Announcements ● All students: email me this week ( [email protected] ) telling me whether you want...

AnnouncementsAnnouncements

● All students: email me All students: email me this weekthis week ( ([email protected]@artic.edu) ) telling me whether you want to:telling me whether you want to:

– Do a presentation Mar 12Do a presentation Mar 12– Do a presentation at end of termDo a presentation at end of term– Do a research paperDo a research paper– Do an art projectDo an art project

● And a couple possible topics you are interested in covering. And a couple possible topics you are interested in covering. ● List of some possible topics on course web page List of some possible topics on course web page

((http://flash.uchicago.edu/~ljdursi/SETIhttp://flash.uchicago.edu/~ljdursi/SETI) ) under blog.under blog.

Marks – Reading Quizzes and AssignmentsMarks – Reading Quizzes and Assignments

● Reading Quiz:Reading Quiz:

– 5 NCRs, 9 CRs, 1 CR+ 5 NCRs, 9 CRs, 1 CR+ ● Assignments:Assignments:

Review: The Distance LadderReview: The Distance Ladder

● Different `realms' of distance in Universe, each Different `realms' of distance in Universe, each requiring different units, techniques of requiring different units, techniques of measurement:measurement:

– Solar SystemSolar System– Nearby starsNearby stars– Galactic distancesGalactic distances– Extra-galactic distancesExtra-galactic distances

● Measurement for each realm depends on Measurement for each realm depends on knowing distances from the nearer realmsknowing distances from the nearer realms

● `Rungs' on Distance Ladder`Rungs' on Distance Ladder


● Geometric Geometric measurement of measurement of distance to Sun distance to Sun depends on knowing depends on knowing distance to Moondistance to Moon

– Solar system Solar system `rung' depends on `rung' depends on Earth-Moon Earth-Moon `rung'`rung'

Sun

Earth

Moon


● Parallax distance Parallax distance measures of nearby measures of nearby stars REQUIRES stars REQUIRES knowing how big an knowing how big an AU isAU is

– `nearby star' rung `nearby star' rung depends on `solar depends on `solar system' rungsystem' rung

Summary of Last Class: LightSummary of Last Class: Light● Light is a form of electromagnetic radiationLight is a form of electromagnetic radiation● All EM radiationAll EM radiation

– Dims with distance as the inverse square lawDims with distance as the inverse square law– Forms a broad spectrumForms a broad spectrum

● Dense, opaque material glows when hot as a Dense, opaque material glows when hot as a blackbodyblackbody

– Hotter glows more, and at shorter (blue-er) Hotter glows more, and at shorter (blue-er) wavelengthswavelengths

● Other processes give rise to distinctive line spectrum Other processes give rise to distinctive line spectrum which can be used to determinewhich can be used to determine

– CompositionComposition– Speed (by Doppler shift)Speed (by Doppler shift)

Summary of Last Class: GalaxiesSummary of Last Class: Galaxies

● Galaxies are `island universes' which contain Galaxies are `island universes' which contain most of the matter, stars in the Universemost of the matter, stars in the Universe

● Can be spiral, elliptical, or irregularCan be spiral, elliptical, or irregular● Star formation continues in galaxies, particular in Star formation continues in galaxies, particular in

spiral galaxiesspiral galaxies● Galaxies also contain gas clouds, dustGalaxies also contain gas clouds, dust● Galaxies are separating over time: expanding Galaxies are separating over time: expanding

universeuniverse

Feedback:Feedback:● Most unclear item from last week's readings?Most unclear item from last week's readings?

What we're going to cover todayWhat we're going to cover today

● The Stellar Cycle: Birth, Life, and Death of the StarsThe Stellar Cycle: Birth, Life, and Death of the Stars

– Birth: turbulent collapse of clouds of gasBirth: turbulent collapse of clouds of gas– Life: ignition; burning; balance between Life: ignition; burning; balance between

gravity and pressuregravity and pressure– Death: gravity begins to win; but burning has Death: gravity begins to win; but burning has

one last hurrah. one last hurrah.

Stars are crucial for lifeStars are crucial for life

● Stars are the main engines in the UniverseStars are the main engines in the Universe● Stars are where planets are foundStars are where planets are found● Stars produce energy that can power lifeStars produce energy that can power life● Stars produce all the heavy elements (Stars produce all the heavy elements (egeg Carbon) Carbon)

that build lifethat build life

Stellar CycleStellar Cycle

The Birth of StarsThe Birth of Stars

● At end of this, we'll know:At end of this, we'll know:

– Where stars are formedWhere stars are formed– How they formHow they form– What has to happen for a star to What has to happen for a star to

`turn on'`turn on'– How planets form around starsHow planets form around stars

TurbulenceTurbulence● Happens when flow velocities are too Happens when flow velocities are too

large to be kept smooth by viscositylarge to be kept smooth by viscosity

TurbulenceTurbulence

● Gas clouds in the galaxy are Gas clouds in the galaxy are turbulent, tooturbulent, too

– Very wispy, tenuous Very wispy, tenuous gasgas

– No viscosity to speak ofNo viscosity to speak of– `Stirred' by energetic `Stirred' by energetic

events in the galaxyevents in the galaxy

Gas CloudsGas Clouds

● Two broad types of clouds:Two broad types of clouds:

– Gas cloudsGas clouds● WarmWarm● Very wispyVery wispy

– Molecular cloudsMolecular clouds● ColderColder● Much denserMuch denser● Gas has condensed Gas has condensed

enough that complex enough that complex molecules have formedmolecules have formed

Molecular CloudsMolecular Clouds

● Because molecular clouds are cooler and denser, Because molecular clouds are cooler and denser, atoms collide more oftenatoms collide more often

● Can form complex moleculesCan form complex molecules● Greatly helped by presence of grainsGreatly helped by presence of grains● Provides sites for atoms to latch ontoProvides sites for atoms to latch onto● Region of high atom density; atoms more easily find Region of high atom density; atoms more easily find

other atoms to interact withother atoms to interact with


● All of these gas clouds are turbulentAll of these gas clouds are turbulent● Random motions, eddiesRandom motions, eddies● Where fluid comes together, dense Where fluid comes together, dense

regionsregions● Fluid is moving fast enough that Fluid is moving fast enough that

can compress very dense spotscan compress very dense spots


● Gravity acts to try to pull Gravity acts to try to pull these dense spots togetherthese dense spots together

● However,However,

– Pressure in gas clouds Pressure in gas clouds – RotationRotation


● If a large enough, dense If a large enough, dense enough region is formed, enough region is formed, gravity can start to win and gravity can start to win and core starts collapsing inwardcore starts collapsing inward

● Nearby material can also Nearby material can also start falling instart falling in

● As collapses, `spins up' and As collapses, `spins up' and disk can formdisk can form


● Collapse will usually Collapse will usually happen in many places happen in many places throughout the cloud at the throughout the cloud at the same time same time

● This is why stars tend to This is why stars tend to be clusteredbe clustered

● Amount of stars depends Amount of stars depends on size of gas cloud on size of gas cloud producing starsproducing stars


● As core collapses, gets hotter As core collapses, gets hotter and denserand denser

● Begins to glow Begins to glow ● Begins to evaporate nearby Begins to evaporate nearby

complex moleculescomplex molecules● Any particularly dense Any particularly dense

regions can (for a while) regions can (for a while) protect columns in their protect columns in their shadowshadow


● Nearby gas evaporates, but Nearby gas evaporates, but disk remainsdisk remains

● Flattens out at spinning Flattens out at spinning increases with collapseincreases with collapse

● Can begin to coalesce as star Can begin to coalesce as star begins to form begins to form

Protoplanetary Protoplanetary DisksDisks

● These protoplanetary These protoplanetary disks can be seen around disks can be seen around very young protostarsvery young protostars

Protoplanetary DisksProtoplanetary Disks

Jets and OutflowsJets and Outflows

● As core collapses further, As core collapses further, heat increases and so gas heat increases and so gas pressure increasespressure increases

● If core is small enough, this If core is small enough, this ends the processends the process

● If core is large enough, If core is large enough, burning can `turn on' and burning can `turn on' and begins rather violentlybegins rather violently

● Under some circumstances, Under some circumstances, enormous jet can form enormous jet can form perpendicular to diskperpendicular to disk

SummarySummary

The Life of StarsThe Life of Stars


– The structure of starsThe structure of stars– How stars burnHow stars burn– How stars ageHow stars age– Our Sun's life storyOur Sun's life story

Failed StarsFailed Stars

● `Stars' that are too small (~8% of `Stars' that are too small (~8% of the mass of the Sun, or ~80 Jupiter the mass of the Sun, or ~80 Jupiter masses) never ``turn on''masses) never ``turn on''

● Central temperatures never get hot Central temperatures never get hot enough for nuclear burning to enough for nuclear burning to begin in earnestbegin in earnest

● Nuclear burning is what powers Nuclear burning is what powers the star through its lifethe star through its life

● Star sits around as a brown dwarf – Star sits around as a brown dwarf – too big and hot to be a planet, too too big and hot to be a planet, too small and cold to be a real starsmall and cold to be a real star

Failed StarsFailed Stars

● Such brown dwarfs have been Such brown dwarfs have been observedobserved

● ~100,000 times fainter than Sun~100,000 times fainter than Sun● Stars, failed or otherwise, often Stars, failed or otherwise, often

observed in binary systemsobserved in binary systems

– (~30% of all stars in binary (~30% of all stars in binary systems?)systems?)

● Turbulent collapse makes it very Turbulent collapse makes it very likely that two cores form nearby likely that two cores form nearby or large core splits into two.or large core splits into two.

Hydrostatic EquilibriumHydrostatic Equilibrium● Once collapse has halted in a star, Once collapse has halted in a star,

force inward (gravity) must be force inward (gravity) must be balanced by force outward (gas balanced by force outward (gas pressure)pressure)

● (Much of the rotation has been (Much of the rotation has been taken away by the planetary disk taken away by the planetary disk by this point)by this point)

● Central region is hottest because Central region is hottest because pressure from the entire star is pressure from the entire star is pushing down on itpushing down on it

● Star as a whole is hot enough that Star as a whole is hot enough that no molecules are left; everything is no molecules are left; everything is broken into componentsbroken into components

Nuclear ReactionsNuclear Reactions● Nuclei of atoms themselves Nuclei of atoms themselves

interactinteract● Change the elements: alchemyChange the elements: alchemy● The star, like the cloud it came The star, like the cloud it came

from, is mostly hydrogenfrom, is mostly hydrogen● So hot the electrons are stripped So hot the electrons are stripped

off; left with bare protons off; left with bare protons (hydrogen nuclei)(hydrogen nuclei)

● Under extreme heat, protons can Under extreme heat, protons can fuse together to produce helium: fuse together to produce helium: and more heat!and more heat!

● Higher temperatures – faster Higher temperatures – faster reactionsreactions

QuestionQuestion

● What happens if an external force What happens if an external force `squishes' the star a little bit?`squishes' the star a little bit?

Built In ThermostatBuilt In Thermostat

● If star is squished in,If star is squished in,

– Central region gets hotterCentral region gets hotter– Reactions speed upReactions speed up– Star gets hotterStar gets hotter– Gas pressure increasesGas pressure increases– Star fluffs outStar fluffs out– Central temperature returns Central temperature returns

to normalto normal

Built In ThermostatBuilt In Thermostat

● If star is pulled out a little,If star is pulled out a little,

– Central region gets coolerCentral region gets cooler– Reactions slow downReactions slow down– Star gets coolerStar gets cooler– Gas pressure decreasesGas pressure decreases– Star falls backStar falls back– Central temperature returns Central temperature returns

to normalto normal● Star is STABLEStar is STABLE

Given that burning is stable,Given that burning is stable,

● What effects how hot a star is?What effects how hot a star is?

Given that burning is stable,Given that burning is stable,

● What effects how hot a star is?What effects how hot a star is?

– MASSMASS● The bigger the star that forms from the The bigger the star that forms from the

collapsecollapse

– More pressure on the central regionMore pressure on the central region– More burningMore burning– Hotter Hotter – BrighterBrighter

● What color are more massive stars?What color are more massive stars?

HR diagram and Main SequenceHR diagram and Main Sequence

● From previous, expect that From previous, expect that hotter stars should be brighterhotter stars should be brighter

– BlackbodyBlackbody– More massive -> bigger More massive -> bigger

● When temperature vs brightness When temperature vs brightness is plotted, see `Main Sequence'is plotted, see `Main Sequence'

● Other populated regions show Other populated regions show later stages in stellar evolutionlater stages in stellar evolution

Stellar EvolutionStellar Evolution

● Nuclear reactions are very Nuclear reactions are very sensitive to temperaturesensitive to temperature

– Massive stars burn MUCH Massive stars burn MUCH faster than smaller starsfaster than smaller stars

● Even though massive stars have Even though massive stars have more fuel (hydrogen) to begin more fuel (hydrogen) to begin with, it is exhausted more with, it is exhausted more quicklyquickly

● Everything happens faster with Everything happens faster with more massive stars because more massive stars because pressure is higherpressure is higher


● As burning in core progresses, As burning in core progresses, Hydrogen in center becomes Hydrogen in center becomes depleted (Sun: ~10 billion depleted (Sun: ~10 billion years)years)

● Core of Helium `ash' left behindCore of Helium `ash' left behind● Shell of Hydrogen burning Shell of Hydrogen burning

slowly moves outwardsslowly moves outwards● As heat source moves further As heat source moves further

out, star `puffs out'out, star `puffs out'● Outer regions cool, reddenOuter regions cool, redden● Red Giant (Sun: 1 billion years)Red Giant (Sun: 1 billion years)


● Eventually Helium core gets so Eventually Helium core gets so hot that even it can burn, to hot that even it can burn, to CarbonCarbon

● New energy source: star gets New energy source: star gets hotter and bluer, and shrinks hotter and bluer, and shrinks back to more normal sizeback to more normal size

● Burning happens faster with Burning happens faster with heavier elements; soon Helium heavier elements; soon Helium becomes exhausted, a Carbon becomes exhausted, a Carbon core forms; becomes giant core forms; becomes giant againagain

Low Mass stars: envelope ejectionLow Mass stars: envelope ejection

● Helium burning can be very Helium burning can be very unstableunstable

● Outer layers begin pulsing; Outer layers begin pulsing; blows most of the envelope off blows most of the envelope off of the starof the star

● (so called) `Planetary nebula' (so called) `Planetary nebula' formsforms

● Only the core is left behind, still Only the core is left behind, still glowing (because hot) but inertglowing (because hot) but inert

● White dwarfWhite dwarf

High Mass Stars: Continue BurningHigh Mass Stars: Continue Burning

● Slightly more massive stars (4 Slightly more massive stars (4 to 8 solar masses):to 8 solar masses):

– Everything happens fasterEverything happens faster– Carbon can burn, as well; Carbon can burn, as well;

one more stage of burningone more stage of burning– Then again leave (larger) Then again leave (larger)

white dwarf and planetary white dwarf and planetary nebula behindnebula behind

Very High Mass Stars: Continue BurningVery High Mass Stars: Continue Burning

● Very massive stars burn VERY fast Very massive stars burn VERY fast

– Main sequence stage – 10 million yearsMain sequence stage – 10 million years● Burning happens so quickly that outer layer Burning happens so quickly that outer layer

can't go unstablecan't go unstable● Burning progresses faster and faster through Burning progresses faster and faster through

higher and higher elements until Ironhigher and higher elements until Iron● No further burning is possibleNo further burning is possible● Left with a large envelope and very heavy coreLeft with a large envelope and very heavy core

Life Story of Our SunLife Story of Our Sun

● Formed in ~50 million yearsFormed in ~50 million years● Began life about 5 billion years agoBegan life about 5 billion years ago

– A little dimmer (¾ current brightness)A little dimmer (¾ current brightness)– A little cooler, smallerA little cooler, smaller

● Slowly getting bigger and hotter:Slowly getting bigger and hotter:

– 1 billion yrs from now: 10% brighter1 billion yrs from now: 10% brighter– Greenhouse effectGreenhouse effect– 5 billion years from now: 40% brighter5 billion years from now: 40% brighter– Earth like Venus todayEarth like Venus today– Still main sequenceStill main sequence

Life Story of Our SunLife Story of Our Sun● Red giant branch beginsRed giant branch begins● Next 700 million years; sun doubles in energy Next 700 million years; sun doubles in energy

outputoutput● Doubles in size, gets little redderDoubles in size, gets little redder● Next 600 million years; very strong wind; planets Next 600 million years; very strong wind; planets

pushed somewhat outwardspushed somewhat outwards● At biggest, sun almost out to Venus' orbitAt biggest, sun almost out to Venus' orbit● Helium Flash!!Helium Flash!!● Helium begins burning, process repeats itself but Helium begins burning, process repeats itself but

10x faster10x faster● Ends with ½ of suns mass blown away; white Ends with ½ of suns mass blown away; white

dwarf remainsdwarf remains

The Old Age and Death of StarsThe Old Age and Death of Stars


– The final stages of stellar lifeThe final stages of stellar life– How stars of different mass dieHow stars of different mass die– How they feed back material into How they feed back material into

the interstellar environment, to be the interstellar environment, to be made into new starsmade into new stars

The Old Age and Death of StarsThe Old Age and Death of Stars

● Small stars end their life quietly Small stars end their life quietly

– White dwarf remnantsWhite dwarf remnants● Massive stars continue burning in outer layers even Massive stars continue burning in outer layers even

when they have burned all the way to iron in the when they have burned all the way to iron in the core.core.

● New ash from burning continues to pile onto iron New ash from burning continues to pile onto iron core until pressure cannot support it any morecore until pressure cannot support it any more

Type II SupernovaType II Supernova

● The result is a collapse to a different The result is a collapse to a different form of matter – a neutron star, or a form of matter – a neutron star, or a black hole -- and a release of energyblack hole -- and a release of energy

● Energy release can be equal to the Energy release can be equal to the entire energy of the host galaxyentire energy of the host galaxy

● Entire envelope is blown apart Entire envelope is blown apart

– Heavy elements from burning Heavy elements from burning blown into surrounding gasblown into surrounding gas

Type Ia SupernovaType Ia Supernova

● Almost as much energy can come Almost as much energy can come from another kind of supernovafrom another kind of supernova

● If a star which ended up as a white If a star which ended up as a white dwarf has a companion, matter can dwarf has a companion, matter can `rain in' on the inert white dwarf until `rain in' on the inert white dwarf until it gets hot enough to burnit gets hot enough to burn

● Can burn catastrophically, exploding Can burn catastrophically, exploding and releasing heat, heavy elements and releasing heat, heavy elements into surrounding gasinto surrounding gas

Supernova FeedbackSupernova Feedback● Originally, gas was all hydrogen and heliumOriginally, gas was all hydrogen and helium

– No planets, lifeNo planets, life● Generations of stars produced all the heavy elements Generations of stars produced all the heavy elements

which make up planets and living thingswhich make up planets and living things● Supernova explosions release these heavy elements into Supernova explosions release these heavy elements into

the galaxythe galaxy

– New stars are formedNew stars are formed– Can make planets, lifeCan make planets, life

● Supernova energy contributes to the turbulence in the Supernova energy contributes to the turbulence in the gas clouds, and can compress gas to start new cycle of gas clouds, and can compress gas to start new cycle of star formationstar formation

Stellar Cycle RevisitedStellar Cycle Revisited

Reading for Next WeekReading for Next Week

● Chapter 7, 8 – origins of life on earthChapter 7, 8 – origins of life on earth

Announcements ● All students: email me this week ( [email protected] ) telling me whether you want...

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