Compact Objects Astronomy 315 Professor Lee Carkner Lecture 15

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

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

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 K  Three times hotter than Sirius A  but much fainter than Sirius A

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

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

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

Accretion onto a White Dwarf

Acceleration of Gravity  How much force would you feel if you stood on a white dwarf?  Acceleration of gravity (units: m/s 2 ) g = GM/r 2   M is the mass of the star or planet (in kilograms)   High mass and small radius means stronger gravity

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   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

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

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”

A Rotating, Magnetized N.S.

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

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

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

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

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