Compact Objects

1. Astronomy 315 Professor Lee Carkner Lecture 15 Compact Objects

2. What are Compact Objects? • The densest objects in the universe • Can produce strong, high-energy radiation and outbursts when in binary systems

3. White Dwarf • Mass: • Size: earth-sized (~10000 km diameter) • Density: • Supported by: electron degeneracy pressure • Progenitor: • Example: nova

4. Sirius B • In 1844 Bessel determines Sirius is a 50 year binary via astrometry • In 1862 Alvan G. Clark finds Sirius B in a telescope test • In 1915 Walter Adams uses spectroscopy to get a surface temperature for Sirius B of 27000 K • Three times hotter than Sirius A • but much fainter than Sirius A

5. Observing White Dwarfs • White dwarfs are very faint • We can only see the near-by ones • Hard to find if they aren’t in an interacting binary

6. Mass Transfer • Stars in a binary can transfer mass • have to be close together • This material ends up in a accretion disk • Friction makes the disk very hot • Material will accrete onto the white dwarf

7. Cataclysmic Variables • Material gets hot as it is compressed by new material • White dwarf has strong gravitational field • Called a cataclysmic variable • We see the star brighten as a nova • Cataclysmic variables brighten and fade periodically

8. Accretion onto a White Dwarf

9. Acceleration of Gravity • How much force would you feel if you stood on a white dwarf? • Acceleration of gravity (units: m/s2) g = GM/r2 • M is the mass of the star or planet (in kilograms) • High mass and small radius means stronger gravity

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

11. 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

12. 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 • A trillion times strong than the sun’s

13. 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? • A large object could not spin fast enough without flying apart • Only neutron stars are small enough

14. 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 • When the beam is pointed at us, the pulsar is “on”, when it is pointed away it is “off”

15. A Rotating, Magnetized N.S.

16. 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 • Beam is very narrow so some pulsars are undetectable

17. Millisecond Pulsars • Near the break-up speed • Many are found in very old clusters • Should have spun down by now

18. Pulsars in Binary Systems • Mass adds angular momentum to the pulsar and counteracts the natural spin down • In extreme cases can produce an powerful magnetically collimated jet • Like a T Tauri star

19. X-Ray Burster • The strong gravitational pressure on this material causes an explosive burst of fusion • Produces a burst of X-rays • Each burst is about 1000 times as luminous as the sun

20. Next Time • Read Chapter 22.5-22.8