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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
White Dwarf
Size: earth-sized (~10000 km diameter)
Supported by: electron degeneracy
Example: nova
Sirius B
 In 1844 Bessel determines
Sirius is a 50 year binary via
 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
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
White dwarf has strong gravitational field
Called a cataclysmic variable
We see the star brighten as a nova
Cataclysmic variables brighten and fade
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/s2)
g = GM/r2
M is the mass of the star or planet (in
High mass and small radius means stronger
Neutron Star
Size: 10 km radius
Supported by: neutron degeneracy
Example: pulsar
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
Cannot occupy the same state
Neutron Star Properties
Small size means low luminosity and high
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 are radio sources that blink on and off
with very regular periods
Each pulse is very short
What could produce such short period
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
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
We can only see pulsars if the beam is
pointing at us
Beam is very narrow so some pulsars are
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
Next Time
Read Chapter 22.5-22.8