Stellar Deaths II

Neutron Stars and Black Holes 17

CPS Question

● The pressure that prevents the gravitational collapse of white dwarfs is a result of ______ .

– A) Conservation of energy

– B) Conservation of angular momentum

– C) Einstein's principle of equivalence

– D) The Pauli exclusion principle

CPS Question

● A nova is a result of _____ .

– A) a supergiant star shedding its envelope

– B) the explosion of an extremely high-mass star

– C) fusion on the surface of an accreting white dwarf

– D) pulsation of an isolated brown dwarf

CPS Question

• A low mass object that failed to start nuclear fusion is known as a ______ dwarf.

– A) brown– B) red– C) white– D) black

1. White Dwarf If initial star mass < 8 M

Sun or so.

2. Neutron Star If initial mass > 8 M

Sun and < 25 M

Sun .

3. Black Hole If initial mass > 25 M

Sun .

Final States of a Star

Supernova remnants

• What type of remnant does a carbon-detonation supernova leave behind?

• What about a core-collapse supernova?

Neutron StarsType I supernova (Carbon-detonation): no remnant

Remnant of core-collapse supernova - a tightly packed ball of neutrons.

Diameter: only ~20 km!

Mass: 1.4 - 3 MSun

Incredible densities!

Conservation of Ang. Mom.=> what?

A neutron star over the Sandias?

Neutron StarsType I supernova (Carbon-detonation): no remnant

Remnant of core-collapse supernova - a tightly packed ball of neutrons.

Diameter: only ~20 km!

Mass: 1.4 - 3 MSun

Incredible densities!

Conservation of Ang. Mom.=> Rapid rotation rate

Magnetic field: 1012 x Earth's!

A neutron star over the Sandias?

Pulsars

Objects that give off periodic pulses of radiation Frequencies: ~one to several hundred pulses/sec

What are they?

Pulsars

• What produces the beam of radiation received from a pulsar?

• Do we see all neutron stars as pulsars? Why or why not?

The Lighthouse Model of a Pulsar• Rapid rotation =>

huge magnetic field• Accelerates

charges• Narrow “beam” of

radiation• Not all neutron

stars are pulsars• Can be too old • Orientation may

be wrong

Pulsars are incredibly accurate clocks!

Accurate to within a few seconds in a million years!

Better than best atomic clocks on Earth!

Observed fluctuations in period can be used to detect planets orbiting a pulsar.

1. White Dwarf If initial star mass < 8 M

Sun or so.

2. Neutron Star If initial mass > 8 M

Sun and < 25 M

Sun .

3. Black Hole If initial mass > 25 M

Sun .

Final States of a Star

Concept Review

• What determines whether or not an object is capable of retaining an atmosphere?

• How might this concept be related to black holes?

Review of Escape Speed

Speed needed to escape the gravitational pull of an object.

vesc

= 2GM R

Escape speed from Earth's surface is 11 km/sec.

If Earth were crushed down to 1 cm size, escape speed would be the speed of light.

Schwarzschild Radius = if an object is crushed to within the Sr, not even light can escape its gravitational pull

Black Holes

If core with > 3 MSun

collapses, not even neutron pressure (Pauli Exclusion Principle) can stop the collapse (initial mass of star > 25 M

Sun).

Collapses to a point, a "singularity". Still searching for a theory of quantum gravity.

Gravity is so strong that nothing can escape, not even light (so no information) => black hole.

So, how might we detect them?

Event horizon: “Surface” of black hole. Point of no return.

Event horizon

Schwarzschild Radius

Black Holes

• What are some of the strange phenomena we might encounter if we fell into a black hole?

Effects around Black Holes

Near event horizon:

1) Enormous tidal forces.

2) Bending of light:

2) Gravitational redshift.

3) Time dilation.

Examples:

1. Bending of light. If light travels in straight lines in free space, then gravity causes light to follow curved paths.

First observed in 1919 eclipse.

Gravitational lensing. The gravity of a foreground cluster of galaxies distorts the images of background galaxies into arc shapes.

• Gravitational Red shift• Light emitted from a large gravity

well object (large star) will appear red shifted on smaller object

• Photons need energy to leave gravity well so photons lowers in frequency to provide that energy and become red shifted

Einstein's Principle of Equivalence

• What phenomenon produces all of the same physical effects as being in gravity?– (Hint: Think about riding in an elevator.)

Einstein's Principle of Equivalence

• According to Einstein, the effects of gravity and acceleration are indistinguishable from one another!

• The laws of physics are identical in either case.– => acceleration should be able to

reproduce effects of a black hole!

Do Black Holes Really Exist? Good Candidate: Cygnus X-1

- Binary system: 30 MSun

star with unseen companion.

- Binary orbit => companion ~10 MSun

.

- X-rays => million degree gas falling into black hole.