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Lifetimes of StarsEstimate a star’s lifetime based on howmuch fuel it has, and how fast it uses upits fuel

T =

M = Mass of star in _____ ______ (MSun)T = Lifetime of star in _____ _____ (TSun)

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M2.5____

Hydrostatic Equilibrium

• ______ is pulling the outer part of a stars toward the center

• Thermal ________ can resist the pull of gravity.

• Nuclear fusion in the core of a star releases hugeamounts of energy.

• This energy (heat) creates the thermal pressure.

• Hydrostatic Equilibrium occurs when gravity and pressure forces ______

Hydrostatic equilibrium keepsstars stable for billions of years.

The inward force of ______ isbalanced by the force of________ pushing out.

All stars on the Main Sequence (ofthe HR Diagram) are ______ and in

____________.

Pressure-TemperatureThermostat

• Main sequence stars are self-regulatingsystems, small changes get corrected

If thermal pressure drops:1. Star _______ a little2. Density _________3. Temperature _________4. Nuclear reactions ______ __5. Thermal pressure _____ again

Leaving the Main Sequence

Stars live mostof their lives on

the MainSequence

When they run out ofHydrogen Fuel, they

leave the _____________ and begin

to ____.

Star Formation and LifetimesLecture-Tutorial: Pg. 111-112

• 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.

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Lifetimes of Stars• Low-mass stars: Economy Cars• High-mass stars: Gas Guzzlers

LifetimeMass

Creating Elements withNuclear Fusion

• ______ _____ is the source of energy for all stars.

• Hydrogen (H) can be fused into Helium (He) intwo ways:

– Proton-Proton Chain– C-N-O Cycle

• Stars can also fuse He into: Carbon, Nitrogen &Oxygen

• Anything heavier than _______ is only made instars!

Examples of Nuclear FusionThe “Proton-Proton” Chain

Used by the Sun tofuse protons (H nuclei)into He

4 H −> He + energy

CNO Cycle: Another way tofuse H -> He

CNO cycle isused in massivestars and involves:Carbon (C)Nitrogen (N) &Oxygen (O)

Examples of Nuclear FusionMaking Heavy Elements

Starting with H, He, C, O ,or N fusion can create many

other elements:

Mg, Na, Al, Si, etc.

Stellar Recycling

The universestarts out with H,He, and a little Li:everything else isformed in stars!

Material getsblown away fromdying stars and

recycled intonebulae, new

stars, and planets

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• How do the deaths of stars differ based onthe stars’ masses?

• What happens to a star when fusion stops?

• How do giants, supergiants, and whitedwarfs form?

An Epic Battle: Gravity vs. Pressure

What happens at the end of a star’s life?

3 possible outcomes:

•______ wins: the star collapses completely intoa black hole•________ wins: the entire star explodes intospace•____: The center of the star contracts, while theouter layers expand.

Low-Mass Stars(0.4 Msun or less)

• HUGE _________zone!

• Hydrogen & Helium get_____ throughout star’slifetime

• T > 100 billion years– Longer than the age of

the ________

Average-Mass StarsStars like the Sun will expand, and turn into red_______

They lose their outer layers which expand to becomea __________ nebula

All that remains is the hot core of the star: a ______dwarf

Average-Mass Stars• Lifetime of the Sun: About 10 billion years total• After H fuel is used up in the core, fusion _____• Core _______: heats up area around the core• H shell fusion around He core• Energy from shell fusion forces outer layers to

expand, ____ ___: the Sun becomes a Red Giant

Average-Mass Stars• The Sun becomes a Giant,

leaves the Main Sequence(doesn’t ____ __ _____!)

• It runs out of fuel, theThermal Pressure in the coredecreases, and gravity willcause the core to shrink

• If the core can shrink enough,it can start ______ fusion

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The Sun’s Last Gasp

• Helium Fusion– Produces Carbon and Oxygen– Is ______ than hydrogen fusion.

• So the star’s outer layers heat up andexpand even more….until they are lost tospace.

• These layers form great ______ called: planetary nebulae

Planetary NebulaeThe last gasps of dying stars

Helix Nebula (close-up view)

(not related to planets!)

Planetary Nebulae• Helium burning

ends with a pulsethat ejects the Hand He into spaceas a planetarynebula

• The core leftbehind becomes a_____ ____

White Dwarfs

• The core of the dying star is leftbehind.

• It is very hot: _______K• It is blue or even white, and called a

White Dwarf

• White dwarfs are ____ _____!

• The Sun will end its life as a WhiteDwarf, slowly cooling down.

White DwarfsA white dwarf is hot, but very____.

White dwarfs are only about asbig as the Earth, but have themass of the ___!

So, they are incredibly ______.

One teaspoon of white dwarfmaterial would weigh 5 tons!!!

Sirius B(White Dwarf)

Sirius A (Main Sequence Star)

What’s holding it up?• No fusion is happening in a white dwarf• So what’s stopping it from collapsing?

Ele

ctro

n en

ergy Degeneracy Pressure:

Counteracts gravity inwhite dwarfs, keepingthem stable

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The Death ofHeavier Stars

Heavyweight stars expand andturn into _________

A supergiant runs out of fuel &causes a massive explosioncalled a ___________.

Could become neutron staror a black hole

Death of Massive Stars:Red Supergiants

• Very massive stars burn up their H fuel______.

• They also expand dramatically.– _____ times larger than the Sun!

Now called red supergiants.

Betelgeuse is a redsupergiant in theconstellation Orion

Evolution of Massive Stars

A massive star is mostlyunfused ______ &_______

In its core, Helium isfusing into Carbon &Oxygen

Around the core a _____of Hydrogen can fuse toHelium.

Death of Massive Stars• Massive stars can fuse

Helium into Carbon andOxygen after their Hydrogenfuel has run out.

• They can also create heavierand heavier elements:Neon, Magnesium, … andeven Iron.

• However this process stopswith _____.– Fusing Iron will not produce

additional ______.

Close-up of Core

Fe Si O C He H

Core-Collapse Supernovae

• Once fusion stops,the core begins to_______ quickly

•Outer core layers_______ off the ironcenter

•Rebound causes anenormous explosion:A Type II __________

Low-mass Stars:0.4 MSun or less

• Only fuse Hydrogen• Remain a star (no planetary nebula)• Stay on Main Sequence

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Medium-mass Stars:0.4 MSun - 8 MSun

• Fuse H & He, some Carbon• Become cool giants• Expel outer layers -> planetary nebulae

& WD

High-mass Stars:more than 8 MSun

• Fuse heavy elements up to Iron• Become supergiants• End as Supernova & Neutron Star or BH

The Crab Supernova Remnant

• Today in that same partof the sky we find the_____ _______

• It has a ______ ____inside.

• It is also expanding insize.

• In 1054 AD observers in China, Japan, andKorea recorded a “Guest Star”

• Bright enough to be seen during the ______!

Supernova Remnants

The Cygnus Loop

The Crab Nebula:

Remnant of asupernova observed

in a.d. 1054

How Do Supernovae Explode?

Supernovae are very complex and not wellunderstood.

_______ ______ of the explosion must betested by observations of a real supernova.

No supernovae in our galaxy since Kepler’sSupernova (1604)

Observing Supernovae• We observe supernovae in other _______.• ____ __ ____ supernovae per century per galaxy• No supernova in our Galaxy for 400 years…. We

are overdue!

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Different types of supernovae:• Mass-______: white dwarfs in binaries

gaining mass & exploding– Type Ia, no H lines

• Mass-____: large star loses outer layers in abinary, core explodes– Type Ib, no H lines

• ____-________: deaths of massive stars– Type II, Hydrogen lines present

Fig. 10-12, p. 212

Mass Transfer

What’s left after asupernova?

• Depends on the ____ of the originalstar!

• 8-20 MSun: Core collapses to a ______star– Spinning? -> _______

• More than 20 MSun: _____ ____!

Chapter 11: Neutron Starsand Black Holes

Neutron Stars• Form from a 8-20 MSun star

• Leftover ___ - __ MSun coreafter supernova

• Neutron Stars consist entirelyof ________ (no protons)

Neutron Star (tennis ball) andWashington D.C.

Neutron Stars• About the size of a large ____(5-10 miles), Several times themass of the _____• So they are incredibly dense!• One teaspoon of a neutronstar would weigh ___ ______tons!

Neutron Star (tennis ball) andWashington D.C.

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

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What’s holding it up?E

lect

ron

ener

gy

White Dwarfs and NeutronStars are made of degeneratematter.

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

White dwarfs and neutron stars are held up by___________ pressure

Pulsars: Stellar Beacons• _________ neutron stars

• Strong _______ 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

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 _____ ____!

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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!

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

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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

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.

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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