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Chapter 11: 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?

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White Dwarfs and NeutronStars are made of degeneratematter.

Degenerate matter cannot becompressed….the neutronsare already as close aspossible.

White dwarfs and neutron stars are held up byDegeneracy pressure

Pulsars: Stellar Beacons• Rotating 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 _________ Model of Pulsars

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

Neutron star’s magnetic field

A pulsar is a ______neutron 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 _________ 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 > ___ Msun

If the core has more than 3 solar masses…

It will collapse completely to _____ _____ –

=> 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 ____ Msun

Similarly, Neutron stars cannotbe larger than about ___ M Sun

They will collapse completelyand turn into a _____ ____!

Black Holes: Overview

•A total victory for _______.

•Collapsed down to a single point.

•This would mean that they have ______density

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

Escape VelocityEscape Velocity (vesc) is the speed requiredto escape _______’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 the_____ of the planet or star…

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

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

A Black Hole is so massive thatVesc = the _____ __ _____.

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 ______ by mass• So a star like the Sun should _____ 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!

________ can getout once it’s insidethe event horizon

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

not _______.

Rs =2GM____

c2

Rs = Schwarzschild radius

If something is _________ smaller than Rs it will turn intoa 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 ______ than your head.

You would experience “tidalforces” pushing & pulling

____ 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 _____.

If its mass > __ Msunthen it’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

“_______ ______”

Cygnus X-1: A black hole

The mass of the compact object ismore than ___ 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 ______.If X-rays are emitted outside the event horizon we can seethem.

Supermassive Black Holes

• Stellar black holes comefrom the collapse of a star.

• They have masses ofseveral Msun

• Bigger mass = bigger BH!

• This happens in the centerof most 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 = _____ ______– 1.4 MSun < Core < 3 MSun = _______ _____– Core > 3 MSun = _____ ____

Lifetime Mass

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The Milky Way “Milky Way”: A band of ____ and a _______

Milky Way probably looks likeAndromeda.

The band of light we see isreally 100 billion stars

Milky Way Composite Photo

• ______ in the center• Dark strip in the middle, from _____

Milky Way

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

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

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

Finding the Center

• Harlow Shapely studied_______ ________.

• He theorized that they mustorbit the true ______ of thegalaxy

A Globular Cluster

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

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

…but are centered on a point about _______ light years fromthe Solar System.

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The Milky WaySize: The Milky Way isroughly _______ lightyears across, and about_____ light years thick.

Stars: The Milky Way iscomprised of over __________ stars!

Almost everything visiblewith the naked eye isinside the Milky Way

Parts of Our Galaxy

Disk: The ____ Resides in theDisk

Nuclear Bulge: The dense_______ region

Halo: Spherical regionsurrounding the disk where the_______ ________ live.

Parts of OurGalaxy

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.

Questions:

• How big is the Milky Way?• Where are stars forming (or not forming)?• How much mass is in the Milky Way?• What’s going on at the center?

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Milky Way: A Spiral Galaxy• Our galaxy seems to be _______: it has spiral arms

• These are dense concentrations of _____ and ____.

• Stars orbit the ___________, pass through thespiral arms as they go.

• Stars ____ _____ andpile up in the spiral arms,like cars in a traffic jam.

The Disk contains ___,so stars are still formingthere. (Population Istars)

The Halo has very little___, and no new starsare forming there.

The halo of the galaxyis populated by ___stars. (Population IIstars)

Star Formation in the Milky Way

Stellar Populations

• Pop. I: Newer, disk & spiral arm stars,with _____ percentage heavy elements

• Pop. II: Older, bulge and halo stars, with_____ percentage of heavy elements

• Heavy elements (metals): anything thatisn’t H, He, or Li

Measuring Distances

•To map the Milky Way Galaxy, we need tomeasure _______ to stars.

• Parallax only works for nearby stars (withinabout ____ light years)

• For more distant stars, we use StandardCandles

Standard Candles

Car Headlights are standard candles: We use them to determine the car’s distance

Standard Candles

• We can easily measure how bright a star appears(________ magnitude)

•If we knew how bright the star really was (its ________magnitude) then we could calculate its distance.

• We need a star whose absolute magnitude is alwaysthe same, wherever we observe it.

• Such a star is called a “standard candle”

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Cepheid Variables

In 1908, astronomer Henrietta Leavitt discovered a newstandard candle using ______ stars

These stars are called _________

They are named for δ Cephei, thefirst example of such a star.

Henrietta Leavitt

Delta Cephei

Measuring Distances with CepheidsCepheid stars change in brightness.

They pulsate in a very regular way.

Large, bright Cepheids pulsate_____, while small, dim Cepheidspulsate _______.

If we observe the period ofpulsation, we can figure out theabsolute magnitude &luminosity.

If we compare this to the apparentmagnitude, we find the distance!

The Structure of the Milky Way

By measuring the distances to various parts of theMilky Way Galaxy, we map out its structure

The Milky Way is a_______ ______ Galaxy

It has a straight structureat the center called a Bar

Mapping the Milky Way

A modern map of the Milky Way (computer-generated diagram)

The Sun is about_________ out fromthe center

Measuring the Mass of the Milky WayWe use the Sun’s ______around the center of the MilkyWay

The greater the mass insidethe orbit, the ______ the Sunhas move around the center.

This way we can measure themass of the Milky Way.

Total mass: about ___ _______ MSun

Mass of the Milky Way

• The mass of the Milky Way is between ___ billionand MSun and ___ billion MSun

• Stars & Gas we see in the Milky Way can onlyaccount for a fraction of the total mass.

-What is it?

- Why can’t we see it?

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The Center of the Milky Way The Center of Our Galaxy• The ______ of stars in the Galactic Center is

much greater than in the Sun’s neighborhood.• They appear to be orbiting a ____________ black

hole at the center of the galaxy

Its mass is over__ _____ MSun !

Chapter 13

Galaxies

We now realize that our galaxy is onlyone of billions of galaxies we can see.

These galaxies come in three maintypes:

Spiral, ________ & _______

Spiral Galaxies

M 100 NGC 300

• Typically very bright, _____ in color• Look like _________ (sometimes with

____)

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… are ____, notflat like spirals

They are typically___ in color.

Less gas and dustthan spirals.

EllipticalGalaxies

Irregular Galaxies• Lack any distinct shape• Are generally ______ than spirals and ellipticals

Hubble Tuning Fork_______ Galaxies (S): Classified according to spiral arms (a,b,c)

and presence of a bar (“B”)

_______ Galaxies (E): Classified according to shape (E0-E9)

_______ Galaxies(Irr): Basicallyanything funky-looking!