Compact Objects
Astronomy 315Professor Lee
CarknerLecture 15
What is a Compact Object?
The densest objects in the
universe
Responsible for many unusual phenomena
White Dwarf
Mass: Size: earth-sized (~13000 km
diameter) Density: Supported by: electron degeneracy
pressure Progenitor: Example:
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 material
Eventually fusion reactions occur, blasting the outer layers away
New material begins to collect and the process stars over Cataclysmic variables brighten and fade periodically
Accretion onto a White Dwarf
Nova Cygni Ejected Ring
Black Hole
Mass: Size: singularity Density: Supported by: unsupported Progenitor: 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 hole
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: Nova:
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 fields
Neutron Star
Mass: Size: 10 km radius Density: Supported by: neutron degeneracy
pressure Progenitor: 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
temperature
Neutron stars are spinning very rapidly
Neutron stars have strong magnetic fields
A trillion times strong than the sun’s
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.
Pioneer 10 Plaque
The Crab Pulsar
Viewing Pulsars Pulsars can be associated with supernova
remnants
The periods of pulsars increase with time
Beam is very narrow so some pulsars are undetectable
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