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Stellar Remnants
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Stellar Remnants
White Dwarfs
Discovery for White Dwarfs
 The first white dwarf was discovered when the star Sirius (the
Dog Star) was suspected to have an unseen binary companion in
1850.
 It was discovered in 1862 and measured to be 10,000 times
fainter than Sirius and became known as Sirius B (or the Pup).
 However, its mass was 0.98 M⊙ and appeared highly underluminous according to mass-luminosity relationships for main
sequences stars.
 In the early 20th century, the temperature of Sirius B was
measured to be that of an A star (10,000 K) and therefore was
warmer per square meter than the Sun.
 The only way it could be this faint and still be so hot was that the
star was very small and hence was named a white dwarf.
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Stellar Remnants
White Dwarfs
White Dwarfs – Basic Properties
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White dwarfs have observed surface temperatures between
4,000 K and 85,000 K. Their masses range from perhaps
0.5 solar masses up to 1.4 M⊙.
When the nature of white dwarfs were realized,
astronomers realized that the gas making up white dwarfs
was extremely dense and was likely degenerate.
In 1930, S. Chandrasekar calculated that a star made of
pure degenerate material could support the gravity of the
entire star if it were about the size of the Earth.
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Stellar Remnants
White Dwarfs
White Dwarfs - Strange Properties
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Chandrasekar found other surprising properties of white
dwarfs
If you add heat to this degenerate gas it does not expand
(unlike gas in this room)
As you add more mass to the white dwarf, it gets smaller!
Theory predicts that white dwarfs radius will go to zero
when the mass of the white dwarf becomes 1.4 M⊙.
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This is the Chandrasekhar limit
Most isolated stars lose enough mass to avoid this
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Stellar Remnants
White Dwarfs
End State of White Dwarfs
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A typical white dwarf has 1.0 M⊙, a 12,000 km diameter (90% of
Earth’s), and a teaspoon of white dwarf material would weigh 2
tons.
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Larger stars form more massive white dwarfs
After it forms, no nuclear reactions are possible and the white
dwarf simply cools off.
A black dwarf is the theorized “final” state of a star with a main
sequence mass less than about 8 solar masses, in which all of its
energy sources have been depleted so that it emits no radiation.
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However, this could take billions of years: a 0.6 M⊙ white dwarf
would become 1/10,000 as luminous as the Sun after 6 billion years.
White dwarfs less then 0.6 M⊙ are rare since single stars would take
over 10 billion years to these type of white dwarfs.
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Stellar Remnants
White Dwarfs in Binary Systems
An Alternate End-Game for White Dwarfs
 A close binary system of a white dwarf and a newly
formed red giant will result in the formation of an
accretion disk around the white dwarf.
 Hydrogen build-up on the white dwarf can ignite an
explosive fusion reaction blowing off a gas shell that
causes the white dwarf to brighten by 10 mags in a
few days - the brightening is called a nova.
 The explosion does not disrupt the binary system.
Infalling H ignition can recur with periods ranging from
months to thousands of years.
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Stellar Remnants
Supernova – Type I
A White Dwarf Supernova Event
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If accretion brings the white dwarf mass above the
Chandrasekhar limit, electron degeneracy can no longer
support the star, and it collapses.
The collapse raises the core temperature and runaway
carbon-fusion begins, which ultimately leads to the star’s
explosion.
Such an exploding white dwarf is called a Type I
supernova. (or white dwarf supernova)
While a nova may reach an absolute magnitude of -8
(about 150,000 Suns), a Type I supernova attains a
magnitude of -19 (5 billion Suns).
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Stellar Remnants
Neutron Stars
Neutron Stars – Just Theory
 A neutron star is the end result of a 6-12 M⊙ star
where its core has collapsed during a massive star
supernova.
 Most of the star is composed of neutrons in a
superfluid state and supported by neutron degeneracy.
 The outer crust of a neutron star is largely electrons
and positively charged atomic nuclei.
 The diameter of a typical neutron star is only 0.2% of
the diameter of a white dwarf (about 20 km diameter)
and the neutron star is a billion times more dense.
 Neutron stars have masses between 1.4 and 3 solar
masses.
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Stellar Remnants
Neutron Stars
Observation - The Discovery of Pulsars
 In 1967, Jocelyn Bell discovered an unknown
source of rapidly pulsating radio waves.
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Subsequent discoveries of similar sources gave
rise to the name pulsar.
A pulsar is a celestial object of small angular size
that emits pulses of radio waves with a regular
period between about 0.0015 and 8.5 seconds,
though nearly all fall between 0.1 and 2.5 seconds.
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Stellar Remnants
Neutron Stars
Back to Theory
 Objects that emit pulsing signals with a duration of 0.001
sec cannot have a diameter any greater than 0.001 lightsecs, which is a few hundred kms.
 Such a small size ruled out white dwarfs (Earth-sized
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objects), leaving the hypothesized neutron star as the
explanation for pulsars.
The lighthouse model is a theory that explains pulsar
behavior as being due to a spinning neutron star whose
(synchrotron) radiation beam we see as it sweeps by.
The high spin rate of a neutron star is obtained from the
original star’s spin as a result of angular momentum
conservation.
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Stellar Remnants
Neutron Stars
Data and Expectations
 More than 1000 pulsars have been
discovered.
 The Crab pulsar spins so rapidly because it
formed so recently. Over time it will lose
rotational energy, slow down, and emit less
energy.
 The Crab pulsar is slowing down because of
the “drag” of the electrons propelled out into
the nebula surrounding the pulsar.
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Stellar Remnants
Neutron Stars
Neutron Stars in Close Binaries
 Accretion disks may form as with white dwarfs in close
binaries.
 X-ray Binaries
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Huge amount of energy release of in-falling matter heats inner
regions of accretion disk to point of x-ray emission – 100,000
times more the Sun’s combined wavelength energy output.
Pulse rate similar to radio pulsation, but accelerates with time
(to millisecond pulses) as neutron star gains angular
momentum.
X-ray Bursters
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Accreted hydrogen fuses to helium and builds up.
Helium fuses rapidly, an x-ray burst occurs, and may repeat
every few hours to every few days with durations of seconds.
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Stellar Remnants
Black Holes
Creation of Black Holes
 A black hole is the end result of a
supernova explosion of a star with
initial mass greater than about 12 M⊙.
 Neutron degeneracy cannot support a
neutron star whose mass is greater
than about 3 solar masses.
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Stellar Remnants
Black Holes
Black Hole Structure
 The Schwarzschild radius is the radius of a
spherical region in space within which no light
can escape:
RS = 3M (RS in km; M in solar masses)
 The size of the Schwarzschild radius depends
on the mass within the sphere.
 A black hole is a spherical volume of space with a radius
given by the Schwarzschild formula above and with an
escape velocity that exceeds the speed of light.
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Stellar Remnants
Black Holes
More Structure
 The event horizon is the spherical surface of radius RS
around a black hole from which nothing can escape.
 Inside a black hole, an object will eventually be
subjected to extreme tidal forces pulled apart.
 The final destination of an object (or so it is thought) in
a black hole is to be crushed out of existence at a
central singularity.
 Spinning and charged black holes are more complex.
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Stellar Remnants
Black Holes
Detecting Black Holes
 If a black hole has a close binary companion, material
may be pulled from the companion to form an accretion
disk around the black hole.
The accretion disk radiates x-rays and gamma rays as the
gas is heated to very high temperatures as it approaches the
event horizon.
 From their x-ray emissions, Cygnus X-1 and AO620-00 are
two very good black hole candidates.
 V404 Cygni - Observations of X-rays and and in the visible
have shown that this is a binary system in which a late G or
early K star revolves every 6.47 days around a compact
companion with a mass between 8 to 15 solar masses.
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Stellar Remnants
Black Holes
Black Holes Forever?
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Jacob Bekenstein discovered that the black
holes can be assigned a temperature.
Soon thereafter in 1974, Stephen Hawking
showed that this temperature meant that a
black hole emits thermal radiation through a
quantum/gravity energy exchange.
Hawking radiation means black holes radiate
away, although a one-solar-mass black hole
will take 1067 years to do so.
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Stellar Remnants
Black Holes
Visiting a Black Hole
 The physics of black holes, both inside and outside is best
understood through Einstein’s General Theory of Relativity
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The theory employs spacetime: a 4-dimensional construct that blends
the 3 dimensions of space with the dimension of time
Objects with mass bend spacetime
The curvature of spacetime then dictates how objects move
Consequences as you approach the event horizon of a black
hole feet first:
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You age at a slower rate relative to someone far from the black hole
(although you do not sense such a slowing yourself)
To an outside observer, you never appear to cross the event horizon as
your emissions shift from the visible to the infrared to the radio due to
gravitational redshift
You are stretched in length and squeezed thinner, and eventually the
tidal forces overwhelm the molecular bonds in your body (i.e., you die!)
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Stellar Remnants
Gamma-Ray Bursts
Observations of Gamma-Ray Burst
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First detected in 1960s by U.S. military using satellites to
detect gamma-ray signatures of nuclear bomb tests.
Gamma-ray bursts (GRBs) with dramatic fluctuations over
just a few seconds were found to be non-terrestrial.
By 1991, GRBs were found to be randomly distributed and
not correlated to the Milky Way’s disc as X-ray bursts were.
And by 1997, correlations between numerous GRBs and
galaxies very far away was established – making GRB
sources the most powerful bursts of energy we observe in
the universe.
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Stellar Remnants
Gamma-Ray Bursts
Two Possible Causes of Gamma-Ray Bursts
 The required energy to create the burst
suggests that they are created when certain
kinds of black holes are formed.
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They may be created during unusually powerful
supernova explosions that create black holes
Or perhaps they are caused when two neutron
stars collide
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