Compact Objects

Astronomy 315Professor Lee

CarknerLecture 15

What is a Compact Object?

The densest objects in the

universe

Responsible for many unusual phenomena

White Dwarf

Mass: Size: earth-sized (~13000 km

diameter) Density: Supported by: electron degeneracy

pressure Progenitor: Example:

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?

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 Cataclysmic variables brighten and fade periodically

Accretion onto a White Dwarf

Nova Cygni Ejected Ring

Black Hole

Mass: Size: singularity Density: Supported by: unsupported Progenitor: Example: high mass X-ray binaries

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

The object is called a black hole

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

High mass, small radius means you need a

high velocity to escape

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=3X108 m/s), the light cannot escape and the object is a black hole

nothing can travel faster than light

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 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

Cygnus X-1

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

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 Msun), 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

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

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.

Pioneer 10 Plaque

The Crab Pulsar

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

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