1. Compact Objects Astronomy 315 Professor Lee Carkner Lecture 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

2. What is a Compact Object?  The leftover core of a dead star   Objects that are supported by (strange) physics rather than thermal pressure 

3. White Dwarf  Mass:  Size:  Density:  Supported by:  Progenitor:  Example:

4. Observing White Dwarfs  White dwarfs are very faint   We can only see the near-by ones  However, most stars are in multiple systems 

5. 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

6. 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

7. Accretion onto a White Dwarf

8. Nova Cygni Ejected Ring

9. Neutron Star  Mass:  Size  Density:  Supported by:  Progenitor:  Example:

10. 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 

11. 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 

12. 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

13. 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 

14. A Rotating, Magnetized N.S.

15. 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 

16. The Crab Pulsar

17. Black Hole  Mass:  Size :  Density:  Supported by:  Progenitor:  Example:

18. 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 

19. 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  Escape velocity ~ (M/R) ½ 

20. General Relativity  According to Einstein energy and mass are the same thing (E=mc 2 )    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  If light can’t escape, nothing can 

21. 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 

22. 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:

23. Finding Black Holes  We can detect compact objects by finding X-ray binaries   If the mass of the compact object is greater than 3M sun, it must be a black hole 

24. Cygnus X-1

25. Next Time  Quiz 2  Covers everything since Quiz 1  Lectures 10-15, Stellar Interiors through Compact Objects  Same format (multiple choice and short answers)