White Dwarf in a Planetary Nebulae
White Dwarfs, Neutron Stars & Supernova
A. Kranf
Once a star runs out of fuel gravity has to win. The victory is often violent
The victory may not be final however. While the loss of burn-able nuclear fuel may mean the end of a stars lifetime producing energy "exotic" quantum mechanical forces can halt the total collapse of a star.
The final fate of a star depends on its mass.
Black Holes are so weird we will
save them for later.
When a star like the sun exhausts all its fuel what is left is an ultra dense core of carbon and perhaps oxygen.
There is not enough matter above the core to get the carbon to burn. The core is inert.
Contraction squeezes the core until it reaches a density of 3x109 kg/m3
A solar mass white dwarf would have the same size as the Sun!
A spoonful of this stuff would weigh as much as a car!
At these densities the electrons are squeezed so tightly that that uncertainty principle comes into play.
The gas is degenerate and the degeneracy pressure provides the support to keep the star (which now just the core and a little bit of H and He above it) against its own gravity.
The carbon nuclei are probably arranged in an ordered crystal pattern.
In a sense the star is a diamond in the sky.
White Dwarf in a Planetary Nebulae
In 1935 Subrahmanyan Chandrasekhar produced the first theoretical models of a white dwarf (they were not observed yet).
He found that there was an upper
limit to the mass of a white dwarf
Mwd < 1.4 Solar Masses.
This is called the Chandrasekhar limit. Anything bigger than 1.4 Solar Masses can not be held up by electron degeneracy pressure.
He also found that the more massive a white dwarf is the smaller it is!!! Why is this?
White dwarfs still produce light as thermal energy slowly leaks out into space. Eventually all white dwarfs cool and become black dwarfs ( as dead as a dead CINDER of a star can get!)
About 1000 white dwarfs have been
observed.
If the core of star has greater than 2 solar masses what happens? If degenerate electron pressure can not support the core against gravity then what can?
When the collapse of a medium mass or massive star occurs (during a process which leads to a supernova) the core is squeezed so tightly that a process called inverse beta decay occurs.
Beta decay is a standard reaction
in radioactive elements: a neutron spontaneously "decays" into a proton
and an electron with a neutrino produced in the process.
n -> p+ + e- + neutrino
Inside a collapsing star with Mcore
> 1.4 solar masses the densities are so high that the protons and electrons
get squeezed together and are forced to form a neutron.
p+ + e- -> n + neutrino
This process is also called neutronization.
Since all the stuff with electrical charge is gone its now possible to squeeze everything even tighter together.
The whole core reaches the same
density as an atomic nucleus
Density = 1017 kg/m3.
In a sense the star becomes one giant nucleus!!!
One teaspoon of this stuff weighs a billion tons. One thimble full would weigh more than all the skyscrapers in Manhattan!
The neutrons now become a degenerate
gas! It is their degeneracy pressure which now supports the star.
Neutron stars are very strange places.
A 1.4 solar mass star would only be 10 km across.
Size Comparison Of Neutron Star To NY City - Neutron stars are not much larger than many of the Earth's major cities. In this fanciful comparison, a typical neutron star sits alongside Manhattan Island.
An atmosphere of gas surrounds a crystal crust.
Gravity has a maniacal grip on the star. Its so strong that nothing on the crust could be more than a centimeter above the surface.
There is a Inner Crust which is mostly neutrons.
Below that there is a superfluid core. The densities and pressures in the core of the core are so high that they may also contain totally exotic states of matter.
A marshmallow falling from 1 AU would hit the surface with the force of a multimegaton hydrogen bomb!
There seems to be an upper limit
to Neutron Star masses as well. Anything with more than 2 solar masses
can not be supported by neutron degeneracy pressure.
If the core of a massive star has more than 2 solar masses no none force can stop its collapse.
Gravity wins finally and forever.
The star collapses all the way.
In 1967 two astronomers (a prof and a grad student) working with radio telescopes discovered a source of intense but also periodic bursts of radio emission.
The pulses with astonishing regularity. One pulse every 1.33730113 seconds
Pulses From Neutron Star - Pulsars emit periodic bursts of radiation. This recording shows the regular change in the intensity of the radio radiation emitted by the first such object known. It was discovered in 1967. Some of the pulses are marked by arrows.
At first it was thought that extra-terrestrial intelligence was found.
Later it was realized that these cosmic clocks where really a form of neutron star.
The became known as Pulsars.
One of the astronomers the prof,
got the noble prize. The other (Jocelyn Bell) got the shaft.
Optical View Of Pulsar In Crab Nebula - (1 of 2) - The pulsar in the core of the Crab Nebula blinks on and off about 30 times each second. In this pair of closely spaced optical images, the pulsing can be seen clearly.
X-Ray View Of Pulsar In Crab Nebula - (2 of 2) - The same phenomenon is also detected in X rays.
A pulsar is a spinning neutron star with an intense magnetic field ( 109 stronger than the suns field).
The axis or rotation and magnetic axis are not aligned.
As the star rotates radio waves
produced by hot gas in the atmosphere is channeled by the magnetic field.
Lighthouse Model Of Neutron Star - This diagram of the "lighthouse model" of neutron-star emission accounts for many of the observed properties of pulsars. Charged particles, accelerated by the magnetism of the neutron star, flow along the magnetic field lines, producing radio radiation that beams outward.
The pulsar forms a rotating radio beam - this is the lighthouse effect.
If the earth is in the path of the beam then with every sweep of the beam we catch a pulse of the pulsar.
So far more than 350 pulsars have been found.
Pulsars make incredibly accurate clocks. Each pulse repeats itself with a timing that is better than one part in a million.
Most pulses repeat on the order of a second. The pulses themselves last for only a millisecond (10-3 sec).
Sometimes we see jumps in the pulses called "glitches". These may be due to neutron star quakes where the surface cracks under the enormous gravity.
Pulsars also slow down! This happens
because the pulsar is losing rotational energy as it emits energy. The
Crab Nebula - a great example.
Remnants
Of The Crab Nebula - This remnant of an ancient supernova is called
the Crab Nebula (or M1 in the Messier catalog). It resides about 1800 pc
from the Earth and has an angular diameter about one-fifth that of the
full Moon. Because its debris is scattered over a region of "only" 2 pc,
the Crab is considered to be a young supernova remnant. In A.D. 1054 Chinese
astronomers observed the supernova explosion itself. The center frame shows
the Crab in visible light.
Some pulsars have super short periods - 1 millisecond. These are not slowing down much at all.
PSR 137+214 loses only 3.2x10-12 ms/year. These are the best clocks in the universe even better than atomic clocks which lose a microsecond each year!)
These pulsars are probably have
a binary companion.
Transfer Of Matter Onto A Neutron Star - Matter
flows from a normal star toward a compact neutron-star companion and falls
toward the surface in an accretion disk. As the gas spirals inward under
the neutron star's intense gravity, it heats up, becoming so hot that it
emits X rays. In at least one instance the peculiar object SS433-some material
may be ejected in the form of two high-speed jets of gas.
Jets From SS433 - False-color radiographs of SS433, made at monthly intervals (left to right), show the jets rotating under the gravitational influence of the companion star.