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ASTRASTR 1120: Stars & Galaxies1120: Stars & Galaxies

Prof. Prof. JuriJuri ToomreToomre TA: TA: LiciaLicia RayRayLecture 34 Wed 7 Apr 04Lecture 34 Wed 7 Apr 04

zeus.colorado.edu/astr1120zeus.colorado.edu/astr1120--toomretoomre

NGC 1232NGC 1232

Today’s Topics & EventsToday’s Topics & Events•• Today we look at the challenge of Today we look at the challenge of measuring big measuring big

distances distances to figure out where things are in to figure out where things are in universeuniverse

•• Class meets inClass meets in FiskeFiske Planetarium this Planetarium this FridayFriday(April 7) (April 7) –– go there directly:go there directly: John Bally: “Birth of John Bally: “Birth of Stars and Planets”Stars and Planets”

•• New New Homework Set 7Homework Set 7 onon Cosmic Distances and Cosmic Distances and Hubble Law Hubble Law still available still available –– due next Mondaydue next Monday

•• Homework 6Homework 6 returned (returned (graded+answersgraded+answers) today, ) today, along with more of the extraalong with more of the extra--credit projectscredit projects

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Clicker Clicker ---- reading on galaxiesreading on galaxies

•• How might you classify this galaxy?How might you classify this galaxy?

•• A.A. SaSa•• B.B. SBbSBb•• C.C. EE•• D.D. SOSO

B.B.

Cepheid variableCepheid variablestarsstars as distanceas distanceindicators:indicators:“standard candle”“standard candle”

Vital discovery byVital discovery byHenrietta Leavitt (1912)Henrietta Leavitt (1912)

ReminderReminder

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Cepheid starsCepheid stars in Hin H--R diagramR diagram•• ““Instability strip”Instability strip” ----

region in Hregion in H--R diagram R diagram with large, with large, bright bright starsstars

•• Outer regions of star are Outer regions of star are unstable and tend to unstable and tend to pulsatepulsate

•• Star expands and Star expands and contracts, getting brighter contracts, getting brighter and fainterand fainter

ReminderReminder(Fig 16.13)(Fig 16.13)

(Chap 16, p 536)(Chap 16, p 536)

Cepheid variable Cepheid variable starsstars

brighter brighter CepheidsCepheidshave longer periodshave longer periods

Period Period --LuminosityLuminosityrelationrelation

DISTANCEDISTANCEESTIMATE 2 ESTIMATE 2

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Andromeda found to be far Andromeda found to be far outsideoutside Milky Way Milky Way –– another “island universe” : galaxy!another “island universe” : galaxy!

•• Edwin Hubble in 1924 Edwin Hubble in 1924 identified identified CepheidsCepheids in in Andromeda (M33) Andromeda (M33) showed they were far showed they were far outside of Milky Way!outside of Milky Way!

•• His first big discovery. His first big discovery. Next was expansion of Next was expansion of the universe the universe –– wow!wow!

Hubble using new Hubble using new 100” Hooker telescope100” Hooker telescopeat Mt. Wilson (above LA)at Mt. Wilson (above LA)

Andromeda Andromeda –– M31M31

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Hubble: showed universe appeared to Hubble: showed universe appeared to be expanding!be expanding!

•• VestoVesto SlipherSlipher (1912) reported that most (1912) reported that most galaxies showed Doppler galaxies showed Doppler redshiftsredshifts

•• Edwin HubbleEdwin Hubble, using new 100” telescope, , using new 100” telescope, started busily measuring galaxy started busily measuring galaxy redshiftsredshifts

•• HubbleHubble (1929) announced that (1929) announced that redshiftsredshifts of of galaxiesgalaxies appear to appear to increase with distanceincrease with distancefrom usfrom us

•• This was startling: suggests an This was startling: suggests an EXPANDING UNIVERSEEXPANDING UNIVERSE !!

Hubble andHubble andrecession ofrecession ofgalaxiesgalaxies::

Further away,Further away,greater greater redshiftredshift !!

Hubble Hubble guessedguessedtheir distancestheir distances bybysize and brightnesssize and brightness---- underestimatedunderestimatedby factor 10!by factor 10!

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““Hubble’s Law”Hubble’s Law”

Hubble’s (1929) originalHubble’s (1929) original

Scatter here from randomScatter here from randomvelocities of nearby galaxies,velocities of nearby galaxies,unreliable distance estimatesunreliable distance estimates

Best currentBest currentvalues forvalues forexpansionexpansion

HHoo = = 71 +/71 +/-- 44km/km/s/Mpcs/Mpc

Hubble (1929) plot extended only to 2 Hubble (1929) plot extended only to 2 MpcMpc, H, Hoo was ~500!was ~500!

““HUBBLE CONSTANT”HUBBLE CONSTANT”

velocityvelocity

distancedistance

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Universe expandsUniverse expandslike raisin bread !like raisin bread !

Mapping the universe: need Mapping the universe: need distancesdistancesto galaxies!to galaxies!

•• Identify (and calibrate) properties of galaxies Identify (and calibrate) properties of galaxies that could serve as that could serve as “STANDARD CANDLES”“STANDARD CANDLES” ----beyond direct measure by beyond direct measure by trigonometric parallaxtrigonometric parallax

•• 1. Make some measure of an object which 1. Make some measure of an object which identifies its luminosity (like period in Cepheid)identifies its luminosity (like period in Cepheid)

•• 2. Use this luminosity and measure apparent 2. Use this luminosity and measure apparent brightness to brightness to infer distanceinfer distance to itto it

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MainMain--Sequence FittingSequence Fitting

•• Start with cluster Start with cluster AA(upper)(upper) whose whose distance known via distance known via parallax parallax

•• Compare with other Compare with other cluster cluster B B (lower)(lower)

•• Use brightness Use brightness difference to difference to estimate estimate B distanceB distance

DISTANCEDISTANCEESTIMATE 1ESTIMATE 1

AA

BB

MM--S Fitting “pinned to” S Fitting “pinned to” HyadesHyades Cluster, Cluster, 151 151 lyly awayaway

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CepheidsCepheids variables as variables as standard candlesstandard candles

1. Measure period 1. Measure period of variabilityof variability

2. From period2. From period--luminosity luminosity relation, infer the relation, infer the luminosityluminosity

3. Compare with 3. Compare with apparent apparent brightness and brightness and thus determine thus determine distancedistance

Cepheid variable in M100 (HST)Cepheid variable in M100 (HST)

DISTANCEDISTANCEESTIMATE 2ESTIMATE 2

Clicker Clicker –– CepheidsCepheids and distanceand distance

•• Two Cepheid stars, Fred and Barney, Two Cepheid stars, Fred and Barney, have the have the same apparent brightnesssame apparent brightness. Fred . Fred has a period of 5 days, and Barney of 10 has a period of 5 days, and Barney of 10 days. Which is days. Which is closercloser ??

•• A.A. FredFred•• B.B. BarneyBarney

A.A.

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Why Why A.A. Fred ?Fred ?

•• Fred has a Fred has a shorter shorter periodperiod and so must and so must be be less luminousless luminous

•• Less luminous but Less luminous but the same apparent the same apparent brightness means brightness means that Fred is closer that Fred is closer to usto us

PeriodPeriod--Luminosity RelationLuminosity Relation

Even brighter: Even brighter: White dwarf supernovaeWhite dwarf supernovae

• “Standard Standard explosion”explosion” = = fusion of 1.4 solar fusion of 1.4 solar masses of masses of materialmaterial

•• Always the same Always the same amount of energy amount of energy releasedreleased

DISTANCE DISTANCE ESTIMATE 3ESTIMATE 3

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Bright enough to be seen halfway Bright enough to be seen halfway across observable universeacross observable universe

Useful for mapping the Useful for mapping the universe to the largest universe to the largest

distancesdistances

Practical difficulty: White dwarf SNPractical difficulty: White dwarf SN

•• Need to catch them Need to catch them within a day or two within a day or two of the explosionof the explosion

•• About 1 per galaxy About 1 per galaxy per century per century

•• Need to monitor Need to monitor thousands of thousands of galaxies to catch a galaxies to catch a few per year few per year galaxy clusters are galaxy clusters are usefuluseful

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TullyTully--Fisher RelationFisher Relation

•• Fast rotation speeds in Fast rotation speeds in spiral galaxies spiral galaxies

•• Lots of mass Lots of mass •• High luminositiesHigh luminosities

Measure rotation speeds Measure rotation speeds to infer the luminosityto infer the luminosity

Need bright Need bright ““edgeedge--onon””spiralsspirals

DISTANCEDISTANCEESTIMATE 4ESTIMATE 4

““Distance Distance ladder”ladder”

OverlappingOverlapping“standard “standard candles”candles”

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Distance ladder to measure universeDistance ladder to measure universe

Different standard candles are useful Different standard candles are useful for different distancesfor different distances

Reading for Next LectureReading for Next Lecture

•• Complete your reading of Complete your reading of 20.3 20.3 Measuring Cosmic Distances Measuring Cosmic Distances

•• ReadRead 20.4 Measuring Cosmic Ages20.4 Measuring Cosmic Ages

•• Remember: go directly toRemember: go directly to FiskeFiskePlanetariumPlanetarium for thisfor this Friday lecture Friday lecture