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Chapter 14: Neutron Starsand Black Holes

Neutron Stars• Form from a 8-20 MSun star

• Leftover 1.4-3 MSun core aftersupernova

• Neutron Stars consist entirelyof neutrons (no protons)

Neutron Star (tennis ball) andWashington D.C.

Neutron Stars• About the size of a large city(5-10 miles), Several times themass of the Sun• So they are incredibly dense!• One teaspoon of a neutronstar would weigh 100 milliontons!

Neutron Star (tennis ball) andWashington D.C.

•Held up by degeneracy pressure: theneutrons don’t like to be squished closetogether!

What’s holding it up?

Ele

ctro

n en

ergy

White Dwarfs and NeutronStars are made of degeneratematter.

Degenerate matter cannot becompressed….the electrons (orneutrons) are already as closeas possible.

White dwarfs and neutron stars are held up bydegeneracy pressure

Pulsars: Stellar Beacons• Spinning neutron stars

• Strong magnetic field emits a beamradio waves along the magneticpoles

• These are not aligned with the axisof rotation.

• So the beam of radio wavessweeps through the sky as theNeutron Star spins. Model of a Pulsar

(a rotating Neutron Star)

The Lighthouse Model of Pulsars

If the beam shines on Earth, then we seea Pulse of energy (radio waves)

Neutron star’s magnetic field

A pulsar is a rotatingneutron star.

A pulsar’s beam is like a lighthouse

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The Crab Pulsar

Inside the Crab Supernova Remnant, a Pulsar has been found

A massive star dies in a Supernova explosion.

Most of the star is blasted into space.

The core that remains can be a neutron star.However…

Neutron stars can not existwith masses M > 3 Msun

If the core has more than 3 solar masses…

It will collapse completely to single point –

=> A black hole!

Degenerate MatterIf a White Dwarf gets tooheavy it will collapse… into aNeutron Star (this triggers asecond type of Supernovaexplosion)

White dwarfs cannot be moremassive than 1.4 Msun

Similarly, Neutron stars cannotbe larger than about 3 M Sun

They will collapse completelyand turn into a black hole!

Black Holes: Overview

•A total victory for gravity.

•Collapsed down to a single point.

•This would mean that they have infinitedensity

•Their gravity is so strong, not even lightcan escape!

Escape VelocityEscape Velocity (vesc) is the speed requiredto escape gravity’s pull.

On Earth vesc ≈ 11.6 km/s.

vesc

If you launch a spaceshipat v= 11.6 km/s or faster, it

will escape the Earth

But vesc depends on themass of the planet or star…

Why Are Black Holes Black?On planets with more gravity than Earth,Vesc would be larger.

On a small body like an asteroid, Vescwould be so small you could jump intospace.

A Black Hole is so massive thatVesc = the speed of light.

Not even light can escape it, so it givesoff no light!

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Black Holes & Relativity• Einstein’s theory of General Relativity says space is

curved by mass• So a star like the Sun should bend space, and light

traveling past it will get thrown off course• This was confirmed during a solar eclipse in 1919

Light Can be Bent by Gravity

Event Horizon

We have no way offinding out what’shappening inside!

Nothing can get outonce it’s inside the

event horizon

The Schwarzschild RadiusIf Vescape > c, then nothing can leave the star, not light,

not matter.

Rs =2GM____

c2

Rs = Schwarzschild radius

If something is compressed smaller than Rs it will turninto a black hole!

G = gravitational constantM = mass

c = speed oflight

We can calculate the radius of such a star:

Vesc = c

Black Holes: Don’t Jump Into One!If you fall into a Black Hole, you

will have a big problem:

Your feet will be pulled withmore gravity than your head.

You would experience “tidalforces” pushing & pulling

Time is also distorted near ablack hole

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How do we know they’re real?

• Black holes:– Kepler’s Laws, Newton’s Laws– Accretion disks

• Pulsars:– Observe radio jets– Strong magnetic fields

Evidence for Black HolesNo light can escape a black hole, so

black holes can not be observed directly.

However, if a blackhole is part of a binarystar system, we canmeasure its mass.

If its mass > 3 Msun thenit’s a black hole!

Evidence for Black Holes• Cygnus X-1 is a source of X rays• It is a binary star system, with an O type supergiant & a compact

object

Cygnus X-1: A black hole

The mass of the compact object ismore than 20 Msun

This is too massive to be a whitedwarf or neutron star.

This object must be a black hole.

Evidence for Black Holes: X-rays

Artists’ drawings ofaccretion disks

Matter falling into a black hole may form an accretion disk.As more matter falls on the disk, it heats up and emits X-rays.If X-rays are emitted outside the event horizon we can seethem.

Supermassive Black Holes

• Stellar black holes come fromthe collapse of a star.

• They have masses of severalMsun

• Bigger mass = bigger BH!

• This happens in the center ofmost galaxies.

A supermassive black holedevours a star, releasing X-rays

Life Cycles of Stars• Low-mass stars: Fade out, stay on Main Sequence• Sun-like stars: White dwarf & planetary nebula• High-mass stars: Supernova -> SN remnant & dense

core– Core < 1.4 MSun = W.D.– 1.4 MSun < Core < 3 MSun = Neutron Star– Core > 3 MSun = Black Hole

Lifetime Mass

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Stellar Evolution Lecture-Tutorial: Page 121

• Work with a partner or two• Read directions and answer all questions carefully.

Take time to understand it now!• Come to a consensus answer you all agree on before

moving on to the next question.• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I will

come around.

The Milky Way

“Milky Way”: A band of light and a Galaxy

Milky Way probably looks likeAndromeda.

The band of light we see isreally 100 billion stars

Milky Way Composite Photo

• Bulge in the center• Dark strip in the middle, from dust

Milky Way

Before the 1920’s, astronomers used a “grindstonemodel” for the galaxy

Tried to estimate our location in the galaxy by countingstars in different directions

Because some stars are blocked by dust, the trueshape of this group of stars was unclear.

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Finding the Center

• Harlow Shapely studiedglobular clusters.

• He theorized that they mustorbit the true center of thegalaxy

A Globular Cluster

Finding the CenterShapely plotted the locations of the globular star clusters.

He found that they are are not centered on the Sun….

…but are centered on a point about 25,000 light years from theSolar System.

The Milky WaySize: The Milky Way isroughly 100,000 light yearsacross, and about 1,000light years thick.

Stars: The Milky Way iscomprised of over 100billion stars!

Almost everything visiblewith the naked eye isinside the Milky Way

Parts of Our Galaxy

100,000light years

1,000light years

Disk: The Sun Resides in theDisk

Nuclear Bulge: The densecentral region

Halo: Spherical regionsurrounding the disk where theglobular clusters live.

Top View

Side View

Parts of OurGalaxy

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Milky Way Scales LectureTutorial: Page 123

• Work with a partner or two• Read directions and answer all questions carefully.

Take time to understand it now!• Discuss each question and come to a consensus

answer you all agree on before moving on to the nextquestion.

• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I will

come around.