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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: _____ ____!