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Hydrostatic Equilibrium

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

• Thermal Pressure 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 balance

Hydrostatic equilibrium keepsstars stable for billions of years.

The inward force of gravity isbalanced by the force ofpressure pushing out.

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

equilibrium.

Pressure-TemperatureThermostat

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

If thermal pressure drops:1. Star shrinks a little2. Density increases3. Temperature increases4. Nuclear reactions speed up5. Thermal pressure rises again

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.

Leaving the Main Sequence

Stars live mostof their lives on

the MainSequence

When they run out ofHydrogen Fuel, they

leave the MainSequence and begin to

die.

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

LifetimeMass

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Creating Elements withNuclear Fusion

• Nuclear 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 Lithium 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.

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

The Deaths of Stars

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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:

•Gravity wins: the star collapses completely intoa black hole•Pressure wins: the entire star explodes intospace•Tie: The center of the star contracts, while theouter layers expand.

Low-Mass Stars(0.4 Msun or less)

• HUGE convectionzone!

• Hydrogen & Helium getmixed throughout star’slifetime

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

the universe

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

They lose their outer layers which expand to becomea planetary nebula

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

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

cool off: the Sun becomes a Red Giant

Average-Mass Stars• The Sun becomes a Giant,

leaves the Main Sequence(doesn’t move in space!)

• 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 Helium fusion

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The Sun’s Last Gasp

• Helium Fusion– Produces Carbon and Oxygen– Must be hotter than hydrogen fusion.

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

• These layers form great clouds 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 awhite dwarf

White Dwarfs

• The core of the dying star is leftbehind.

• It is very hot: 10,000K• It is blue or even white, and called a

White Dwarf

• White dwarfs are very small!

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

White DwarfsA white dwarf is hot, but verydim.

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

So, they are incredibly dense.

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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How do we know when a starwas born?

• We don’t!

• We couldcomparestars, iftheyformed atthe sametime…

Perseus Double Cluster

HR Diagram: A Young Cluster

HR Diagram: An Old Cluster The Death ofHeavier Stars

Heavyweight stars expand andturn into supergiants

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

Could become neutron staror a black hole

Death of Massive Stars:Red Supergiants

• Very massive stars use up their H fuelquickly.

• They also expand dramatically.– 1000 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 Hydrogen &Helium

In its core, Helium isfusing into Carbon &Oxygen

Around the core a shellof Hydrogen can fuse toHelium.

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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 Iron.– Fusing Iron will not produce

additional energy.

Close-up of Core

Fe Si O C He H

Core-Collapse Supernovae

• Once fusion stops,the core begins tocollapse quickly

•Outer core layersrebound off the ironcenter

•Rebound causes anenormous explosion:A Type II Supernova

Supernova 1987A

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

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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 theCrab Nebula

• It has a neutron starinside.

• It is also expanding insize.

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

• Bright enough to be seen during the daytime!

The Crab Supernova Remnant• Radius: 4.4 light years

– About the distance fromthe Sun to ProximaCentauri!

• Speed: 1400 km/s– That’s about 3 million

miles per hour!

Supernova Remnants

The Cygnus Loop

The Veil Nebula

The Crab Nebula:

Remnant of asupernova observed

in a.d. 1054

Cassiopeia A

How Do Supernovae Explode?

Supernovae are very complex and not wellunderstood.

Computer models 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 galaxies.• One or two supernovae per century per galaxy• No supernova in our Galaxy for 400 years…. We

are overdue!

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Supernovae are Rare!

• Most numerous stars: red dwarfs &white dwarfs

• Most rare stars: Blue giants• White Dwarfs result from most common

main sequence stars• Supernovae result from least common

main sequence stars

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

gaining mass & exploding– Type Ia, no H lines

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

• Core-collapse: deaths of massive stars– Type II, Hydrogen lines present

Fig. 10-12, p. 212

Mass Transfer

What’s left after asupernova?

• Depends on the mass of the originalstar!

• 8-20 MSun: Core collapses to a neutronstar– Spinning? -> Pulsar

• More than 20 MSun: Black Hole!