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The Exotic Realm of
Stellar Remnants
“Do not go gentle into that good
night, Old age should burn and
rave at close of day; Rage, rage
against the dying of the light” –
Dylan Thomas
White Dwarfs
Sirius B is the closest white dwarf to us
Sirius A + B in X-rays
White Dwarfs
• Degenerate matter obeys different laws of physics.
• The more mass the star has, the smaller the star becomes!
• increased gravity makes the star denser
• greater density increases degeneracy pressure to balance gravity
Limit on White Dwarf Mass
• Chandra formulated the laws of
degenerate matter.
– for this he won the Nobel Prize
in Physics
• He also predicted that gravity
will overcome the pressure of
electron degeneracy if a white
dwarf has a mass > 1.4 M
– energetic electrons, which cause
this pressure, reach the speed of
light
Chandrasekhar Limit
Subrahmanyan Chandrasekhar
(1910-1995)
Novae
• Term comes from the Latin Stella Nova for
‘new star’
• In reality the star is not new, it just gets much
brighter in a matter of days (Greeks didn’t have
telescopes)
• Totally different event than
a supernova
Novae
• They typically increase in brightness by 5 to 10
magnitudes for a few days, then fade.
• An accretion disk is a
rotating disk of gas
orbiting a star.
– formed by matter falling
onto the star.
• The hydrogen build-up
on the surface of the
white dwarf can ignite
into an explosive fusion
reaction that blows off a
shell of gas.
Artist’s impression of a nova
Novae
• Because so little mass
is blown off during a
nova, the explosion
does not disrupt the
binary system.
• Ignition of the infalling
hydrogen can recur
again with periods
ranging from months
to thousands of years.
recurrent nova T Pyxidis
viewed by Hubble Space Telescope,
past eruptions in 1890, 1902, 1920,
1944, 1966, 2011
Neutron Stars
Chandra X-ray image of the neutron star
created during a supernova observed in A.D.
386. The remnant is known as G11.20.3.
Pulsars
• In 1967, graduate student Jocelyn Bell and
her advisor Anthony Hewish accidentally
discovered a radio source in Vulpecula.
• Sharp pulse recurred every 1.3 sec.
• They determined it was 300 pc away.
• They called it a pulsar, but what was it?
Light Curve of Jocelyn Bell’s Pulsar
Prof. Jocelyn Bell
1968: mystery solved when a pulsar was
discovered in the heart of the Crab Nebula
supernova remnant.
The Crab pulsar also
pulses in visual light (and
gamma-rays).
Pulsars
• Discovered in radio, x-rays, gamma-rays
• About 2000 known, expected population much
higher in our galaxy.
• Periods range from milliseconds to 10 seconds
– younger pulsars have faster periods
– fastest periods require very small rotators
– rotator must be very dense to avoid breaking up
• http://www.jb.man.ac.uk/pulsar/Education/Sounds/sounds.html
Pulsars and Neutron Stars
All pulsars are neutron
stars, but all neutron
stars are not pulsars!!
Crab jet movie
Pulsar Timing
Pulsars are among the most
accurate clocks known
Testing general relativity
PSR B1913+16
Binary pulsar
Discovered 1974
Supernova
•
The neutron core collapses
until abruptly stopped by
neutron degeneracy
– this takes less than a second
•
Core recoils and sends a shock
wave outward
– shock wave hits still infalling outer
layers and accelerates them
outward at 1/10th speed of light
– Accelerated material heated
above 109 K  explosive
nucleosynthesis
– http://www.youtube.com/watch?v=2RxIwtxdE
nQ
Supernova
The amount of energy released
is so great, that most of the
elements heavier than Fe are
instantly created
In the last millennium, four
supernovae have been observed
in our part of the Milky Way
Galaxy: in 1006, 1054, 1572, &
1604
Likely occur every ~40 y in our
Crab Nebula in Taurus
galaxy, but they are obscured by
supernova exploded in 1054 dust and gas
Supernova
Remnants
Cas A: exploded 300 y
ago but no one saw it!
(obscured by dust)
“superluminous supernovae”
When outer layers reach R ~ 100 AU, opacity drops and optical
photons escape
- huge initial luminosity (109 Lsun)
- slow decay due to radioactivity
Type II : Hydrogen absorption lines,
from massive star core collapse
Type Ib : He but no H absorption lines,
Massive star first loses outer layers via
a wind, then core collapse occurs
(Type II)
(Type Ia)
Type Ia : No H or He absorption lines,
from white dwarf core collapse
Type Ia (White Dwarf) Supernova
• While a nova may reach an absolute magnitude of –8 (about 100,000 Suns)…
• a white dwarf supernova attains an absolute magnitude of –19 (10 billion
Suns).
• Can use light curves of Type Ia supernovae to get true luminosity
– they are more luminous than Cepheid variable stars
– so they can be used to measure out to greater distances than Cepheid
variables
– led to discovery of dark energy in early 2000s
Feb 23, 1987
A few hours
later…
Supernova 1987A
Apparent
Magnitude
• Underluminous Type II supernova
• Progenitor: massive blue giant star
• Still no sign of the neutron star
(Days)
• SN1987A emitted 1058 neutrinos
– and Kamiokande II detected… 11 of them
SN1993J
VLBA
movie
Stellar Evolution Summary
Approx. mass range
Fusion stages
End product
No deuterium fusion
Planet
0.01 < M/Msun < 0.08
No H fusion
Brown dwarf
0.08 < M/Msun < 0.5
No He fusion
He white dwarf
No C or O fusion
C+O white dwarf
No Si fusion
Ne+Mg white dwarf
Elements fused up to
Fe
Supernova, then neutron
star or black hole
M/Msun < 0.01
0.5 < M/Msun< 5
5 < M/Msun< 7
M/Msun > 7
Black Holes
• After a massive star supernova, if the core has a
mass > 3 M, the force of gravity will be too strong
for even neutron degeneracy to stop it.
• The star will collapse into oblivion (gravity wins,
finally)
• The neutron star becomes infinitely small.
– it creates a “black hole” in the Universe, from which
nothing can escape, even light
• Equations of general relativity break down at the
infinitely dense ‘singularity’ point
Black Holes
• According to
Einstein’s Theory of
Relativity, gravity is
really the warping of
spacetime about an
object with mass.
• This means that even
light is affected by
gravity.
Warping of Space by Gravity
• Gravity imposes a curvature on space.
– even though it has no mass, light will be affected by gravity
– its path through space will be bent
– within the event horizon, it can not climb out of the hole
“Size” of a Black Hole
• Spacetime is so highly warped around a black hole, even light can
not escape.
•
Schwarzchild Radius – the distance from a black hole where the
escape velocity equals the speed of light.
• A sphere of radius Rs around the black hole is called
the event horizon.
Artist’s
impression
“Interstellar” Movie Black Hole
http://dneg.com/dneg_vfx/blackhole/
Are Black Holes Suckers?
• Far from the event horizon, a black hole exerts gravitational
force according to Newton’s Law, just like any star of the same
mass
• Only at a distance of 3 Rs from the black hole will the gravity
increase from what Newton’s Law predicts.
– then one could eventually fall into the black hole, if you get rid of your
angular momentum
• Angular momentum
is transferred from
inner accretion disk
outward via viscous
torques
How to have a ripping good time at a
black hole
• Gravity falls off as (1/distance)2, while
tidal forces drop off as (1/distance)3
• Your feet are pulled much harder than
your head, spagettification ensues…
– occurs well outside the event horizon (160
Rs for a 1 Msun black hole)
• Tidal forces are (non-intuitively) less
strong for supermassive black holes
Warping of Time by Gravity
• In the vicinity of the black hole, time slows down.
• If we launched a probe to it, as it approached the event horizon:
– e.g., it takes 50 min of time on mother ship for 15 min to elapse on probe
– from the mother ship’s view, the probe takes forever to reach event horizon
– probe would eventually disappear as light from it is red-shifted beyond radio
• From the probe’s
view:
– it heads straight
into the black hole
– light from the
mother ship is
blue-shifted
Finding Black Holes
If they are black, how do we know they exist?
• Indirect evidence seen in:
–
–
–
–
–
x-ray binary stars (3 to 15 Msun)
gamma-ray bursts (3 to 100? Msun)
our galactic center (~106 Msun)
centers of other (active) galaxies (106 to 109 Msun)
centers of globular clusters? ( ~103 Msun?)
No rock solid evidence yet for a black hole
< 3 Msun or between 100 and 106 Msun
Cygnus X-1 X-ray binary
• First good candidate for a black hole
– Strong x-ray source
– In 1971 found to be a spectroscopic binary by Webster, Murdin
and Bolton
– Kepler’s 3rd Law gives a mass > 3 M for the unseen
companion
Supermassive Black Holes
NGC 4258:
Extragalactic
Megamaser
• Doppler (radial) velocities and proper motions of maser spots indicate
Keplerian orbits
• Central object is a 35 million Msun black hole 7.6 Mpc from Earth
Gamma-Ray Bursts
Discovered in late 1960s by Vela satellites
 CsI gamma-ray detectors to monitor atomic
bomb test-ban treaty compliance
 found ~1 burst/day
 distributed over entire celestial sphere.
 discovery remained classified by the
military until 1973
 Origin was a mystery for 25 years
 Occur during massive supernova collapse or
mergers of extremely compact objects
NASA’s CGRO: 1991-2000
Compton Gamma
Ray Observatory
Eight NaI scintillators are oriented like the eight faces
of an octahedron. Position of burst determined by
relative counts in the 8 different detectors.
Isotropy
CGRO Findings
• Each burst has unique
light curve
– durations 10-2 to 103 s
• Many have sharp rise
times and exponential
falloff
– ctrise ~ 30 km
– led to suspicion that
bursts might involve
neutron stars
GRB: galactic or extragalactic?
- Bright bursts were localized by CGRO to an
accuracy of 2°, dim ones to 10°.
- prevented identifying X-ray or optical
counterparts.
- gamma-rays decayed too fast to point other
telescopes
1 deg.
GRB 970508 – Optical Counterpart
BeppoSAX X-ray telescope
found sky position of GRB
Ground optical telescopes
detected transient while still
on the rise and obtained
spectrum.
Host galaxy located for GRB 970508 :
4 Gpc from Earth
- implied 1045 J of energy released
- comparable to a core-collapse supernova
• If GRBs are extragalactic, how
do you convert a stellar mass of
material into gamma-rays in
under 1 second?
• Solution: gamma-rays from a
very high velocity outflow are
beamed into narrow cone
– can reduce true energetics by a
factor of 105
• But how do you create a high
velocity outflow?
Core-collapse / GRB connection
• 1998: GRB 980425
detected in a galaxy 40
Mpc from Earth:
– type Ib/c supernova
SN1998bw detected at
same location
– although SN was
particularly energetic (5 x
1045 J), GRB was not (8 x
1040 J)
– energetics of the SN
suggested a 3 Msun core
collapsed into a black hole
– Produced a very shortlived, fast jet
Long vs Short GRBs
Bursts come in ‘long’ and ‘short’ types (based on time to reach
90% of maximum flux)
Long bursts: associated with core-collapse supernovae
Short bursts: result of neutron star/neutron star or neutron star/black hole mergers.
Collapsar model (long duration gamma ray bursts)
Real-time GRB Sky Map
• http://grb.sonoma.edu/