1. Compact Objects Astronomy 315 Professor Lee Carkner Lecture 15

2. What is a Compact Object?   The densest objects in the universe   Responsible for many unusual phenomena

3. White Dwarf  Mass:  Size: earth-sized (~13000 km diameter)  Density:  Supported by: electron degeneracy pressure  Progenitor:  Example:

4. 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?

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  Cataclysmic variables brighten and fade periodically

7. Accretion onto a White Dwarf

8. Nova Cygni Ejected Ring

9. Black Hole  Mass:  Size: singularity  Density:  Supported by: unsupported  Progenitor:  Example: high mass X-ray binaries

10. 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

11. 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

12. 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

13. 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 

14. 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

15. Cygnus X-1

16. 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

17. Neutron Star  Mass:  Size: 10 km radius  Density:  Supported by: neutron degeneracy pressure  Progenitor:  Example: pulsar

18. 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

19. 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

20. 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

21. 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 

22. A Rotating, Magnetized N.S.

23. Pioneer 10 Plaque

24. The Crab Pulsar

25. 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

26. 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