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Compact Objects
Astronomy 315
Professor Lee Carkner
Lecture 15
What is a Compact Object?
The densest objects in the universe
Responsible for many unusual
White Dwarf
Size: earth-sized (~13000 km diameter)
Supported by: electron degeneracy
Observing White Dwarfs
White dwarfs are very faint
We can only see the near-by ones
What happens if the white dwarf is in a
system with a normal star?
Mass Transfer
Stars in a binary can transfer mass
This material ends up in a accretion disk
Friction makes the disk very hot
Material will accrete onto the white dwarf
Cataclysmic Variables
Material gets hot as it is compressed by new
Eventually fusion reactions occur, blasting
the outer layers away
New material begins to collect and the
process stars over
Cataclysmic variables brighten and fade
Accretion onto a White Dwarf
Nova Cygni Ejected Ring
Black Hole
Size: singularity
Supported by: unsupported
Example: high mass X-ray binaries
Limits of Neutron Degeneracy
If a stellar core has more than about 3 Msun,
not even neutron degeneracy pressure can
support it
A huge mass in such a tiny space creates a
powerful gravitational field
The object is called a black hole
Escape Velocity
What is required for an object to escape from
a mass (planet or star)?
Velocity is related to kinetic energy (KE =
½mv2) , so the object must have more kinetic
energy than the gravitational energy that
holds it back
High mass, small radius means you need a high
velocity to escape
General Relativity
Thus, if mass is affected by gravity, so is light
If the escape velocity of an object is greater
than the speed of light (c=3X108 m/s), the
light cannot escape and the object is a black
nothing can travel faster than light
Structure of a Black Hole
Once you get closer to a black hole than the event
horizon, you can never get back out
The radius of the event horizon is called the
Schwarzschild radius:
Compressing a mass to a size smaller than its
Schwarzschild radius creates a black hole
X-ray Binaries
Compact objects in binary systems can exhibit
many properties due to mass transfer from
the normal star to the compact object:
X-ray Burster:
X-ray Binary: X-rays emitted from the inner
accretion disk around the compact object
Cygnus X-1
Finding Black Holes
By getting the Doppler shifts for the stars we
can find the orbital parameters
Even though the black holes are invisible, they
manifest themselves by their strong gravitational
Neutron Star
Size: 10 km radius
Supported by: neutron degeneracy
Example: pulsar
Above the Limit
If a stellar core has mass greater than the
Chandrasehkar limit (1.4 Msun), electron
degeneracy pressure cannot support it
Supernova breaks apart atomic nuclei
Neutrons also obey the Pauli Exclusion principle
Cannot occupy the same state
Neutron Star Properties
The properties of a neutron star are extreme
Small size means low luminosity and high
Neutron stars are spinning very rapidly
Neutron stars have strong magnetic fields
A trillion times strong than the sun’s
Pulsars are radio sources that blink on and off
with very regular periods
Each pulse is very short
What could produce such short period
Only something very small
Only neutron stars are small enough
Pulsar in Action
The strong magnetic field of a pulsar
accelerate charged particles to high velocities
The radiation is emitted in a narrow beam
outward from the magnetic poles
These two beams are swept around like a
lighthouse due to the star’s rotation
A Rotating, Magnetized N.S.
Pioneer 10 Plaque
The Crab Pulsar
Viewing Pulsars
Pulsars can be associated with supernova
The periods of pulsars increase with time
Beam is very narrow so some pulsars are
Next Time
Next class is Tuesday, April 18
Read Chapter 23.1-23.7
Observing List #2 due
Observing Tonight, 8:30-9:30 pm
If clear
Only for those that missed last time