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Black Holes! And other
collapsed stars
The Hertzsprung-Russell Diagram
What will happen to
the Sun, once its
fuel (hydrogen)
Mass of
Size of Star
Evolution phases of a star like the
Sun today: age
~4.5 Gyrs; will last another
~5 Gyr (hydrogen burning)
Red giant
Yellow Giant (helium
Second red giant
Planetary nebula
Example of a Planetary Nebula
The Helix
White Dwarfs
A White Dwarf is a dying star, which has
terminated its nuclear fuel, and has contracted to
roughly the size of the Earth.
WDs are prevented to collapse further by electron
degeneracy (electrons, because of quantum
properties, cannot be crowded more then certain
This fate is shared by all stars with masses below
8 MSun, and they end up with masses below 1.4
MSun [the Chandrasekar limit]. Most WDs have
masses around 0.6 MSun
The core of a WD is commonly a mixture of
Carbon and Oxygen, and is releasing as light the
contraction heat.
When cold (~6,000-8,000 K) they may crystallize
into `giant diamonds’ (first confirmed
observationally from WD oscillations in 2004).
As the heat is releases, the WD cools down and
will end up a Brown, and then Black, Dwarf
This is the
fate of our
White Dwarfs can Flare back to life
If they have
a `younger’
Novae are nuclear explosions on the
surface of white dwarf and neutron stars
Brightness changes by a factor of 4000!
White Dwarf Supernova
• If a White Dwarf accretes enough matter from a
companion star, it will eventually nova.
• If, after the nova, it does not shed all the mass it
gained, it will continue to accrete mass until it novas
• If this process continues (accretion, nova, accretion,
nova, etc.) such that the WD continues to gain mass,
once it has a mass of 1.4Msun, the core will collapse,
carbon fusion will occur simultaneously throughout
the core, and the WD will supernova.
Another distance indicator: White Dwarf
(Type Ia) Supernovae in distant galaxies.
L=4D2 l
Evolution Phases of a Star Much More
Massive than the Sun (>8 Msun)
Massive star, main sequence
(H burning)
Massive star, He burning
Neutron star
or black hole
Red giant/supergiant
Supernova explosion (SN Type II)
Example of a Type II Supernova
The Crab
The supernova
explosion that
created the Crab
was seen on
about July 4,
1054 AD.
Lifetimes of Stars
• On the Main Sequence, stars appear to obey a MassLuminosity relation:
L  M3.5
•For example, if the mass of a star is doubled, its luminosity
increases by a factor 23.5 ~ 11.
•Thus, stars like Sirius that are about twice as massive as
the Sun are about 11 times as luminous.
•The more massive a Main Sequence star is, the hotter
(bluer), and more luminous, the star, and the shorter its life.
• For instance, Sirius is `only’ twice the mass of the Sun,
but is 11 times more luminous, implying its life will be about
5.5 times shorter than that of the Sun:
T(Sirius) ~ 2/11 T(Sun))
Neutron star
• A neutron star --- a giant
nucleus --- is formed
from the collapse of a
massive star, with
Mcore > 1.44 Msun.
• Supported by neutron
degeneracy pressure.
• Only about 10 km in
• A teaspoon full would
contain 108 tons!
• Typically with very strong
magnetic field
SNR N157B in the LMC
• 16ms period
• The fastest young
pulsar known
• A fast rotating,
neutron star.
• Emits both strong
radiation and highenergy particles.
The Limit of Neutron
• The upper limit on the mass of stars
supported by neutron degeneracy
pressure is about 3.0-3.2 MSun.
• If the remaining core contains more
mass, neutron degeneracy pressure is
insufficient to stop the collapse.
• In fact, nothing can stop the collapse,
and the star becomes a black hole.
What is a black hole?
• Well … mostly nothing.
• When the ball of neutrons collapses, it
forms a singularity – a point in space
with infinitely small volume and the
mass of the parent material.
A singularity has infinite density!
The most interesting aspects of a black hole are not what it’s
made of, but what effect is has on the space and time around it.
A collapsed star can be sufficiently dense to trap light in its
The Size of a Black Hole
• The extent of a black hole is
called its event horizon.
Nothing escapes the event
• The radius of the event
horizon is the
Schwarzschild radius given
Rs = 2GM/c2
Some Examples of Black Hole Sizes
• A 3MSun black hole would have a Schwarzschild
radius of ~10km. It would fit in Amherst.
• A 3 billion MSun black hole would have a radius of 60
AU – just twice the radius of our solar system.
• Some primordial black holes may have been created
with a mass equal to that of Mount Everest. They
would have a radius of just 1.5x10-15 m – smaller than
a hydrogen atom!
What would a Black Hole
look like?
Gravitational lensing
Some Odd Properties of
Space Around a Black Hole
• Light emitted near the surface of a black hole
is redshifted as it leaves the intense
gravitational field.
• For someone far away, time seems to runs
more slowly near the surface of a black hole.
An astronaut falling into a black hole would
seem to take forever to fall in.
Gravitational Redshifts
A photon will
give up energy
while climbing
away from a mass.
It is trading its
own energy for
potential energy.
Black Holes Don’t Suck You In!
• Many people are under the impression
that the gravity of black holes is so
strong that they suck in everything
around them.
• Imagine what would happen if the Sun
were to instantly turn into a black hole.
What would happen to the Earth?
Black Holes Don’t Suck You In!
• Since the mass of the Sun and Earth
don’t change, and the Earth is no further
from the Sun than it was before, the
force on the Earth would remain exactly
the same. The Earth would continue to
orbit the black hole at a distance of 1
Black Holes Don’t Suck You In!
• So why are black holes so infamous?
– The reason is that the mass is so compact
that you can get within a few kilometers of
a full solar mass of material. Today, if you
stood on the surface of the Sun, much of
the material is hundreds of thousands of
kilometers away. With a black hole, the
mass is so concentrated that you can get
very close to the full mass.
The tidal forces
near a moderate
sized black hole
are lethal!
How Do We See A Black
• Short answer … we don’t. But we can
see radiation from the material falling
into one.
Evidence for Black Holes
• If black holes are black, how do we
know that they exist?
• The star HD 226868 is an excellent
example. It is a B supergiant.
• The spectral lines in the star clearly
show that it is in a binary system with a
period of 5.6 days, however, we see no
companion star.
The companion is one of the brightest Xray sources in the sky and is called
Cygnus X-1
HD 226868
Cygnus X-1
The blue supergiant is so large, that its outer atmosphere can be
drawn into the black hole. As the material spirals into the black
hole, it heats up to millions of degrees and emits X-ray radiation.
Stellar Evolution on the HertzsprungRussel Diagram
Stellar Evolution in a Nutshell
M < 8 MSun
M > 8 MSun
Mcore < 3MSun
Mass controls the
evolution of a star!
Mcore > 3MSun