Compact Objects

Astronomy 315Professor Lee

CarknerLecture 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

What is a Compact Object?

The leftover core of a dead star Objects that are supported by

(strange) physics rather than thermal pressure

White Dwarf

Mass: Size: Density: Supported by: Progenitor: Example:

Observing White Dwarfs

White dwarfs are very faint

We can only see the near-by ones However, most stars are in

multiple systems

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

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

Accretion onto a White Dwarf

Nova Cygni Ejected Ring

Neutron Star

Mass: Size Density: Supported by: Progenitor: Example:

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

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

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

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

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

The Crab Pulsar

Black Hole

Mass: Size : Density: Supported by: Progenitor: Example:

Limits of Neutron Degeneracy

If a stellar core has more than about 3 Msun, not even neutron degeneracy pressure can support it

A huge mass in such a tiny space

creates a powerful gravitational field

Escape Velocity What is required for an object to escape

from a mass (planet or star)?

Velocity is related to kinetic energy (KE = ½mv2) , so the object must have more kinetic energy than the gravitational energy that holds it back

Escape velocity ~ (M/R)½

General Relativity According to Einstein energy and mass are

the same thing (E=mc2)

If the escape velocity of an object is

greater than the speed of light (c=3X108 m/s), the light cannot escape and the object is a black hole If light can’t escape, nothing can

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

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:

Finding Black Holes We can detect compact objects by

finding X-ray binaries If the mass of the compact object

is greater than 3Msun, it must be a black hole

Cygnus X-1

Next Time

Quiz 2 Covers everything since Quiz 1

Lectures 10-15, Stellar Interiors through Compact Objects

Same format (multiple choice and short answers)