Lecture Chap18 v1

8/10/2019 Lecture Chap18 v1

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Chapter 18

The Bizarre Stellar Graveyard



What Happens At the End?

Very high-mass star

High-mass star

Low-mass star

Black Hole

Neutron Star

White Dwarf

Core evolves into:



18.1 White Dwarfs

Our goals for learning

• What is a white dwarf?

•

What can happen to a white dwarf in a close binary system?



What is a white dwarf?

Very high-mass star

High-mass star

Low-mass star

Black Hole

Neutron Star

White Dwarf

Core evolves into:



Off Main Sequence

! Planetary Nebula and White Dwarf

•

Double-shell burning ends with a pulse that ejects the

H and He into spaceas a planetarynebula

•

The core left behind becomes a whitedwarf



White Dwarfs•

White dwarfs are

the remaining cores

of dead stars

• Electron

degeneracy pressure

supports themagainst gravity



White dwarfs

cool off and

grow dimmer with time

No fusion in white dwarf,

but radiation at low level

! losing energy gradually.



Size of a White Dwarf

• White dwarfs with same mass as Sun areabout same size as Earth

• Higher mass white dwarfs are smaller



The White Dwarf Limit• Degeneracy pressure is not strong enough to support a

white dwarf with >1.4 M Sun against its gravity

• A white dwarf cannot be more massive than 1.4 M Sun,the white dwarf limit (or Chandrasekhar limit )

What happens if white dwarf mass exceeds

the white dwarf limit?



If nothing happens,

white dwarfs cool off

and grow dimmer

with time, and fade

away forever .



What can happen to a white

dwarf in a close binary system?



Binary system

Two stars next to each other – do they somehow interact?



Star that started with less

mass gains mass from its

companion

What happens next?

What happens if the lessmassive star is a white

dwarf? – can the mass

exceed the white dwarf

limit (1.4Msun)?

Massive Less Massive

Off mainsequence first,

and becomes

giants

Mass transfer



Accretion Disks

•

Mass falling toward

a white dwarf from

its close binary

companion hassome angular

momentum

•

The matter

therefore orbits the

white dwarf in an

accretion disk

Mass does not get directly onto the secondary star (white dwarf).



Accretion Disk Formation

Star



Accretion Disks

•

Friction between orbiting rings of matter in the disktransfers angular momentum outward and causes the

disk to heat up and glow.

• Material gradually moves inward, and falls onto the star.

Friction: “C” is moving slower than“B”, and drags “B” backward.

! Move angular momentum outward.



Thought Question

What happens to a white dwarf when it accretes enoughmatter to reach the 1.4 M Sun limit?

A. It explodes

B. It collapses into a neutron star

C. It gradually begins fusing carbon in its core

Remember: the question is

What happens if the less massive star is

a white dwarf? – can the mass exceed

the white dwarf limit (1.4Msun)?



Two Types of Supernova

Massive star supernova: -- End of Massive Stars

Iron core of massive star reaches

white dwarf limit and collapses into a

neutron star, causing explosion

White dwarf supernova:

Carbon fusion suddenly begins as white

dwarf in close binary system reachesthe white dwarf limit, causing total explosion

Complete explosion of white dwarf, nothing left behind



One way to tell supernova types apart is with a light

curve showing how luminosity changes with time



Supernova Type:

Massive Star or White Dwarf?

• Light curves differ

• Spectra differ (exploding white dwarfs

don’t have hydrogen absorption lines)



White Dwarf Supernova:

Standard Candle•

White Dwarf Supernova always has the

mass of about 1.4 Msun.

•

That means, we know the amount of fuel forexplosion, and therefore, we know the

luminosity.

• If we compare the brightness measured on

Earth with the predicted luminosity, we can

get distance.

Supernova is bright

It is used to measure the expansion rate of the Universe.



What have we learned?

•

What is a white dwarf?

–

A white dwarf is the inert core of a dead star

– Electron degeneracy pressure balances theinward pull of gravity

• What can happen to a white dwarf in aclose binary system?

– Matter from its close binary companion can

fall onto the white dwarf through an accretiondisk

–

Accretion of matter can lead to novae andwhite dwarf supernovae



18.2 Neutron Stars

Our goals for learning

• What is a neutron star?

•

How were neutron stars discovered?

• What can happen to a neutron star in a close binary system?



Midterm 2

•

Wednesday, Nov 5 (11:00-11:50) • Place – Simons Center 103 (regular class room)

• Chapters: 14, 15, 16, 17, 18, S4 (textbook & notes)

•

Bring: Student ID, Pencils, & Eraser

• NO MAKEUP:

– Only on the basis of

• Valid medical absence (with written evidence)

•

Jury Duty

• Military service

• Ask help before exam, not after.

–

I cannot help after exam (want to be fair to all students



What is a neutron star?

Very high-mass star

High-mass star

Low-mass star

Black Hole

Neutron Star

White Dwarf

Core evolves into:



A neutron star

is the ball ofneutrons left

behind by a

massive-star

supernova



Electron degeneracy pressuregoes away because electrons

combine with protons, making

neutrons and neutrinos

Neutrons collapse to the center,

forming a neutron star

Super Onion Structure

At the end of high-mass star

Angular momentum must be

conserved, so a neutron star is often

spinning very rapidly.



A neutron star is about the same size as a small city (~ 10km)



How were neutron stars

discovered?



Discovery of Neutron Stars

• Using a radio telescope in 1967, Jocelyn Bell noticed

very regular pulses of radio emission coming from asingle part of the sky

•

The pulses were coming from a spinning neutron star

—a pulsar



Pulsar at center

of Crab Nebula pulses 30 times

per second !!

Imagine a star with the

size of Stony Brook is

spinning 30 times per

second.



Series of images observed by the Hubble Space Telescope.

Pulsation



Pulsars

•

A pulsar is a

neutron star that

beams radiation

along a magneticaxis that is not

aligned with the

rotation axis



Pulsars

•

The radiation beams

sweep through

space like

lighthouse beams as

the neutron starrotates

Radiation beam has to be

directed toward us.Otherwise, we cannot see

them.

We have to be lucky to find it.



Why Pulsars must be Neutron Stars

Circumference of NS = 2! (radius) ~ 60 km

Spin Rate of Fast Pulsars ~ 1000 cycles per second

Surface Rotation Velocity ~ 60,000 km/s

~ 20% speed of light

~ escape velocity from NS

If not as compact as (theoretically predicted) neutron star,

it would be torn to pieces!




•

What is a neutron star?

–

A ball of neutrons left over from a massivestar supernova and supported by neutrondegeneracy pressure

•

How were neutron stars discovered?

– Beams of radiation from a rotating neutronstar sweep through space like lighthouse beams, making them appear to pulse

–

Observations of these pulses were the firstevidence for neutron stars




•

What can happen to a neutron star in aclose binary system?

– The accretion disk around a neutron star getshot enough to produce X-rays, making thesystem an X-ray binary

– Sudden fusion events periodically occur on athe surface of an accreting neutron star, producing X-ray bursts



What is a black hole?

Very high-mass star

High-mass star

Low-mass star

Black Hole

Neutron Star

White Dwarf

Core evolves into:



A black hole is an object whose gravity is so powerful

that even light cannot escape from it!

What is a black hole?

This is not a science fiction, so it’s not like



What does “no escape” mean?



Thought Question

A. It increases

B. It decreases

C. It stays the same

Escape velocity?

What happens to the escapevelocity from the surface of an

object if you shrink it?



“Surface” of a Black Hole

•

The “surface” of a black hole is the radius at which

the escape velocity equals the speed of light, not the

surface of the star itself.

• This spherical surface is known as the event horizon .

• The radius of the event horizon is known as the

Schwarzschild radius .



“Surface” of a Black Hole

Schwarzschild radius

event horizon



The event horizon of a 3 M Sun black hole is also about

as big as a small city



Event horizon is

larger for black

holes of larger mass

Larger mass

! Stronger gravity

! More difficult to escape

! Larger radius of event horizon



A black hole’s mass

strongly warps

space and time in

vicinity of event

horizon



NO Escape

• Nothing can escape from within the event

horizon because nothing can go faster than light.

•

No escape means there is no more contact with

something that falls in. It increases the black hole

mass, changes the spin or charge, but otherwise

loses its identity.



Neutron Star Limit

• Strong force (force between nucleon) can no

longer support a neutron star against gravity if its

mass exceeds about 3 M sun

•

Some massive star supernovae can make a black

hole if enough mass falls onto core

Remember: The White dwarf limit is 1.4 Msun. Beyond that, theelectron degeneracy pressure is not strong enough to support the white

dwarf against gravity.

Neutron star is supported by “strong force” – force between nucleons



Singularity• Beyond the neutron star limit, no known force can

resist the crush of gravity.

• As far as we know, gravity crushes all the matter into

a single point known as a singularity .



What would it be like to visit a

black hole?



Light waves take extra time to climb out of a deep hole in

spacetime, leading to a gravitational redshift



Remember: Time in Gravity

•

Light pulses travelmore quickly fromfront to back of anaccelerating spaceshipthan in other direction

• Everyone on shipagrees that time runsfaster in front than in

back

• Effects of gravity areexactly equivalent to

those of acceleration



Time passes more slowly near the event horizon



Tidal forces near the

event horizon of a

3 M Sun black holewould be lethal to

humans

Tidal forces would be

gentler near a

supermassive blackhole because its radius

is much bigger



Do black holes really exist?



Black Hole Verification• Need to measure mass

—

Use orbital properties of companion

—

Measure velocity and distance of orbiting gas

•

It’s a black hole if it’s not a star and its mass

exceeds the neutron star limit (~3 M Sun

)



Some X-ray binaries contain compact objects of mass

exceeding 3 M Sun which are likely to be black holes



One famous X-ray binary with a likely black hole is in

the constellation Cygnus




•

What is a black hole? –

A black hole is a massive object whose radiusis so small that the escape velocity exceedsthe speed of light

•

What would it be like to visit a black hole?

– You can orbit a black hole like any otherobject of the same mass—black holes don’tsuck!

–

Near the event horizon time slows down andtidal forces are very strong




•

Do black holes really exist? –

Some X-ray binaries contain compact objectstoo massive to be neutron stars—they arealmost certainly black holes

Lecture Chap18 v1

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