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Protostars on the HR Diagram•Once a protostar is hotenough to start _____, itcan blow away thesurrounding gas

•Then it is visible: crossesthe “____ ____” on the HRdiagram

•The more _______ thecloud, the ______ it willform stars

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____

Lifetimes of Stars: ExampleHow long will a star with 1/5 of the Sun’s mass (0.2

MSun) live?

T =

So if the Sun’s lifetime is about 10 billion years, thisstar will last ___ _________ years!

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M2.5____

Hydrostatic Equilibrium

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

• ______ ________ 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

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Leaving the Main Sequence

Stars live mostof their lives on

the MainSequence

When they run out ofHydrogen Fuel, they

leave the _____________ and begin

to ____.

Lifetimes of Stars• Low-mass stars: ________ ____• High-mass stars: ____ ________

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

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.

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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

• 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 ___________

They lose their outer layers which expand to becomea __________ ________

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

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

_____, ____ ___: the Sun becomes a Red Giant

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Average-Mass Stars• The Sun becomes a Giant,

leaves the Main Sequence(_____ ____ __ _____!)

• 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

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 burningends 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 _____, 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)

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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

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 __________

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Low-mass Stars:0.4 MSun or less

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

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)

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

are overdue!

Supernovae are _____!

• Most ________ stars: red dwarfs &white dwarfs

• Most ____ stars: Blue giants• White Dwarfs result from ____ common

main sequence stars• Supernovae result from _____ common

main sequence stars

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: _____ ____!