Download Chapter 13: Neutron Stars and Black Holes - Otto

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 1017 to 1018 kg/m3
• 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 13.10
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 13.11
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 13.12
Gamma-Ray Burst Models
Figure 13.13
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 13.14
Speed of Light
More Precisely 13-1.1
Special Relativity
More Precisely 13-1.2
Special Relativity
Figure 13.15
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 13.16
Curved Space
Figure 13.17
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 13-2.1
Tests of General Relativity
More Precisely 13-2.2
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 13.18
Black Hole Heating
Figure 13.19
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 13.20
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 13.21
Cygnus X-1 - likely
black hole
Figure 13.22
Black Hole
Figure 13.23
Intermediate-Mass Black
Holes?