Title: James J Marie, Astronomy, 2005
1Stellar Exotica
James J Marie, Astronomy, 2005
2White 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
3Planetary 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
4Composition 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
5Size 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
6Degenerate 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
7White Dwarf Matter
- A piece of white dwarf matter the size of a
sugar cube would weigh 5 tons!
James J Marie, Astronomy, 2005
8White 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
9Chandrasekhar 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
10White Dwarf Binary Systems
- Many white dwarfs are part of a binary system.
? Hot spot
James J Marie, Astronomy, 2005
11Close 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
12Accretion 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
13Renewed 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.
James J Marie, Astronomy, 2005
14Nova
- 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
15White 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
16Standard 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
17Neutron 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
18Neutron 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
19Nature 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
20Inside 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
21Relative Size of a Neutron Star
James J Marie, Astronomy, 2005
22Lone 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
23Pulsars
- 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
24Beacons 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
25Magnetar
- 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
26Quark Star
- A quark star is a hypothetical star thought to
be composed entirely of free - quarks.
James J Marie, Astronomy, 2005
27Relative Size of a Quark Star
James J Marie, Astronomy, 2005
28Black Holes
- Black holes are the most bizarre and exotic
objects in the universe!
James J Marie, Astronomy, 2005
29A 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
30A 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
31A Glimpse From Relativity
- Black holes are mathematically predicted by
Einsteins general theory - of relativity.
James J Marie, Astronomy, 2005
32Curved 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
33What 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
34Event 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
35Schwarzschild 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
James J Marie, Astronomy, 2005
36Schwarzschild 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
37Singularity
- 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
38Core 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
39RXJ1242-11
- 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
40V4641 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
41James J Marie, Astronomy, 2005