Transcript Neutrons and Pulsars - George Mason University

Neutrons and Pulsars
Lab 8
Neutron Stars
• Neutron stars are the collapsed cores of
massive stars, ~15 to 30 times the mass of our
sun
• masses << 15 solar masses = the star becomes
a white dwarf
• masses >> 30 solar masses becomes a black
hole
• typical mass of a neutron star is ~1.4 solar
masses, and the radius is probably ~10 km
Neutron Stars form when….
• The central part of the star fuses its way to
iron
• it can't go any farther because at low
pressures Fe 56 has the highest binding
energy per nucleon of any element
• which means fusion or fission of Fe 56 will
require an energy input
so…..
• So the iron core just accumulates until it
gets to about 1.4 solar masses (the
"Chandrasekhar mass")
• Then the electron degeneracy pressure
that had been supporting it against gravity
gives up the ghost and collapses inward
then…..
• At the very high pressures involved in this
collapse, it is energetically favorable to combine
protons and electrons to form neutrons +
neutrinos
• About 1057 neutrinos are made in the iron core,
as the protons are converted to neutrons
• The neutrinos escape after scattering a bit and
making more supernovae, and the neutrons
settle down to become a neutron star, with
neutron degeneracy managing to oppose gravity
What’s inside a Neutron Star?
TOP
• In the atmosphere and upper crust, there are lots of
nuclei, so it is not primarily all neutrons yet
• At the top of the crust, the nuclei are mostly Fe56 and
lighter elements, but deeper down the pressure is high
enough that the atomic weights rise
• At densities of 106g/cm3 the electrons become
degenerate, i.e., electrical and thermal conductivities are
huge because the electrons can travel great distances
before bumping into each other
The “Neutron Drip” Layer
• Deeper yet, at a density around 4x1011
g/cm3, is the "neutron drip" layer
• At this layer, it becomes energetically
favorable for neutrons to float out of the
nuclei and move freely around, so the
neutrons "drip" out
• Even further down, mainly free neutrons,
with a 5%-10% sprinkling of protons and
electrons
“Pasta-Antipasta” Layer
• As the density increases, the "pasta-antipasta"
sequence starts
• At relatively low (about 1012 g/cm3) densities, the
nucleons are spread out like meatballs that are
relatively far from each other
• At higher densities, the nucleons merge to form
spaghetti-like strands, and at even higher
densities the nucleons look like sheets (such as
lasagna)
• Increasing the density further brings a reversal
of this sequence, where there are mainly
nucleons but holes form (in order of increasing
density) anti-lasagna, anti-spaghetti, and antimeatballs (also called Swiss cheese)
Pulsars, Neutron Stars
• Simply put, pulsars are rotating neutron stars.
And pulsars pulse because they rotate!
• http://www.astro.umd.edu/~miller/Images/pulsar
Small2.gif
• rotate very rapidly, up to 600 times per second
• have the strongest magnetic fields in the known
universe
• center of neutron stars are believed to be 100
million K
Spinup and spindowns
• Neutron stars are born rotating fast
• Magnetic field exerts a torque which slows
it down for ever after
• But “glitches” can briefly spin it back up
again
Accretion disks
• Stars usually exist as binary systems, so a
neutron star can accrete from its
companion
• If the companion is relatively small, matter
tends to flow towards the neutron star and
forms a disk around it
• If companion >10 solar masses, matter
flows towards the neutron star as a low
angular momentum wind
What happens to the Neutron Star?
• The fate of the hot neutron core depends upon
the mass of the progenitor star
• If the progenitor mass is ~10x mass of the Sun,
the neutron star core will cool to form a neutron
star or "pulsars", powerful beacons of radio
emission
• If the progenitor mass is larger, then the
resultant core is so heavy that not even nuclear
forces can resist the pull of gravity and the core
collapses to form a black hole
BLACK HOLES!
• Black holes are usually formed when an extremely
massive neutron star
• A black hole is a region of space in which the matter is
so compact that nothing can escape from it, not even
light
• the "surface" of a black hole, inside of which nothing can
escape, is called an event horizon
• The matter that forms a black hole is crushed out of
existence
• Just as the Cheshire Cat disappeared and left only its
smile behind, a black hole represents matter that leaves
only its gravity behind
Strange Facts About Black Holes
• Light bends so much near black holes that if you were
near one and looking away from the hole, you would see
multiple images of every star in the universe, and could
actually see the back of your own head!
• Inside a black hole the roles of time and radius reverse:
just as now you can't avoid going into the future, inside a
black hole you can't avoid going in to the central
singularity
• Singularity: in a black hole, the "center point", at which
densities, tidal forces, and other physical quantities
become infinite (our current physical theories break
down at this point)
Forces at black holes
• Black holes, like any gravitating objects, exert a
tidal force
• If you approach a black hole feet first, the
gravitational force at your feet is greater than the
force at your head
• The tidal force at the event horizon is smaller for
larger black holes
• You would get torn to shreds far outside a black
hole the mass of our sun, but at the event
horizon of a billion solar mass black hole the
tidal force would only be a millionth of an ounce!
Your friend at a Black Hole
• If you stood a safe distance from a black
hole and saw a friend fall in, he would
appear to slow down and almost stop just
outside the event horizon
• His image would dim very rapidly
• Unfortunately for him, from his point of
view he would cross the event horizon just
fine, and would meet his doom at the
singularity
really cool neutron star websites
• http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html
• http://cosmology.berkeley.edu/Education/BHfaq.html
• http://imagine.gsfc.nasa.gov/docs/science/know_l1/pulsa
rs.html
• http://www-astronomy.mps.ohiostate.edu/~ryden/ast162_5/notes21.html
• http://chandra.harvard.edu/xray_sources/neutron_stars.h
tml
• http://www.herts.ac.uk/astro_ub/a41_ub.html
• http://map.gsfc.nasa.gov/m_uni/uni_101stars.html