Compact Objects Astronomy 315 Professor Lee Carkner Lecture 15 “How will we see when the sun goes...
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Transcript of Compact Objects Astronomy 315 Professor Lee Carkner Lecture 15 “How will we see when the sun goes...
Compact Objects
Astronomy 315Professor Lee
CarknerLecture 15
“How will we see when the sun goes dark?”
“We will be forced to grope and feel our way.”
--Jack Vance, The Dying Earth
What is a Compact Object?
The leftover core of a dead star Objects that are supported by
(strange) physics rather than thermal pressure
White Dwarf
Mass: Size: Density: Supported by: Progenitor: Example:
Observing White Dwarfs
White dwarfs are very faint
We can only see the near-by ones However, most stars are in
multiple systems
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 material
Eventually fusion reactions occur, blasting the outer layers away
New material begins to collect and the process stars over
Accretion onto a White Dwarf
Nova Cygni Ejected Ring
Neutron Star
Mass: Size Density: Supported by: Progenitor: Example:
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
Neutron Star Properties Small size means low luminosity and high
temperature
Neutron stars are spinning very rapidly
Neutron stars have strong magnetic fields Field is trapped in the collapsing star and is
compressed to great strength
Pulsars Pulsars are radio sources that blink on and off
with very regular periods
Each pulse is very short
What could produce such short period signals?
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.
Viewing Pulsars Pulsars can be associated with
supernova remnants
The periods of pulsars increase with time
We can only see pulsars if the beam is pointing at us
The Crab Pulsar
Black Hole
Mass: Size : Density: Supported by: Progenitor: Example:
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
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
Escape velocity ~ (M/R)½
General Relativity According to Einstein energy and mass are
the same thing (E=mc2)
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 hole If light can’t escape, nothing can
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 As we have seen, compact objects
in binary systems can exhibit many properties due to mass transfer from the normal star to the compact object: Nova: X-ray Burster: X-ray Binary:
Finding Black Holes We can detect compact objects by
finding X-ray binaries If the mass of the compact object
is greater than 3Msun, it must be a black hole
Cygnus X-1
Next Time
Quiz 2 Covers everything since Quiz 1
Lectures 10-15, Stellar Interiors through Compact Objects
Same format (multiple choice and short answers)