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 anti-meatballs (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/pulsarSmall2.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/

pulsars.html• http://www-astronomy.mps.ohio-state.edu/~ryden/

ast162_5/notes21.html• http://chandra.harvard.edu/xray_sources/

neutron_stars.html• http://www.herts.ac.uk/astro_ub/a41_ub.html• http://map.gsfc.nasa.gov/m_uni/uni_101stars.html