Transcript PowerPoint Presentation - Brighter Than a Trillion Suns

Neutron Stars and
Black Holes
Remnant cores of massive stars:
produce pulsars, jets and
gamma-ray bursts
Relativity theory is needed for full
understanding, but …
NEUTRON STARS
• CORE REMNANTS of
stars with masses between
about 8 and 30 M .
• Outer layers blasted off in
SN explosion.
• Remaining NS masses are
about 1.4 to 2 M
• Radii of about 10-12 km
(city sized)
• Densities above 1014g/cm3
or 108 tons/teaspoon
• Immense surface gravity:
you would weigh billions of
tons
Properties of Neutron Stars
• Interior composition: 7/8ths neutrons + p's and e's
• The inner core may have lots of PIONS and be a
superfluid and superconductor.
• Neutron stars have an "atmosphere" only a few
centimeters thick and a "crust" a few meters thick,
both mostly made of iron.
• Conservation of ANGULAR MOMENTUM means
NSs are formed spinning very fast, with
PERIODS of < 10 ms (~ 100 rotations/sec!)
• Conservation of MAGNETIC FIELD means
B above 1012 Gauss!
Isolated NS and X-ray Emission
• NS found by Xrays and then
HST moving >
100 km/s
• Accretion disk
around NS fed
by companion
• SS 433: radio
maps show jets
ejected from disk
around NS or BH
Finding Neutron Stars
• Isolated NSs are hard to detect by thermal X-ray
emission from very hot surfaces. They are so small
that their black-body emission, L = 4πR2T4, is weak.
• However, NSs in binaries will accrete from
companions, just like WDs do.
• Since NS's are much smaller, gas falls in further
through the accretion disk, and gets much hotter:
disk emits LOTS of X-RAYS.
• Nuclear burning on NS surface can 
X-RAY BURSTERS
X-ray Bursters
• Globular cluster
Terzian 2 w/
central dot
indicating
location of X-ray
bursts
• X-ray images
showing great
increase during
burst
NASA Neutron Star
Resources
• X-rays from the Supernova Remnant Cas A:
a neutron star inside
• Chandra monitors Crab Nebula during
powerful Gamma-ray flare
Chandra X-ray Observatory Education Resources
http://chandra.harvard.edu/edu/formal/age_snr/ds9.html
A very nice lab, but only works on Macs or Linux.
Thought Question
According to conservation of angular momentum, what
would happen if a star orbiting in a direction
opposite the neutron’s star rotation fell onto a
neutron star?
A.
B.
C.
The neutron star’s rotation would speed up.
The neutron star’s rotation would slow down.
Nothing, the directions would cancel each other out.
Thought Question
According to conservation of angular momentum, what
would happen if a star orbiting in a direction
opposite the neutron’s star rotation fell onto a
neutron star?
A.
B.
C.
The neutron star’s rotation would speed up.
The neutron star’s rotation would slow down.
Nothing, the directions would cancel each other out.
Discovery of Neutron Stars: Pulsars
• NS's were discovered only in 1967, since
some of them are PULSARS. Jocelyn Bell &
Ant. Hewish discovered them; only Hewish
shared Nobel Prize (with Martin Ryle) in 1974.
• But they were predicted to exist in the 1930’s
by nuclear physicists: astronomers scoffed
back then!
• Strong B field plus fast rotation generates
powerful forces: accelerate particles
(mainly e's) close to speed of light.
• Fast acceleration in magnetic fields
SYNCHROTRON RADIATION
Pulsar Light Curves and Periods
• Pulsars are excellent clocks, with very accurately
measured periods between ~1 ms and ~10 s. Some
pulses stronger than others.
• Young, single pulsars start out spinning very fast, but
slow down to periods of a few seconds over 106
years. WHY?
• Rotational energy and angular momentum are
radiated away with photons.
Pulsars are Beamed
• A pulsar is a
neutron star
that beams
radiation along
a magnetic axis
that is not
aligned with the
rotation axis
Pulsars,
Rare
• The radiation
beams sweep
through space
like lighthouse
beams as the
neutron star
rotates
• So, visible from
only certain
directions: many
unseen ones
should be out
there!
Why Pulsars Must be Neutron
Stars
Circumference of NS = 2π (radius) ~ 60 km
Spin Rate of Fast Pulsars ~ 1000 cycles per second
Surface Rotation Velocity ~ 60,000 km/s
~ 20% speed of light
~ escape velocity from NS
Anything else would be torn to pieces!
The
Crab
Pulsar
•
•
•
•
33 ms period (30 flashes/second)
At center of Crab nebula SNR
Seen in optical, X-ray
Shows inter-pulse, probably from 2nd beam
Gamma-Ray Pulsars
• Geminga and Crab both emit gamma-rays but
Geminga is much weaker in optical and invisible in
radio; its 0.24 s period is shown on top
Binary Pulsars
• A few pairs of pulsar binaries have been
found. Accurate clocks mean Doppler shifts
measured remarkably well.
• Their orbits are decaying because of
gravitational radiation and allow for sensitive
tests of General Relativity
• Led to Nobel prize for Joe Taylor and Russell
Hulse in 1993 for 1974 discovery of the first
binary pulsar
Pulsar(s) with Planets?
• The first (2) extrasolar planets detected were found in
1992 around a pulsar (of all places!)
• They caused tiny changes in the radial-velocity
because of their tugs on the NS, showing periods of
67 and 98 days (at 0.4 and 0.5 AU)
• Could planets survive a SN? Form from debris after?
Perhaps NS was in an
exchange encounter
with a MS binary;
could allow easier ms
pulsar spin-up; maybe
planet capture too?
Millisecond Pulsars
• Old pulsars in binary systems can be rejuvenated into
MILLISECOND pulsars, if they accrete enough mass
and angular momentum from companion.
• Typically much lower magnetic fields (so pulsar is old)
A (Very) Short Course in Relativity
• In 1905 Einstein published the Special Theory of Relativity
(along with photoelectric effect proving light was a photon;
and Brownian motion proving atoms exist!)
• An improvement on Newton’s laws of motion when things
move close to c
• Key postulates:
1) speed of light is constant in a vacuum and the same
in all directions; and nothing can go faster than light
2) equations of physics should be the same in all
inertial frames (those moving relatively with constant
velocities)--the Principle of Relativity
• TOGETHER THESE LEAD TO IMPORTANT RESULTS:
• 3) There is no absolute frame of reference -- no preferred
observer AND
• 4) Space and time can’t be considered independently: we
have SPACE-TIME: different observers, different values
Proof of Constancy of c
• Michelson & Morley
(1887) used an
interferometer to
see how much
faster light was
moving with and
against the earth’s
motion
• Answer: NO
DIFFERENCE!
Adding Velocities Relativistically
Lorentz Contraction and Time Dilation
• A moving object
appears shorter
• A moving clock
appears to tick slower
• Lorentz factor, 

1
1 v 2 /c 2
L  L0 /
 t  t 0
Special Relativity Works!
• E=mc2 : tested in nuclear fission and fusion
• Lifetimes of cosmic ray muons: they decay in 2.0
microseconds at rest, but travel big distances,
implying longer lives (like 44 s) in our frame if they
move at 0.999c.
• Effective mass increases from rest mass as v  c:
meff = m
• So it’s harder to accelerate a particle that is moving
faster (a = F/meff), explaining why so much energy is
needed in cyclotrons and other “atom smashers”.
GENERAL RELATIVITY
• In 1916 Einstein
published the final form
of the General Theory of
Relativity
• Equivalence between
gravity and acceleration:
you are weightless in a
plummeting elevator
• Improves on Newtonian
gravity and motion laws
when masses are big
Space-Time Warped Near Masses
• In GR, matter warps
space-time, so that
the straightest and
shortest path
(geodesic) looks like
a curve to us.
• Mass tells space
how to curve.
• Space tells matter
how to move.
• Analogy: weight on
a tight rubber sheet
depresses it, so a
ball is deflected
General Relativity Works Too!
• GR predicts that light will appear to bend as it follows a
curved path near a mass
• Measure small displacement of stellar positions near Sun
during a solar eclipse (done in 1919): 1.75” at limb
• Made Einstein world famous since it agreed very nicely!
Other Tests of GR
• Mercury’s perihelion was
found to advance some
574”/century but planetary
perturbations explained
only 531”/cent
• GR perfectly explained
the excess 43”/century
• Later tests: radar ranging
to planets; Global
Positioning Satellite
(GPS) system; dragging
of inertial frames by
rotating earth (Gravity
Probe B)
Gravity Waves:
a GR Prediction
• Gravity radiates energy away
as waves, causing orbits to
shrink: perfect fit to binary
pulsar orbit decay (Noble Prize
to Hulse and Taylor in 1993)
• Detectors (LIGO now; LISA in
space may happen in your
lifetimes) may “see” :
• NS-NS mergers,
NS-BH collisions,
Supernova explosions;
providing a new “window on
the universe” (not photons or
neutrinos or cosmic rays)
BLACK HOLES
• A part of space-time divorced from the rest of the universe.
• Not even light can escape if emitted too close to a black
hole (BH); inside event horizon or Schwarzschild radius.
General Relativity and BHs
• A BH is a singularity: finite amount of mass at a
point, so
• Density there is (nominally) INFINITE
• The BH is surrounded by an event horizon or
infinite redshift surface or Schwarzschild radius
M BH 
2GM
RS  2  3 km 

c
M Sun 
So a BH with Earth’s mass has RS = 1 cm!
100,000,000 Msun BH has Rs = 300,000,000 km or
8km = 10-5parsec = 1000 light-seconds
3x10

Too much mass in too little volume!
• Warping of space-time can be so severe that
the region effectively pinches off
• Space-time curvature becomes extremely
strong in the vicinity of a BH’s event horizon
If the Sun shrank
into a black hole,
its gravity would
be different only
near the event
horizon
So black holes don’t
really suck!
Light waves take extra time to climb out of a deep hole
in spacetime leading to a gravitational redshift
Redshifted Emission
• Photons lose energy
as they climb out of
the gravitational pit
established by a BH.
• We observe longer
(redder) wavelengths
(lower frequencies)
compared to those
emitted.
• Time freezes for a
distant observer
watching something
fall past event horizon
Black Hole Applets
•
•
•
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Escape Velocity and Radius
Schwarzschild Radii and Mass
Time Near BH
Spacetime Orbits
Black Holes have no Hair!
A BH is characterized by only:
1. Mass
2. Electric charge (astrophysically unimportant)
3. Angular momentum (spin)  ergosphere
Rotating Black Holes
•
A rotating (Kerr) BH will have a SMALLER EVENT
HORIZON than the same mass non-rotating
(Schwarzschild) BH.
• BUT, outside the Event Horizon there will be an
ellipsoidal STATIONARY LIMIT:
inside of it, everything MUST rotate w/ BH;
outside the Stationary Limit, a powerful enough
rocket could stand still.
• The region between the Event Horizon and the
Stationary Limit is called the ERGOSPHERE: (it is
sort of donut shaped) In principle (and maybe in
practice too!)
More About Kerr BH’s
• In principle (and maybe in practice too!) the
ROTATIONAL ENERGY of a BH can be
EXTRACTED by PARTICLES or MAGNETIC FIELDS
that penetrate the ERGOSPHERE (Penrose effect).
• A way to make a great garbage disposALL plus
power plant!
• If the SPIN of a BH is too large it could become a
NAKED SINGULARITY, with no EVENT HORIZON;
but the COSMIC CENSORSHIP HYPOTHESIS
argues this never happens and BH's stay clothed
with horizons.
Tidal Stretching & Hawking
Radiation
• Large gravity differences (tides):
“toothpaste tube effect”
• Quantum gravity effect:
Hawking temperature
T=h/162kGM=610-8K(M/M)
• Hawking power:
LR2T4 M2/M4 1/M2
• Incredibly small if BH mass >
1017g (rules out stars/galaxies)
It’s Hard to Find Black Holes
• They don’t emit
(significant) radiation
• Light bending means
they don’t even show
up as dark spots: 
• Unless distance is
close to RS, gravity is
close to that of a
regular star of the
same mass
Origin of Black Holes
• Collapse of very massive stars (>30 M) can
lead to BHs of ~3-25 M (neutron stars must
have masses below about 2 M ).
• A NS could accrete more gas from a binary
companion, kicking it over the upper mass limit
• Collapse of densest regions of forming
galaxies, either directly or through merger of
stars in dense clusters can yield BHs with
M > 1000 M .
• Quantum fluctuations in the early universe
could give primordial BHs of a wide range of
masses.
NASA Black Hole Resources
•
http://nasa.gov/audience/foreducators/912/features/F_Black_Hole_Extreme_Exploration.html
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Black Hole Videos
X-rays from a microquasar: GRS 1915
How a black hole can form in a supernova explosion of a massive star
Accretion Disks
• Form when gas spirals down into a massive
object. Seen in:
• Stars (and planetary systems) being born
• Binary stellar systems with compact component:
white dwarf
neutron star
black hole
• Active Galactic Nuclei (later)
In an Accretion Disk
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•
•
•
Mass moves inward
Angular momentum is carried outward
Friction (viscosity) in the gas heats it up
Usually most of this heat is radiated from the
disk surface giving:
Ultraviolet radiation from white dwarfs
X-rays from neutron stars and stellar mass
BHs
Mostly visible and UV from AGN BHs
• Most logical way to launch jets
One famous X-ray binary with a very likely black
hole is in the constellation Cygnus
Cyg X-1: Radio Image & X-ray light curve
Combining observations: optical of blue giant; Doppler shifted
lines of star and gas stream we conclude star has M>15 M
and X-ray emitting companion has M>10 M, so
Cygnus X-1 is a Black Hole Binary
Accretion Disks are Efficient
• E = mc2
• Complete conversion of mass to energy is only
possible in matter-antimatter annihilation
• But normal accretion disks can convert > 5.7%
but probably < 32% of mass to energy (the
absolute upper limit is ~42%)
• This is far better than chemical reactions
(~ 0.0001 %) or even nuclear fusion (~0.7 %)
• Full conversion of 1 M /year = 5.7  1039W
Jets Launched From Disks
Artist’s rendition of
jets launched from
vicinity of BH in the
center of an AD.
Gamma-Ray Bursts
• Tremendous powers in high energy photons emitted in
just a few seconds
• First discovered by spy satellites in 1960s looking for
atomic bomb tests: isotropic in the sky
• Usually have “afterglows”: emission in X-rays, optical
and radio bands that decay more slowly
• Generic model: a “fireball” of very hot plasma, bursting
out as a very relativistic jet (~100)
• This makes it look even brighter if jet points at us, but
still involves great powers, since many are very distant
Where do gamma-ray bursts come
from?
GRB Light Curves and Locations
GRBs are Far Away
• Since 1997 many have had galaxies identified as
their hosts; at large cosmological redshifts,
therefore billions of parsecs away
Competing Models: NS-NS
Mergers or Hypernovae (or both)?
Some GRBs are Hypernovae
• Light curves brightening and looking like SN have
been seen in a few cases, making it likely that some
(many?, all?) GRBs are exceptionally powerful SN
• But could just be long GRBs, w/ short ones NS-NS
mergers.
End of Compact Stellar
Remnants
• We now turn to collections of stars
• Galaxies, starting with our Home Galaxy
• Focus on Active Galactic Nuclei