Compact Objects Astronomy 315 Professor Lee Carkner Lecture 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 M sun, 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 = ½mv 2 ), 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=3X10 8 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 M sun ), 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 Observing List #2 due Observing Tonight, 8:30-9:30 pm If clear Only for those that missed last time