The Shape of the UniverseThe Shape of the Universe

This logo denotes A102 appropriate

The Progress of Distance The Progress of Distance Measurements Measurements (in Earth Radii)(in Earth Radii)

PtolemyPtolemy CopernicusCopernicus ModernModern

MercuryMercury 100 100 430 (.4 AU) 430 (.4 AU) 9,600 (.4 AU) 9,600 (.4 AU)

VenusVenus 600 600 820 (.75 AU) 820 (.75 AU) 17,000 (.72 AU) 17,000 (.72 AU)

EarthEarth 1,200 1,200 1,100 (1 AU) 1,100 (1 AU) 23,500 (1 AU) 23,500 (1 AU)

MarsMars 5,000 5,000 1,700 (1.5 AU) 1,700 (1.5 AU) 35,700 (1.5 AU) 35,700 (1.5 AU)

JupiterJupiter 11,500 11,500 6,000 (5.5 AU) 6,000 (5.5 AU) 122,000 (5.2 AU) 122,000 (5.2 AU)

SaturnSaturn 17,000 17,000 10,500 (9.5 AU) 10,500 (9.5 AU) 224,000 (9.5 AU) 224,000 (9.5 AU)

StarsStars 20,000 20,000 too far to measure too far to measure 6,300,000,000 to 6,300,000,000 to nearest star nearest star

Starting backwardsStarting backwards

To help with learning how our modern To help with learning how our modern ideas about the cosmos were formed, we’ll ideas about the cosmos were formed, we’ll see the ‘answer’ first and then see how see the ‘answer’ first and then see how these ideas evolvedthese ideas evolved

Therefore, before we delve into the Therefore, before we delve into the historical aspects, let’s study our galaxy: historical aspects, let’s study our galaxy: the Milky Way.the Milky Way.

Quite visibleQuite visible

A long exposure A long exposure picture though a picture though a telescope can get telescope can get you this, but the you this, but the Milky Way is Milky Way is obvious if the sky obvious if the sky is dark enough.is dark enough.

If you could see it from afar…If you could see it from afar…

Where the Sun is:Where the Sun is:

Factinos:Factinos:

A spiral galaxyA spiral galaxy

100-500 billion stars100-500 billion stars

100,000 LY across the disk100,000 LY across the disk

Central bulge 15,000 LY thickCentral bulge 15,000 LY thick

Halo out to 75 kpc ~ 470,000 LY acrossHalo out to 75 kpc ~ 470,000 LY across

Sun located 28,000 LY from nucleusSun located 28,000 LY from nucleus

1 Rotation takes ~ 240-270Myr1 Rotation takes ~ 240-270Myr

Side ViewSide View

View from aboveView from above

DiagramDiagram

Sparke/Gallagher

In the nucleus:In the nucleus:

A supermassive black A supermassive black hole, ~ 4 million solar hole, ~ 4 million solar massesmasses

Likely that most Likely that most galaxies with bright galaxies with bright nucleii have a SMBHnucleii have a SMBH

Right: star movement Right: star movement near the SMBH, near the SMBH, 1992-19961992-1996

Consistently 0.5% of Galactic MassConsistently 0.5% of Galactic Mass

Galaxies were found to Galaxies were found to rotate at speeds that could rotate at speeds that could no be accounted for by no be accounted for by stars alone. (See the stars alone. (See the Doppler shift?)Doppler shift?)

A SMBH accounts for A SMBH accounts for some of the speed, some of the speed, , but , but not all.not all.

Not seeingNot seeing

Seeing!Seeing!

Companions: LMC, GMCCompanions: LMC, GMC

HubrisHubris

We always tend to put ourselves in the We always tend to put ourselves in the middle of thingsmiddle of things

GeocentrismGeocentrism

HeliocentrismHeliocentrism

Watch how this theme takes centuries to Watch how this theme takes centuries to undoundo

Nebulous IdeasNebulous Ideas

We’ve seen early ideas about the shape of the We’ve seen early ideas about the shape of the UniverseUniverse

Descartes’ Vortices circa 1670Descartes’ Vortices circa 1670

The spill of stars is across the sky is quite The spill of stars is across the sky is quite apparent to the naked eye, so the Milky Way apparent to the naked eye, so the Milky Way was well-known to the ancientswas well-known to the ancients– Some Messier objects are even visible (just barely)Some Messier objects are even visible (just barely)

But with the introduction of telescopes many But with the introduction of telescopes many other celestial objects became visibleother celestial objects became visible– What were they?What were they?– How far away?How far away?– Are they all basically the same?Are they all basically the same?

How would How would youyou differentiate these? differentiate these?

Thomas Wright (circa 1750)Thomas Wright (circa 1750)Theologian; thought of the Theologian; thought of the Universe to be made up of Universe to be made up of spheres, but not with the Earth at spheres, but not with the Earth at the centerthe center– We are in a thin shell with Providence We are in a thin shell with Providence

within and the Outer Darkness without, within and the Outer Darkness without, so when you die…so when you die…

Looking along the shell you’d Looking along the shell you’d see the Milky Waysee the Milky Way

Immanuel KantImmanuel Kant

~1755 received a text version of Wright’s ~1755 received a text version of Wright’s ideaideaFrom that he postulated that:From that he postulated that:– The Galaxy (loosely, Greek for Milky Way) was a The Galaxy (loosely, Greek for Milky Way) was a

flattened ring of stars that included our Sunflattened ring of stars that included our SunWith Providence still at the centerWith Providence still at the center

– Also, the Milky Way might not be unique; that is, Also, the Milky Way might not be unique; that is, many disk-like systems exist distributed many disk-like systems exist distributed throughout spacethroughout space

– These disk-like systems were 'island universes' These disk-like systems were 'island universes' and could be observed as faint nebulae and could be observed as faint nebulae

William Hershel, while “star gauging”, saw that William Hershel, while “star gauging”, saw that stars were not evenly distributed across the sky stars were not evenly distributed across the sky and imagined the Galaxy to be this disk-shape, and imagined the Galaxy to be this disk-shape, assuming:assuming:– all stars have the same brightness all stars have the same brightness – the galaxy has a uniform density the galaxy has a uniform density – he can see to the edge he can see to the edge

EstimatesEstimates

The dimensions of the galaxy were given in a The dimensions of the galaxy were given in a unit Herschel christened the unit Herschel christened the siriometersiriometer, the , the distance from the solar system to the star Sirius.  distance from the solar system to the star Sirius.  He estimated the galaxy to be 1,000 siriometers He estimated the galaxy to be 1,000 siriometers in diameter and 100 siriometers thick. The actual in diameter and 100 siriometers thick. The actual distance to Sirius was not known in Herschel’s distance to Sirius was not known in Herschel’s day, but the modern figure is 54 trillion miles or 9 day, but the modern figure is 54 trillion miles or 9 light years.light years.Thus, according to Herschel, the Milky Way is Thus, according to Herschel, the Milky Way is 6,400 light years in diameter and 1300 light 6,400 light years in diameter and 1300 light years thick, roughly 10% the value of modern years thick, roughly 10% the value of modern estimates. estimates.

ReconsiderationReconsideration

Herschel discovers that some double stars are Herschel discovers that some double stars are true star systems, with the stars revolving true star systems, with the stars revolving around a common center of gravityaround a common center of gravityThus, they must be at approximately the same Thus, they must be at approximately the same distance from usdistance from usSince most of these systems contain stars of Since most of these systems contain stars of unequal brightness, not all stars have the same unequal brightness, not all stars have the same luminosity—causes him to change his earlier luminosity—causes him to change his earlier ideas ideas – Out with the lens or grindstone modelOut with the lens or grindstone model

Nebulous NebulaeNebulous Nebulae

Herschel, as did Messier, catalogued many (2500) Herschel, as did Messier, catalogued many (2500) nebulae, faint fuzzy patches, some of which nebulae, faint fuzzy patches, some of which resolved into stars and some of which did notresolved into stars and some of which did not– He tried to distinguish between star systems and ‘true He tried to distinguish between star systems and ‘true

nebulosity’, a luminous fluidnebulosity’, a luminous fluid

Some like Kant thought that they might be distant Some like Kant thought that they might be distant versions of the Milky Way versions of the Milky Way – Island universes,Island universes, coined by Alexander von Humboldt in coined by Alexander von Humboldt in

1845)1845)– That designation figured large in the upcoming Great That designation figured large in the upcoming Great

DebateDebate

Others like Pierre Laplace thought they might be Others like Pierre Laplace thought they might be solar systems in formation solar systems in formation

From all this incomplete data, three From all this incomplete data, three big questions emerge:big questions emerge:

1.1. What is the shape of the Galaxy?What is the shape of the Galaxy?

2.2. What is its size?What is its size?

3.3. Are the other nebulae part of the Milky Are the other nebulae part of the Milky Way or separate from it?Way or separate from it?

– Secondarily:Secondarily:What were these nebula?What were these nebula?

Did they move? How fast? Which way?Did they move? How fast? Which way?

How big?How big?

During the 19During the 19thth Century Century

Parallax angles for other nearby stars were Parallax angles for other nearby stars were found by Bessel and other observers. found by Bessel and other observers. – Alpha Centuri (the nearest at 1.3 parsecs or 4 light Alpha Centuri (the nearest at 1.3 parsecs or 4 light

years) years) – Vega Vega – Altair  Altair 

From the now known distances, it was quickly From the now known distances, it was quickly determined that 61 Cygni was significantly determined that 61 Cygni was significantly lessless luminous than the Sun while Vega and Altair luminous than the Sun while Vega and Altair were were moremore luminous. Only Alpha Centuri had a luminous. Only Alpha Centuri had a luminosity nearly the same as the sun. luminosity nearly the same as the sun.

William HugginsWilliam Huggins

His application of His application of spectroscopy comes to spectroscopy comes to have a critical second use have a critical second use towards the second half of towards the second half of the 19the 19thth century: century: spectroscopic parallaxspectroscopic parallax

The categorization of stars The categorization of stars turns out to reveal their turns out to reveal their distancedistance

Annie Jump Cannon (1863-1941)Annie Jump Cannon (1863-1941)

Attended WellesleyAttended Wellesley– Studied physics and astronomy Studied physics and astronomy

and learned to make and learned to make spectroscopic measurements spectroscopic measurements

1896: she became a member 1896: she became a member of the group of women hired of the group of women hired by Harvard College by Harvard College Observatory director Edward Observatory director Edward Pickering to reduce data and Pickering to reduce data and carry out astronomical carry out astronomical calculations calculations

Edward Pickering Edward Pickering

Directed the Harvard Directed the Harvard College Observatory for College Observatory for forty-two years forty-two years

Instituted the Henry Instituted the Henry Draper *Memorial Draper *Memorial Catalog in 1884 as a Catalog in 1884 as a long-term project long-term project – Obtain optical spectra of as Obtain optical spectra of as

many stars as possible many stars as possible – Index and classify the stars Index and classify the stars

by their spectra. by their spectra.

* Well-known amateur Astronomer

Pickering’s “Harem” Pickering’s “Harem” AKA “computers”AKA “computers” AJC

First System of OrderingFirst System of Ordering

Pickering and Pickering and Williamina Williamina Fleming classified Fleming classified stars in the 1890s stars in the 1890s based on the based on the strength of H linesstrength of H lines– A for strongest A for strongest

H linesH lines– B for H plus HeB for H plus He– C for more He, C for more He,

etcetc

Cannon’s ImprovementCannon’s Improvement

By 1901she had By 1901she had looked at the looked at the spectra of ¼ million spectra of ¼ million stars (!) and stars (!) and rearranged them rearranged them according to according to temperature, temperature, eliminating eliminating redundancy and redundancy and adding subdivisionsadding subdivisionsOOh h BBe e AA FFine *ine *GGirl, irl, KKiss iss MMee

*or Guy

Cannon’s CanonsCannon’s Canons

Published catalogs of variable stars (including Published catalogs of variable stars (including 300 she discovered) 300 she discovered)

First recipient of an honorary doctorate from First recipient of an honorary doctorate from OxfordOxford– She earned a B.S. from WellesleyShe earned a B.S. from Wellesley

First woman elected an officer of the American First woman elected an officer of the American Astronomical SocietyAstronomical Society

Curator of Astronomical Photographs at HarvardCurator of Astronomical Photographs at Harvard

William C. Bond Astronomer at Harvard William C. Bond Astronomer at Harvard

Cecilia Payne Cecilia Payne (1900-1979)(1900-1979)

Studied with AJCStudied with AJC

Her book, Her book, Stellar AtmospheresStellar Atmospheres, , “undoubtedly the most brilliant “undoubtedly the most brilliant Ph.D. thesis ever written in Ph.D. thesis ever written in astronomy.”astronomy.”

She used the new quantum She used the new quantum mechanical understanding of atomic mechanical understanding of atomic structure, elaborating on Planck’s structure, elaborating on Planck’s model, to show how and why the model, to show how and why the spectral lines of the various spectral lines of the various elements varied with respect to elements varied with respect to spectral typespectral type

But the rub…But the rub…

In the chapter entitled “The Relative Abundance of the In the chapter entitled “The Relative Abundance of the Elements”, CP could not account for the fact that, even though Elements”, CP could not account for the fact that, even though H and He are most abundant in stars, they are rare on EarthH and He are most abundant in stars, they are rare on Earth

““If . . . the earth originated from the surface layers of the sun, If . . . the earth originated from the surface layers of the sun, the percentage composition of the whole earth should resemble the percentage composition of the whole earth should resemble the composition of the solar (and therefore of a typical stellar) the composition of the solar (and therefore of a typical stellar) atmosphere. . . . Considering the possibility of atomic atmosphere. . . . Considering the possibility of atomic segregation both in the earth and in the star, it appears likely segregation both in the earth and in the star, it appears likely that the earth’s crust is representative of the stellar that the earth’s crust is representative of the stellar atmosphere.”atmosphere.”

This ultimately meant that the theory of solar system formation This ultimately meant that the theory of solar system formation of the time was incorrectof the time was incorrect

Henry Norris Russell (1877-1922), Henry Norris Russell (1877-1922), and Ejnar Hertzsprung (1873-1967) and Ejnar Hertzsprung (1873-1967)

Working Working independently independently they derived they derived (perfected) this (perfected) this diagram in the diagram in the early 20early 20thth C. C.– See the See the

classifications?classifications?

Astrometrical CSIAstrometrical CSI

Once a star's spectrum is Once a star's spectrum is identified, the star can be identified, the star can be correctly placed on the H-correctly placed on the H-R diagram.R diagram.Knowing a star's proper Knowing a star's proper location on the H-R location on the H-R diagram makes it diagram makes it possible to determine its possible to determine its intrinsic brightnessintrinsic brightness– Absolute magnitudeAbsolute magnitude

Coupled with apparent Coupled with apparent magnitude, simple math magnitude, simple math then gives the distance then gives the distance

Henrietta Leavitt and Henrietta Leavitt and Cepheid VariablesCepheid Variables

Another of Pickering’s Another of Pickering’s groupgroup

““a straight line can readily a straight line can readily be drawn ... showing that be drawn ... showing that there is a simple relation there is a simple relation between the brightness of between the brightness of the variables and their the variables and their periods...”  periods...” 

Actually, all stars are Actually, all stars are variablevariable– The Sun varies 0.07%The Sun varies 0.07%

Cepheids vary Cepheids vary greatly, and their greatly, and their output is closely output is closely correlated with their correlated with their periodperiod

Using the inverse Using the inverse square law, square law, Cepheids are also Cepheids are also good distance good distance markersmarkers– Standard CandlesStandard Candles

The Inverse Square LawThe Inverse Square Law

With this technique she estimated the Small With this technique she estimated the Small Magellanic Cloud to be about 50,000 parsecs Magellanic Cloud to be about 50,000 parsecs away -- making it one of the most distant objects away -- making it one of the most distant objects known at the time (1912). known at the time (1912).

Other observationsOther observations

For some astronomers, For some astronomers, the spirals and nebulae the spirals and nebulae were new planetary were new planetary systems in formationsystems in formation

For others, they For others, they exemplified what a exemplified what a complex star system like complex star system like our own Milky Way might our own Milky Way might look like if we could see it look like if we could see it from a great distance. from a great distance.

The Milky Way star systemCornelius Easton (1900)

The Stage is SetThe Stage is Set

Better stellar distancing and new observations Better stellar distancing and new observations add fuel to what would become known as The add fuel to what would become known as The Great Debate:Great Debate:– Is the Universe just the Milky Way? OrIs the Universe just the Milky Way? Or– Is our galaxy just one of many Is our galaxy just one of many island universesisland universes??

Two camps emerge, one out of the Mt. Wilson Two camps emerge, one out of the Mt. Wilson (LA) observatory, the other out of the Lick (LA) observatory, the other out of the Lick observatory (SJ)observatory (SJ)Civil but intenseCivil but intense– And neither side had it exactly right!And neither side had it exactly right!

Harlow Shapley (Harlow Shapley (leftleft) ) Mt. Wilson ObservatoryMt. Wilson Observatory/ Heber D. Curtis  / Heber D. Curtis  Lick ObservatoryLick Observatory

PositionsPositions

The galaxy is approximately The galaxy is approximately 300,000 light years in 300,000 light years in diameter and the sun is diameter and the sun is located far from the center. located far from the center. The spiral nebulae are The spiral nebulae are associated with the galaxy, associated with the galaxy, although outside the main although outside the main body.  The nature of the body.  The nature of the spirals is not known, but is spirals is not known, but is probably some combination probably some combination of gas and faint stars. of gas and faint stars. The Milky Way and its “halo” The Milky Way and its “halo” of globular clusters and spiral of globular clusters and spiral nebulae is all there is to the nebulae is all there is to the universe. universe.

The galaxy is approximately The galaxy is approximately 30,000 light years in diameter 30,000 light years in diameter and the sun is located near the and the sun is located near the center. center. The spiral nebulae are “The spiral nebulae are “island island universesuniverses”, i.e., other galaxies ”, i.e., other galaxies comparable in size to the Milky comparable in size to the Milky Way. Way. The universe contains a large, The universe contains a large, indeterminate, number of indeterminate, number of galaxies spread out over a galaxies spread out over a large, indeterminate volume of large, indeterminate volume of space. space.

HS HC

In 1915, Shapley used Cepheids contained in In 1915, Shapley used Cepheids contained in globular clusters to estimate the distance to globular clusters to estimate the distance to each clustereach cluster

He calculated that M13 is about 30,000 pc away He calculated that M13 is about 30,000 pc away

In 1916, Adriaan van Maanen (1884-1946) In 1916, Adriaan van Maanen (1884-1946) announced he had photographic proof of rotation in announced he had photographic proof of rotation in face-on spiral nebulae: T* = 85,000 yearsface-on spiral nebulae: T* = 85,000 yearsHe calculated that if this spiral were millions of light He calculated that if this spiral were millions of light years away and comparable to the size of the Milky years away and comparable to the size of the Milky Way, a point on the edge of this galaxy would be Way, a point on the edge of this galaxy would be traveling at a speed greater than the speed of light traveling at a speed greater than the speed of light

M33

Actual T = ~200 million years

Shapley to van Maanen, 1916Shapley to van Maanen, 1916

““Congratulations on the nebulous results!  Congratulations on the nebulous results!  Between us we have put a crimp in the Between us we have put a crimp in the island universes, it seems, -- you by island universes, it seems, -- you by bringing the spirals in [bringing the spirals in [i.e., closeri.e., closer] and I by ] and I by pushing the Galaxy out.  We are indeed pushing the Galaxy out.  We are indeed clever, we are”clever, we are”

Additionally;Additionally;

In 1885 there was a nova in the In 1885 there was a nova in the Andromeda galaxy that almost reached Andromeda galaxy that almost reached naked eye visibility.  If M31 is really a naked eye visibility.  If M31 is really a million light years away, to appear that million light years away, to appear that bright the peak luminosity of this single bright the peak luminosity of this single star would have to equal the combined star would have to equal the combined luminosity of billions of suns, something luminosity of billions of suns, something inconceivable (at the time) inconceivable (at the time)

Shapley’s three basic reasons for rejecting the theory Shapley’s three basic reasons for rejecting the theory

that the spiral nebulae are “island universes”that the spiral nebulae are “island universes”

The spiral nebulae are distributed uniformly about the The spiral nebulae are distributed uniformly about the Milky Way with a very distinct “Milky Way with a very distinct “zone of avoidancezone of avoidance” ” containing no spiral nebulae running exactly along the containing no spiral nebulae running exactly along the galactic equator.galactic equator.– Why would randomly distributed galaxies at vast distances Why would randomly distributed galaxies at vast distances

always avoid only this specific region of the sky? always avoid only this specific region of the sky?

Spectroscopic studies showed conclusively that many Spectroscopic studies showed conclusively that many nebulae are gaseousnebulae are gaseous– Even “stellar” nebulae have a gas component, suggesting that Even “stellar” nebulae have a gas component, suggesting that

there’s a continuum from gas to starsthere’s a continuum from gas to stars

The enormous size of the Milky Way argues against the The enormous size of the Milky Way argues against the island universe theory island universe theory

Shapley’s Picture of the UniverseShapley’s Picture of the Universe

On the other handOn the other hand

1917: George Ritchey 1917: George Ritchey photographed a nova photographed a nova in the spiral nebula, in the spiral nebula, NGC 6946.  NGC 6946.  – NGC 6946 must be NGC 6946 must be

more than just a more than just a glowing cloud of gas glowing cloud of gas and dust.and dust.

– This was seen in 2004This was seen in 2004

Must be a huge, Must be a huge, distant collection of distant collection of stars stars

Curtis argued:Curtis argued:

A large number of novae have been A large number of novae have been identified in spiral nebulae. This makes identified in spiral nebulae. This makes sense if spiral nebulae are island sense if spiral nebulae are island universes, but hard to explain if spirals are universes, but hard to explain if spirals are part of the Milky Way.  Why should this part of the Milky Way.  Why should this region of the Milky Way have such a high region of the Milky Way have such a high concentration of novae? Why not in the concentration of novae? Why not in the galactic plane?galactic plane?

If the spirals are as close as Shapely claims If the spirals are as close as Shapely claims (20,000 light years for M31), then the large (20,000 light years for M31), then the large number of novae observed there have very high number of novae observed there have very high absolute magnitudes (very low absolute absolute magnitudes (very low absolute luminosity), therefore much dimmer than novae luminosity), therefore much dimmer than novae observed in the Milky Way.  Why should these observed in the Milky Way.  Why should these objects and these objects alone have objects and these objects alone have extraordinarily dim novae? extraordinarily dim novae?

On the other hand, if the spiral distance is more On the other hand, if the spiral distance is more like 5 times Shapley’s estimate, then the like 5 times Shapley’s estimate, then the absolute magnitudes correlate well with the absolute magnitudes correlate well with the absolute magnitudes of local Milky Way novae. absolute magnitudes of local Milky Way novae.

If we assume the spirals are galaxies like the Milky Way, If we assume the spirals are galaxies like the Milky Way, we can use their observed properties as analogies for we can use their observed properties as analogies for our own galaxy. Many edge-on spirals, for example, our own galaxy. Many edge-on spirals, for example, show dark bands of obscuring material along the major show dark bands of obscuring material along the major axis of their disks. We see similar regions of obscuring axis of their disks. We see similar regions of obscuring matter in the Milky Way. This offers an explanation for matter in the Milky Way. This offers an explanation for why spirals are never seen in the Milky Way (the so-why spirals are never seen in the Milky Way (the so-called called zone of avoidancezone of avoidance) – they are blocked from view. ) – they are blocked from view.

Jacobus Cornelius Kapteyn Jacobus Cornelius Kapteyn (1851-1922)(1851-1922)

Worked on the photographic Worked on the photographic plates of David Gill for 4 plates of David Gill for 4 years to produce a catalog of years to produce a catalog of 455,000 Southern stars455,000 Southern starsPlan of Selected AreasPlan of Selected Areas – Hoped to organize Astronomers Hoped to organize Astronomers

all over the world for a massive all over the world for a massive star catalogstar catalog

Founded the productive Founded the productive Dutch school of AstronomersDutch school of AstronomersFound evidence that proper Found evidence that proper motion was not randommotion was not random– Two streams oppositely Two streams oppositely

directeddirected– Led to discovery of galactic Led to discovery of galactic

rotationrotation

Hale met Kapteyn at a conference Hale met Kapteyn at a conference in 1904 and invited him to become in 1904 and invited him to become a research associate at the Mount a research associate at the Mount Wilson ObservatoryWilson ObservatoryKapteyn launched a plan for a Kapteyn launched a plan for a major study of the distribution of major study of the distribution of stars in the Galaxy, using counts stars in the Galaxy, using counts of stars in different directions of stars in different directions "First attempt at a theory of the "First attempt at a theory of the arrangement and motion of the arrangement and motion of the sidereal system" was published in sidereal system" was published in 1922 1922 His sidereal models worked for His sidereal models worked for high galactic latitudes but failed in high galactic latitudes but failed in the disk because he didn’t the disk because he didn’t account for extinctionaccount for extinction

George ElleryGeorge Ellery HaleHale

Kapteyn’s Island UniverseKapteyn’s Island Universe

19201920

Harlow Shapley Harlow Shapley presented the case presented the case for his model at a for his model at a meeting of the meeting of the National Academy of National Academy of Sciences Sciences

Heber Curtis argued Heber Curtis argued for Kapteyn's "Island for Kapteyn's "Island Universe" modelUniverse" model

ShapleyShapley CurtisCurtis

Distance to Distance to globular globular clusters?clusters?

Cepheids in Cepheids in globular globular clusters are clusters are the same as the same as those nearby those nearby

how do we how do we know that know that Cepheids in Cepheids in globular globular clusters are clusters are like those that like those that are nearby?are nearby?

What are the What are the spirals?spirals?

rotation of rotation of spirals proves spirals proves they are close they are close they must be they must be true nebulaetrue nebulae

novae have novae have been been observed in observed in some of these some of these nebulae they nebulae they must be must be collections of collections of stars like our stars like our own systemown system

The tie-breakerThe tie-breaker

In 1923 Edwin Hubble In 1923 Edwin Hubble (1899-1953) used the (1899-1953) used the new 100-inch new 100-inch telescope at the Mt. telescope at the Mt. Wilson observatory to Wilson observatory to photograph M31.  photograph M31.  For the first time ever, For the first time ever, individual stars could individual stars could be distinguished on be distinguished on the photographic the photographic plate plate

Var!Var!

Hubble scratched “N” on the plate, first believing Hubble scratched “N” on the plate, first believing the Cepheid was a nova the Cepheid was a nova His discovery of a Cepheid in M31 made it His discovery of a Cepheid in M31 made it possible to apply Henrietta Leavitt's period-possible to apply Henrietta Leavitt's period-luminosity method and determine that M31 is luminosity method and determine that M31 is located at the remarkable distance of 1 million located at the remarkable distance of 1 million light-years (300,000 parsecs)light-years (300,000 parsecs)For most Astronomers this settled the issue of For most Astronomers this settled the issue of the spiral nebulae and the island universe the spiral nebulae and the island universe theory, and also gave a clue as to how really big theory, and also gave a clue as to how really big the cosmos is the cosmos is

Who was right about whatWho was right about what

The galaxy is approximately The galaxy is approximately 300,000 light years in 300,000 light years in diameter and the sun is diameter and the sun is located far from the center. located far from the center. The spiral nebulae are The spiral nebulae are associated with the galaxy, associated with the galaxy, although outside the main although outside the main body.  The nature of the body.  The nature of the spirals is not known, but is spirals is not known, but is probably some combination probably some combination of gas and faint stars.of gas and faint stars. The Milky Way and its “halo” The Milky Way and its “halo” of globular clusters and spiral of globular clusters and spiral nebulae is all there is to the nebulae is all there is to the universe.universe.

The galaxy is approximately The galaxy is approximately 30,000 light years in diameter 30,000 light years in diameter and the sun is located near the and the sun is located near the center.center. The spiral nebulae are “The spiral nebulae are “island island universesuniverses”, i.e., other galaxies ”, i.e., other galaxies comparable in size to the Milky comparable in size to the Milky Way. Way. The universe contains a large, The universe contains a large, indeterminate, number of indeterminate, number of galaxies spread out over a galaxies spread out over a large, indeterminate volume of large, indeterminate volume of space. space.

HS HC

But now new questions aroseBut now new questions arose

Why are globular clusters so different from Why are globular clusters so different from other stars in a galaxy? other stars in a galaxy?

What is the significance of the different What is the significance of the different shapes and structures of these distant star shapes and structures of these distant star systems? systems?

Do galaxies evolve? Do galaxies evolve?

Why do distant galaxies appear to be Why do distant galaxies appear to be moving away from us?moving away from us?

Why Why dodo distant galaxies appear to distant galaxies appear to be moving away from us? be moving away from us?

A whole other can of wormsA whole other can of worms

A whole other PPT!A whole other PPT!