Lecture 21 Student Notes PDF

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Lecture notes on the milky way (Lecture 21)...

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Lecture 21 Our Galaxy – The Milky Way & The Rea Realm lm of Neb Nebulae ulae This collage, clockwise from upper left, shows four outstanding astronomers of the 20th century: Harlow Shapley (1885–1972) discovered our place in the “ suburbs” of the Milky Way. Annie Cannon (1863–1941) classified nearly a million stars over the course of a 50year career. Karl Jansky (1905–1950) first detected radiation at radio wavelengths coming from the Milky Way. And Edwin Hubble (1889–1953) principally discovered the expansion of the universe. (Harvard Observatory; NRAO; Caltech)

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Key$Ideas I."The"Milky"Way"is"our"Galaxy A."Diffuse"Band"of"Light"Crossing"the"Sky B."First"Studies"of"and"Discovering"the"Galaxy C."Light"Absorption"Limits"View"of"Milky"Way II."The"Realm"of"the"Nebulae A."The"Nature"of"Spiral"Nebulae Island"Universes Nebular"Hypothesis B."Leavitt,"ShapleyRCurtis"Debate,"and"Edwin" Hubble’s"analysis"of"Cephied’s"in"Andromeda

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What$does$our$galaxy$look$like?

http://en.wikipedia.org/wiki/File:ESORVLTRLaserRphotR33aR07.jpg

Hershels’ Star$Gauges$(1785$AD) One$of$the$first$attempts$to$measure the$Milky$Way$ was$done$by$William$and$Caroline$Herschel$using$ visible$stars.

Unfortunately,$they$were$not$aware$that$most$of$the$ galaxy,$particularly$the$center,$is$blocked$from$view$ by$vast$clouds$of$gas and$dust.

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Harlow$Shapley$(1915$– 1921)

45 Globular Clusters

45 Globular Clusters

http://www.astro.virginia.edu/class/whittle/astr553/Topic01/t1_GCs_Shapley_Pogge.gif

Harlow$Shapley$(1915$– 1921)

47$Globular$Clusters$above$the$disk$of$M.W.

45$Globular$Clusters$below$the$galactic$disk http://www.astro.virginia.edu/class/whittle/astr553/Topic01/t1_GCs_Shapley_Pogge.gif

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Harlow$Shapley$(1915$– 1921)

Galactic$Center$at$about$17kpc$from$the$Sun$ and$a$total$width$of$80kpc$(260,000$light$years)

http://www.astro.virginia.edu/class/whittle/astr553/Topic01/t1_GCs_Shapley_Pogge.gif

The$Absorption$Problem

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The$Absorption$Problem

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The$Greater$Milky$Way

• We"see"our"galaxy"edgeRon. • Primary"features:"disk,"bulge,"halo,"globular"clusters ©"2014"Pearson"Education,"Inc.

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The$Greater$Milky$Way • We"see"our" galaxy"edgeR on…"but"if"we" could"step" outside"and" look"down," might"look"like" this.

https://www.basicknowledge101.com/photos/2016/milkywaysample.png

The$Whirlpool$Galaxy

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Here$are$two$other$spiral$galaxies,$one$ viewed$from$the$side and$the$other$from$the$ top:

Shapley"– Curtis"Debate: Astronomers"began"to"suspect"some"of"the"“nebulae”"were"other"galaxies… Curtis"argued"that" Shapley"argued"that" Andromeda"was"far" Andromeda"was"part"of" In"1920,"they"held"a"debate away"and"itself"a"galaxy our"star"system

In"1923,"Edwin"Hubble"photographed" individual"stars"in"the"Andromeda"Nebula" and"in"1924"found"the"distance"via" Cepheids Andromeda"Nebula"à ฀ Andromeda"Galaxy

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Pioneers$of$Stellar$Classification

Pickering’s"“Calculators” at"Harvard" College"Observatory"1913 ©"2014"Pearson"Education,"Inc.

Henrietta$Swan$Leavitt

http://en.wikipedia.org/wiki/File:Leavitt_henrietta_b1.jpg

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H.S.L.$measured$≈$25$Cepheid$Variables$in$ the$Large$Magellanic$Cloud$(1912)

http://www.astro.gla.ac.uk/users/martin/outreach/m100_cepheid.gif http://map.gsfc.nasa.gov/media/ContentMedia/M100CphB.jpg

Light$Curve$for$LMC$Cepheid

/outreach/education/senior/astrophysics/variable_cepheids.html

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Light$Curves$of$Variable$Stars The$upper$plot$is$an$RR$ Lyrae star.$All$such$stars$ have$essentially$the$same$ luminosity$curve$with$ periods$from$0.5 to$1 day.

The$lower$plot$is$a$ Cepheid variable;;$Cepheid$ periods$range$from$about$ 1 to$100 days. ©"2011"Pearson"Education,"Inc.

Variable$Stars$&$the$Instability$Strip The$variability$of$ these$stars$comes$ from$a$dynamic balance$between$ gravity and$ pressure—they$have$ large$oscillations around$stability.

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Period$– Luminosity$Relation The$usefulness$of$these$stars$comes$from$their$ period– luminosity$relation:

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2$Types$of$Cepheid,$So$Actually$3$types$of$ Variable$Stars

https://slideplayer.com/slide/3852981/13/images/67/Mira+and+Cepheid+Variables.jpg

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2$Types$of$Cepheid,$So$Actually$3$types$of$ Variable$Stars

= 0.7

……The$Plot$Could$be$Logarithmic

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Another$way$to$plot$the$data….

http://homepages.umflint.edu/~mistark/outreach/cepheid/PL.jpg

And$yet$Another….

http://physics.weber.edu/carroll/phsx1040/HW/PL.gif

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Standard$Candles • After"appropriate"calibrations,"the"PRL"Relation allows"for"a"powerful"distance"measuring"tool. – Measure"the"PERIOD"of"a"Cepheid" Variable"Star. – Use"the"P"– L"relation"to"estimate"the" Luminosity"à ฀ absolute"magnitude. – Measure"the"average"apparent"magnitude" of"the"star. – Apply"the"Distance"Modulus"and"the" Inverse"Square"Law"to"determine"the" distance. tion,"Inc.

An$example$– Zeta$Geminorum • Approx."Period"="____ • Given:"The"measured" apparent"magnitude"is" 3.7"– 4.1 …"we"will"use"""m"="3.9 • Estimate"the"absolute" magnitude"(2"slides" ago)" M"="________" • Distance"Modulus"&"ISL http://www.daviddarling.info/images2/Zeta_Geminorum.gif

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Example$2$mavg =$6.45,$T$=$10$days

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Example$2$Solution • m"="6.45,"T"="10"days,""

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Example$3$mavg =$3.75,$T$=$8$days

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Example$3$Solution • mavg ="3.75,"T"="8"days

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RR$Larea$Example$Mavg =$0.7$(all$RR$Larea) mavg =$10.55

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RR$Larea$Example$Solution • Mavg ="0.7"(all"RR"Larea) mavg ="10.55

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RR$Larea$Example$2$Mavg =$0.7$(all$RR$Larea) mavg =$7.7

http://spiff.rit.edu/classes/phys230/lectures/mw_size/rrlyr_art_nice.gif

RR$Larea$Example$2$Solution • Mavg ="0.7"(all"RR"Larea) mavg ="7.7

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The$Cosmic$Distance$Ladder …now$have$a$new$tool

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The$Puzzle$of$"Spiral$Nebulae" • Before"Hubble,"some"scientists"argued"that" "spiral"nebulae""were"entire"galaxies"like"our" Milky"Way,"while"others"maintained"they"were" smaller"collections"of"stars"within"the"Milky"Way. • The"debate"remained"unsettled"until"Edwin" Hubble"finally"measured"their"distances.

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How$did$Hubble$prove$that$galaxies$lie$far$ beyond$the$Milky$Way?

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1923$thru$1925$– Edwin$Hubble$

http://i.space.com/images/i/000/009/783/original/hubbleRphotosRvariableRstarRv1Rm31RcepheidR5.jpg?1306178768

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1923$thru$1925$– Edwin$Hubble$

http://cdn.phys.org/newman/gfx/news/hires/2011/hubbleviewst.jpg

1923$thru$1925$– Edwin$Hubble$ • Period"of"V1"(Delta"Cepheid)"="31.4"days. • Estimated"M"≈"– 5.4 • Measured"m"≈"+"18.9 • Applying"the"Distance"Modulus" à ฀ 736,100"pc"""or"""2,400,000"ly!! THIS"DISTANCE"IS"24x"the"width"of"the" known"size"of"the"Milky"Way."" In"one"night,"the"universe"became"a"much," much,"bigger"place…..

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Hubble$Deep$Field • Our"deepest"images"of"the"universe"show"a" great"variety"of"galaxies,"some"of"them"billions"of" lightRyears"away.

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Galaxies$and$Cosmology • A"galaxy's"age," its"distance,"and" the"age"of"the" universe"are"all" closely"related. • The"study"of" galaxies"is"thus" intimately" connected"with" cosmology— the"study"of"the" structure"and" evolution"of"the" universe. ©"2014"Pearson"Education,"Inc.

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