astr 1120 general astronomy: stars & galaxies · stars & galaxies!nnouncements ¥ midterm...

ASTR 1120ASTR 1120General Astronomy:General Astronomy:Stars & GalaxiesStars & Galaxies !NNOUNCEMENTS!NNOUNCEMENTS

• Midterm #3 this Thursday 11/17;

covering lect. 15-21 (and corresponding reading in Ch. 19-22)

• last part of class today will be review

• HW #8 due today, 5pm

• HW #9 due Tue, 12/01, by 5pm

ALTERNATE FINAL on Monday, Dec.7th, 5:30pm-ALTERNATE FINAL on Monday, Dec.7th, 5:30pm-

7:00pm,7:00pm, in in Muenzinger Muenzinger E131E131

Numerical simulations of structure formationNumerical simulations of structure formation

REVIEW

Lessons from ImaginaryLessons from Imaginary

UniversesUniverses•• Cold (Slow) dark matterCold (Slow) dark matter

works better than works better than hot (fast)hot (fast)dark matterdark matter

•• Neutrinos are too fastNeutrinos are too fast––structure would bestructure would besmeared outsmeared out

•• What is slow and darkWhat is slow and darkenough? We donenough? We don’’t knowt knowyet!yet!–– Particle experiments underParticle experiments under

wayway……....

Dark Matter and the Fate of theDark Matter and the Fate of the

UniverseUniverse

•• Expansion begins with the Big Bang (weExpansion begins with the Big Bang (we’’ll talk aboutll talk about

this after the break)this after the break)

•• At that point, everything in the universe is flungAt that point, everything in the universe is flung

apart atapart at outrageous speeds!outrageous speeds!

•• Several different models for Several different models for PastPast and and FutureFuture

depending upon the amount of depending upon the amount of dark matterdark matter

Very important diagramVery important diagram

““Average distanceAverage distance

betweenbetween

galaxiesgalaxies””

measure ofmeasure of

““expansion factorexpansion factor

of Universeof Universe””

Hubble constantHubble constant NOWNOW

sets how fastsets how fast

universe isuniverse is

expandingexpanding NOWNOWBig Bang = when distance zeroBig Bang = when distance zero

TIMETIME

SIZ

ES

IZE

NOWNOW

The expansion rate of the universe isThe expansion rate of the universe is

not necessarily constant for all timenot necessarily constant for all time

•• GRAVITYGRAVITY should SLOW should SLOW

expansion rate expansion rate !!

decelerationdeceleration

•• Different models forDifferent models for

different amounts of darkdifferent amounts of dark

mattermatter

–– LetLet’’s ignore s ignore acceleratingaccelerating for for

nownow

Since gravity is what pulls everythingSince gravity is what pulls everything

back in,back in, there must be athere must be a magic numbermagic number

•• Just the right amount of mass (in our currentJust the right amount of mass (in our current

universe) to slow down expansion but notuniverse) to slow down expansion but not

enough toenough to cause cause recollapserecollapse

•• We call this exact amount of matter, theWe call this exact amount of matter, the

CRITICAL DENSITYCRITICAL DENSITY

~10~10-29-29 grams/cm grams/cm33 = a few atoms in a closet = a few atoms in a closet

CriticalCritical Universe Universe

•• Density ofDensity of

matter =matter =

““criticalcritical

densitydensity””

•• Will expandWill expand

foreverforever, but, but

more andmore and

more slowlymore slowly

with timewith time

FlatFlat UniverseUniverse

Recollapsing Recollapsing UniverseUniverse

•• Dark matterDark matterdensity isdensity isgreatergreater than than““critical densitycritical density””

•• Expansion willExpansion willstop in thestop in thefuture, willfuture, willcollapse backcollapse backinin

–– Big CrunchBig Crunch

–– Oscillations?Oscillations?

Closed UniverseClosed Universe

Coasting UniverseCoasting Universe

•• The universeThe universehas alwayshas alwaysexpanded at theexpanded at thesame ratesame rate(no deceleration(no decelerationdue to gravity!)due to gravity!)

•• The age of theThe age of theUniverse = 1/HUniverse = 1/Hoo

Open UniverseOpen Universe

Three models for fates of universeThree models for fates of universe

CLOSEDCLOSED

OPENOPEN

FLATFLAT

Which model predicts theWhich model predicts the youngestyoungest

ageage for the universe today? for the universe today?

A.A. RecollapsingRecollapsing

(closed)(closed)

B.B. CriticalCritical

(flat) (flat)

C.C. CoastingCoasting

(open) (open)

Clicker QuestionClicker Question

•• A.A. RecollapsingRecollapsing

•• Age of universe isAge of universe is

how far to lefthow far to left

curves hitcurves hit

horizontal axishorizontal axis

(distance between(distance between

galaxies = 0)galaxies = 0)

What determines the future averageWhat determines the future average

distance between galaxiesdistance between galaxies??

A. The rate of expansion, with a slower expansionrate meaning a greater average distance

BB.. The rate of expansion, with a faster expansionrate meaning a greater average distance

C.C. Only on the density of matter in the universe.


Which model has the slowest future rate ofWhich model has the slowest future rate of

expansion?expansion?

A.A. RecollapsingRecollapsing

BB.. CriticalCritical

C. CoastingC. Coasting


•• Recollapse Recollapse to big crunchto big crunch::

–– Crushing heatCrushing heat

–– Destruction of all matterDestruction of all matter

–– Rebirth?Rebirth?

•• Eternal expansionEternal expansion::

–– Cold, galaxies dimmingCold, galaxies dimming

–– Star formation slowingStar formation slowing

–– Everything winds up as a brown dwarf, blackEverything winds up as a brown dwarf, blackdwarf, neutron star or black holedwarf, neutron star or black hole

What is the fate of the Universe?What is the fate of the Universe?

Is there enough dark matterIs there enough dark matter

to to recollapse recollapse the universe?the universe?

Baryonic matter:Baryonic matter: only a few % of critical density only a few % of critical density

Dark matter:Dark matter: only about 25% of what is needed only about 25% of what is needed

•• Universe should be in between the Universe should be in between the ““coastingcoasting””and and ““criticalcritical”” models models

The Universe will expand foreverThe Universe will expand forever

Which is it?Which is it?

… and with an unexpected twist…

A New Twist for the 21A New Twist for the 21stst

CenturyCentury

•• Scientists using whiteScientists using white

dwarf supernovae todwarf supernovae to

measure distancesmeasure distances

discovered somethingdiscovered something

quite strangequite strange

UsingUsing supernovae to determinesupernovae to determine

the fate of the Universethe fate of the Universe• Redshifts of the

supernovae givestheir verticalposition– Space has

stretched sincethey gave off theirlight

• Apparentbrightness givestheir horizontalposition– Dimmer=more

distant = highlookback

Supernovae areSupernovae are notnot consistent even withconsistent even with

the expectations of a coasting universe!!!the expectations of a coasting universe!!!

Universe is accelerating?!?!

How can the universe beHow can the universe be

accelerating???????accelerating???????

A force that counteracts gravity?A force that counteracts gravity?

““Dark energyDark energy”–”– outweighs every other form of outweighs every other form of

mass/energy!mass/energy!

Truly an Truly an unknown force in all of physicsunknown force in all of physics

The Cosmological ConstantThe Cosmological Constant actually exists!actually exists!

(“Einstein’s Greatest Blunder” )

FourFour models for fates of universe models for fates of universe

CLOSEDCLOSED

OPENOPEN

FLATFLAT

ACCELERATINGACCELERATING

What is meant by What is meant by ““dark energydark energy””??

A.A. The energy associated with dark matter throughThe energy associated with dark matter throughE=mcE=mc22

B.B. An unknown form of energy thatAn unknown form of energy that counteractscounteractsgravity and causes the expansion of thegravity and causes the expansion of theuniverse to accelerate.universe to accelerate.

C.C. Any unknown force thatAny unknown force that acts like gravityacts like gravity

D.D. Highly energetic particles that are believed toHighly energetic particles that are believed toconstitute dark matterconstitute dark matter

E.E. The total energy in the universe after the BigThe total energy in the universe after the BigBang but before the first starsBang but before the first stars


REVIEW FOR MIDTERM IIIREVIEW FOR MIDTERM III

to followto follow……....

Disk, Bulge & HaloDisk, Bulge & Halo

•• DiskDisk:: includesincludes

spiral armsspiral arms ----

young, new staryoung, new star

formationformation

•• Bulge & HaloBulge & Halo::older stars,older stars,

globular clustersglobular clusters

Artist’s sketch

Disk is very thin!Disk is very thin!

Galaxies: Ultimate Galaxies: Ultimate Recyling Recyling PlantsPlants

Summary of Galactic RecyclingSummary of Galactic Recycling

• Stars make new elements by fusion

• Dying stars expel gas and new elements, producinghot bubbles (~106 K)

• Hot gas cools, allowing atomic hydrogen clouds toform (~100-10,000 K)

• Further cooling permits molecules to form, makingmolecular clouds (~30 K)

• Gravity forms new stars (and planets) in molecularclouds

Gas C

ools

From HOTHOT to COLDCOLD

We observe star-gas-star cycle operating in Milky WayWe observe star-gas-star cycle operating in Milky Way’’ss

disk using many different wavelengths of lightdisk using many different wavelengths of light

Dark matter halo for galaxiesDark matter halo for galaxies

•• Dark matter extendsDark matter extendsbeyond visible part ofbeyond visible part ofthe galaxythe galaxy -- mass is-- mass is~10x~10x stars and gas!stars and gas!

•• Probably not normalProbably not normalmass that we know ofmass that we know of(protons, neutrons,(protons, neutrons,electrons).electrons).

•• Most likely subatomicMost likely subatomicparticles, as yetparticles, as yetunidentifiedunidentified (weakly(weaklyinteracting massiveinteracting massiveparticles particles –– WIMPsWIMPs?)?)

Stars appear to be

orbiting something

massive but

invisible … aa

black holeblack hole!

Orbits of stars

indicate a mass of

about 3-4 million4 million

MMsun sun withinwithin 600600

RRSchwarzchildSchwarzchild

SummarySummary

““Distance LadderDistance Ladder”” to measure universe to measure universe

Different standard candles are usefulDifferent standard candles are useful

for different distancesfor different distances

v = Ho ! d

Velocity of

Recession

(Doppler Shift)

Hubble’s

ConstantDistance

(km/sec) (km/sec/Mpc) (Mpc)

““HubbleHubble’’s Laws Law””

velocityvelocity

distancedistance

Best currentBest current

values forvalues for

expansionexpansion

HHoo = = 7171 +/- 4+/- 4

km/s/Mpckm/s/Mpc

Balloon analogy forBalloon analogy for

expanding universeexpanding universe

•• Each dot on theEach dot on the

balloon can beballoon can be

thought of as athought of as a

galaxy.galaxy.

As the balloonAs the balloon

expands, galaxiesexpands, galaxies

move farther awaymove farther away

from each otherfrom each other

Forming a Forming a diskdisk

with spiralwith spiral

•• As more materialAs more material

collapses,collapses, angularangular

momentummomentum spins itspins it

into a diskinto a disk

•• Stars now formedStars now formed

in in dense spiraldense spiral

armsarms –– disk starsdisk stars

are younger!are younger!

Angular momentum of protogalactic cloud

important in spiral galaxy formation

Making Making ellipticalsellipticals1.1. Higher density:Higher density: muchmuch

faster star formationfaster star formationuses up all the gasuses up all the gas

–– Nothing left to make aNothing left to make adiskdisk

oror

2.2. Lower spinLower spin–– Gas used up beforeGas used up before

angular momentum tookangular momentum tookoverover

•• Now we see a Now we see a spheresphereof old starsof old stars

Or perhaps a different scenarioOr perhaps a different scenario……..

•• Spiral galaxy collisionsSpiral galaxy collisionsdestroy disks,destroy disks, leaveleavebehind behind ellipticalelliptical

•• Burst of star formationBurst of star formationuses up all the gasuses up all the gas

•• Leftovers:Leftovers: train wrecktrain wreck

•• EllipticalsEllipticals more commonmore commonin dense galaxy clustersin dense galaxy clusters(centers of clusters(centers of clusterscontaincontain central dominantcentral dominantgalaxiesgalaxies))

NGC 4038/39 AntennaeNGC 4038/39 Antennae

AGNs AGNs Central EnginesCentral Engines

How doHow do AGNs AGNs emit soemit somuch light in so littlemuch light in so littlespace?space?

•• They are powered byThey are powered byaccretion disks aroundaccretion disks aroundsupermassive supermassive blackblackholesholes

•• In some In some AGNsAGNs, huge, hugejets of material are shotjets of material are shotout at the poles. Theseout at the poles. Thesejets are strong radiojets are strong radiosources.sources.

JETJET

DISKDISK

REVIEW

““Central EngineCentral Engine”” -- artist -- artist’’s conceptions conception

•• Accretion diskAccretion diskaround super-around super-massive blackmassive blackholehole

•• Disk itself may orDisk itself may ormay not be obscuredmay not be obscuredby dustby dust

•• If bright nucleus isIf bright nucleus isvisible, looks like avisible, looks like aquasarquasar,, if not, thenif not, thenits aits a radio galaxyradio galaxy

PrototypicalPrototypical

““radio galaxyradio galaxy””

Giant ellipticalGiant elliptical

galaxygalaxy NGC 5128 NGC 5128

with dust lanewith dust lane

(from spiral galaxy?)(from spiral galaxy?)

+ +

Centaurus Centaurus AA

radio sourceradio source

(color lobes)(color lobes)

astr 1120 general astronomy: stars & galaxies · stars & galaxies!nnouncements ¥ midterm...

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