Chapter 13 Neutron Stars and Black Holes

Optical, Infrared and X-ray Image of Cassiopeia A

Neutron Stars Type I supernovae - star destroyed Type II supernovae (core collapse) - part of core could survive Ball of neutrons remain - neutron star 20 km or so across (size of major city) Mass greater than sun Density to kg/m 3 Thimbleful would weigh 100 million tons 150 lb human on surface would weigh 1 million tons

Figure 13.1 Neutron Star

Rotation and Magnetism Newly formed neutron star rotates rapidly Period of fraction of a second Strong magnetic field Trillions of times stronger than earth’s Collapsing core concentrates magnetic field of original star

Predicted by theory Neutron stars (and black holes) predicted by theory before discovered Fast rotation and strong magnetic fields allowed them to be detected

Pulsars Graduate student Jocelyn Bell at Cambridge University discovered rapid radio pulses in 1967 More than 1500 now known, as pulsars Bell’s thesis advisor, Anthony Hewish, reasoned it was a small rotating source of radiation

Figure 13.2 Pulsar Radiation

Pulsar explanation Accelerated charged particles near magnetic poles of neutron star Emit radiation along magnetic axis Rotation and magnetic axes not aligned Beam sweeps through space - lighthouse model

Figure 13.3 Pulsar Model

Analogy 13.1 A lighthouse beacon

Figure 13.4 Crab Pulsar

Pulsar radiation Most pulsars emit radio wavelengths Some emit visible, X- and gamma-rays

Figure 13.5 Gamma-Ray Pulsars

Pulsars and Neutron stars All pulsars are neutron stars Not all neutron stars are pulsars: Must be aligned just right to be visible from earth Lose rapid rotation and magnetic field over tens of millions of years

Figure 13.6 Isolated Neutron Star

Neutron star binaries Some neutron star binaries Can measure mass - close to 1.4 M 

X-ray bursters Neutron star tears matter from surface of binary companion Accretion disk heats up - emits X-rays Heats enough to fuse H - rapid burst of burning Similar to nova, but much more violent

Figure 13.7 X-Ray Burster

Millisecond pulsars Spin hundreds of times per second Period is several milliseconds Mass of sun, several km in size, spinning almost 1000 times per second Many are old (and should be slow) Are spun-up by infalling matter from binary companion

Figure 13.8 Millisecond Pulsar

Figure 13.9 Cluster X-Ray Binaries

Pulsar planets Several millisecond pulsars have variations in their periods Explained by Doppler shift due to interaction with orbiting planets Planets captured or formed from debris of companion star

Gamma-Ray bursts Gamma-rays are very high energy photons Bursts first discovered in 1960’s by military satellites Made public in 1970’s Bright irregular bursts lasting few seconds Compton Gamma-Ray Observatory - CGRO

Figure Gamma-Ray Bursts

Gamma-Ray burst distances Some optical counterparts measured Distances of billions of parsecs If energy emitted in all directions, then hundreds of times more energetic than supernovae, all in seconds

Figure Gamma-Ray Burst Counterpart

Gamma-Ray burst explanation millisecond variation in intensity Light travels 300 km in 1 millisecond Must be small Relativistic fireball - gases traveling at speeds approaching speed of light Probably jets

Two GRB models Merging stars - binary pair of neutron stars merge Hypernova - collapsing star forming black hole High temperature accretion disk in both cases Relativistic outflow, perhaps in jets

Figure Gamma-Ray Burst Models

Figure Hypernova?

Gravity waves Predicted by Einstein’s theory of gravity Very difficult to detect - wave amplitude smaller than atomic nucleus Most likely candidates: Merger of binary stars Collapse of star into a black hole

Discovery 13.1 Gravity Waves—A New Window on the Universe

Black holes Neutron stars can exist up to about 3 M  Above that, even tightly packed neutrons can’t prevent further gravitational collapse Any main-sequence star above 25 M  will collapse beyond neutron star Gravity so great not even light escapes

Two key facts from Einstein’s Relativity Nothing can travel faster than the speed of light, 300,000 km/s Even light is attracted by gravity

Escape speed How fast do you have to toss an object upward so that it never falls back? 11 km/s ignoring the atmosphere Depends on mass and radius of earth If crushed earth to 1 cm radius, escape speed would be 300,000 km/s so not even light would escape - black hole

Schwarzschild radius Radius for a given mass at which escape speed is speed of light 1 cm for mass of earth 3 km for mass of sun 9 km for 3 M  stellar core Surface of sphere of this radius is called event horizon

Figure Speed of Light

More Precisely Special Relativity

More Precisely Special Relativity

Figure Einstein’s Elevator

Gravity and Curved Space General relativity predicts gravity curves or warps space Objects follow curvature of space At black hole, curvature so great that space folds over and closes off Can picture as a rubber sheet

Figure Curved Space

Figure Space Warping

Tests of General Relativity Deflection of starlight by gravity Observable during solar eclipse Planetary orbits should deviate from ellipses Greatest for Mercury

More Precisely Tests of General Relativity

More Precisely Tests of General Relativity

Space travel near a black hole Beyond event horizon, objects orbit normally, and can escape Once inside event horizon, no escape Tidal forces very strong Objects ripped and stretched apart into pieces Frictional heating

Figure Black Hole Heating

Figure Robot-Astronaut

Robot probe with clock and light source As probe approaches event horizon, light from it more redshifted Gravitational redshift Redshift becomes infinite at event horizon Robot’s clock slows down as approaches event horizon - time dilation

Figure Gravitational Redshift

Singularity General relativity predicts collapse to a point with infinite density - a singularity Probably incomplete theory Quantum gravity in future might properly explain

Black hole observational evidence Black holes in binary systems Large black holes at centers of galaxies (more in later chapters) Intermediate size black holes forming from tight clusters

Figure Cygnus X-1 - likely black hole

Figure Black Hole

Figure Intermediate-Mass Black Holes?