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

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

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 (Type I) supernova leave behind?

What about a core-collapse (Type II) supernova?

Neutron Stars

Remnant of Type II supernova (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?

The Crab Pulsar

Crab nebula created by Type II supernova in 1054 AD

Pulse rate about 30 times/sec

Can see associated supernova remnant

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, RS. R

S m.

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".

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 severely warp or curve the space around them. According to Einstein's General Relativity, all masses curve space.

How does this change our understanding of the gravitational force?

(Grav. Well Demo)

Effects around Black Holes

Near event horizon:

1) Enormous tidal forces.

2) Bending of light.

2) Gravitational redshift.

3) Time dilation.

Examples:

2. 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.

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!

3. Gravitational Redshift

Consider accelerating elevator in free space (no gravity).

time zero, speed=0

later, speed > 0 light received when elevator receding at some speed.

light emitted when elevator at rest.

Gravity must produce same result! But now due to reduction of photon energy required to escape gravitational field, not Doppler effect! Photon Energy Frequency!

4. Gravitational Time Dilation

A photon moving upwards in gravity is redshifted.

Since

the photon's period also gets longer. Observer 1 will measure a longer period than Observer 2.

Observer 1 would say that Observer 2's clock runs slow!

1

2

All these effects are negligible in our daily experience.

Wavelength Period

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.