  • knows).
  • The rotation of this odd
  • ensemble probably
  • produces quantum
  • vortices within the star.

James J Marie, Astronomy, 2005
21
Relative Size of a Neutron Star
James J Marie, Astronomy, 2005
22
Lone Neutron Star
  • The Hubble Space Telescope
  • provided the first direct look in
  • visible light at a neutron star.
  • The surface of the star is very
  • hot (1.2 million ?F).
  • The radius is no larger than 16.8
  • miles.
  • No other object except a neutron
  • star can be this hot, small and
  • dim.

James J Marie, Astronomy, 2005
23
Pulsars
  • The magnetic poles of a
  • neutron star are not
  • generally aligned with the
  • rotation axis.
  • So as the star rotates, the
  • beams of radiation sweep
  • around just like the light
  • beacon from a lighthouse.
  • If a distant observer is in the
  • path of the sweeping beam
  • of radiation, a series of
  • pulses will be observed.
  • Each pulse corresponds to the moment when the
    radiation beam sweeps past.
  • A neutron star observed by the regular pulses
    is known as a pulsar.

James J Marie, Astronomy, 2005
24
Beacons of the Cosmos
  • The radiation pulses are mainly in
  • the form of radio waves and are
  • observed with radio telescopes.
  • Pulsars can spin very fast up to 1
  • rotation every millisecond!
  • The pulses are very regular and can
  • be used as an extremely precise
  • clock!
  • A series of pulses received from
  • the pulsar at the center of the Crab
  • Nebula.

James J Marie, Astronomy, 2005
25
Magnetar
  • A magnetar is a neutron
  • star with an extremely
  • strong magnetic field.
  • The magnetic field of a
  • magnetar would be lethal
  • even if you were 370
  • miles away!
  • Objects known as soft
  • gamma ray repeaters or
  • x-ray pulsars are thought
  • to be explained by
  • magnetars.

James J Marie, Astronomy, 2005
26
Quark Star
  • A quark star is a hypothetical star thought to
    be composed entirely of free
  • quarks.

James J Marie, Astronomy, 2005
27
Relative Size of a Quark Star
James J Marie, Astronomy, 2005
28
Black Holes
  • Black holes are the most bizarre and exotic
    objects in the universe!

James J Marie, Astronomy, 2005
29
A Vanishing Star
  • Most massive stars end their life by imploding
    and creating a neutron star
  • during a supernova.
  • Most of the stars mass is blown into space.
  • But sometimes, matter falling onto the core can
    raise the mass of the core
  • beyond the neutron star mass limit.
  • The degeneracy pressure of the neutrons (or
    quarks?) can no longer support
  • the core against the crushing force of
    gravity.

The core implodes and completely vanishes from
existence!
James J Marie, Astronomy, 2005
30
A Hole in Space
  • The mass and energy of the imploded core
    severely warps spacetime into
  • a hole.
  • It is a hole, because anything that enters into
    a black hole can never return
  • to the observable universe.
  • Gas swirling into a black
  • hole.
  • The accretion disk is warped
  • by spacetime around the
  • black hole.

James J Marie, Astronomy, 2005
31
A Glimpse From Relativity
  • Black holes are mathematically predicted by
    Einsteins general theory
  • of relativity.

James J Marie, Astronomy, 2005
32
Curved Spacetime
  • Spacetime is strongly curved near a black hole.
  • We can try to visualize this curvature with a
    2-dimensional sheet

? flat spacetime
  • curved spacetime near a
  • black hole

Note A black hole does not literally look
like a funnel.
James J Marie, Astronomy, 2005
33
What does a Black Hole Look Like?
  • From general relativity,
  • the visual appearance
  • of a distant black hole
  • can be calculated or
  • simulated.
  • Here is a simulated
  • black hole of 10 solar
  • masses seen from a
  • distance of 372 miles
  • with the Milky Way in
  • the background.
  • (horizontal camera
  • opening angle 90?)

James J Marie, Astronomy, 2005
34
Event Horizon
  • The event horizon is an imaginary surface that
    surrounds the black hole.
  • Anything that passes through the event horizon
    can never return to the
  • observable universe.
  • At the event horizon, the escape velocity is
    equal to the speed of light.
  • Within the event horizon, the escape velocity
    exceeds the speed of light.

James J Marie, Astronomy, 2005
35
Schwarzschild Radius
  • The Schwarzschild radius is the physical size
    of the black hole.
  • The location of the event horizon is determined
    by the Schwarzschild radius
  • and is given by

rs Schwarzschild radius M Mass c speed
of light G Newtons universal gravitation
constant
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36
Schwarzschild Radius
  • All objects have a Schwarzschild radius.
  • It simply means that if you were able to
    squeeze the object of a given mass to
  • the size of its Schwarzschild radius, you
    would create a black hole!

Schwarzschild radius of Sun 1.83 miles
Schwarzschild radius of Earth 0.349 inches
James J Marie, Astronomy, 2005
37
Singularity
  • The center of a black hole is a place where
    gravity has crushed matter into
  • an infinitely small point with infinite
    density.
  • No one really understands the singularity.
  • All of the known laws of physics break down at
    the singularity.
  • A quantum theory of gravity is needed before we
    can begin to understand
  • what is at the singularity.

James J Marie, Astronomy, 2005
38
Core of Galaxy NGC 4261
  • A ring of gas forms around a
  • suspected super massive
  • black hole at the center of
  • NGC 4261.

James J Marie, Astronomy, 2005
39
RXJ1242-11
  • An artists impression of a star being ripped
    apart by a giant black hole as
  • recently seen by the Chandra x-ray telescope.

James J Marie, Astronomy, 2005
40
V4641 Sgr
  • V4641 is the closest candidate black to the
    Earth at only 1500 light years
  • away.
  • Here is a radio image of a dramatic explosion
    from matter falling into the
  • black hole.

James J Marie, Astronomy, 2005
41
James J Marie, Astronomy, 2005
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