The Shape of the Universe This logo denotes A102 appropriate The Progress of Distance Measurements (in Earth Radii) Ptolemy Copernicus Modern Mercury 100 430 (.4 AU) 9,600 (.4 AU) Venus 600 820 (.75 AU) 17,000 (.72 AU) Earth 1,200 1,100 (1 AU) 23,500 (1 AU) Mars 5,000 1,700 (1.5 AU) 35,700 (1.5 AU) Jupiter 11,500 6,000 (5.5 AU) 122,000 (5.2 AU) Saturn 17,000 10,500 (9.5 AU) 224,000 (9.5 AU) Stars 20,000 too far to measure 6,300,000,000 to nearest star Starting backwards To help with learning how our modern ideas about the cosmos were formed, we’ll see the ‘answer’ first and then see how these ideas evolved Therefore, before we delve into the historical aspects, let’s study our galaxy: the Milky Way. Quite visible A long exposure picture though a telescope can get you this, but the Milky Way is obvious if the sky is dark enough. If you could see it from afar… Where the Sun is: Factoids: A spiral galaxy 100-500 billion stars 100,000 LY across the disk Central bulge 15,000 LY thick Halo out to 75 kpc ~ 470,000 LY across Sun located 28,000 LY from nucleus 1 Rotation takes ~ 240-270Myr Side View View from above Diagram Sparke/ Gallagher In the nucleus: A supermassive black hole, ~ 4 million solar masses Likely that most galaxies with bright nucleii have a SMBH Right: star movement near the SMBH, 1992-1996 Consistently 0.5% of Galactic Mass Galaxies were found to A SMBH accounts for rotate at speeds that some of the speed, s, but could no be accounted for not all. by stars alone. (See the Doppler shift?) Not seeing Seeing! Companions: LMC, GMC Hubris We always tend to put ourselves in the middle of things Geocentrism Heliocentrism Watch how this theme takes centuries to undo Nebulous Ideas We’ve seen early ideas about the shape of the Universe Descartes’ Vortices circa 1670 The spill of stars is across the sky is quite apparent to the naked eye, so the Milky Way was well-known to the ancients – Some Messier objects are even visible (just barely) But with the introduction of telescopes many other clumps of stars became obvious – What were they? – How far away? – Are they all basically the same? How would you differentiate these? Thomas Wright (circa 1750) Thought of the Universe as made up of spheres, but not with the Earth at the center He imagined we were wedged between two concentric spheres, seeing the Milky Way in the tangent plane Emmanuel Kant Philosopher ~1755 Postulated that: – The Galaxy (loosely, Greek for Milky Way) was a disk of stars that includes our Sun – The Milky Way might not be unique; that is, many disk-like systems exist distributed throughout space – These disk-like systems were 'island universes' and could be observed as faint nebulae William Hershel, while “star gauging”, saw that stars were not evenly distributed across the sky and imagined the Galaxy to be this disk-shape, assuming: – all stars have the same brightness – the galaxy has a uniform density – he can see to the edge Estimates The dimensions of the galaxy were given in a unit Herschel christened the siriometer, the distance from the solar system to the star Sirius. He estimated the galaxy to be 1,000 siriometers in diameter and 100 siriometers thick. The actual distance to Sirius was not known in Herschel’s day, but the modern figure is 48 trillion miles or 8 light years. Thus, according to Herschel, the Milky Way is 8,000 light years in diameter and 800 light years thick, roughly 10% the value of modern estimates. Reconsideration Herschel discovers that some double stars are true star systems, with the stars revolving around a common center of gravity Thus, they must be at approximately the same distance from us Since most of these systems contain stars of unequal brightness, not all stars have the same luminosity—causes him to change his earlier ideas – Out with the lens or grindstone model Nebulous Nebulae Herschel, as did Messier, catalogued many nebulae, faint fuzzy patches, some of which resolved into stars and some of which did not Some like Kant thought that they might be distant versions of the Milky Way – Island universes, coined by Alexander von Humboldt in 1845) – That designation figured large in the upcoming Great Debate Others like Pierre Laplace thought they might be solar systems in formation From all this incomplete data, three big questions emerge: 1. What is the shape of the Galaxy? 2. What is its size? 3. Are the other nebulae part of the Milky Way or separate from it? – Secondarily: What were these nebula? Did they move? How fast? Which way? How big? During the 19th Century Parallax angles for other nearby stars were found by Bessel and other observers. – Alpha Centuri (the nearest at 1.3 parsecs or 4 light years) – Vega – Altair From the now known distances, it was quickly determined that 61 Cygni was significantly less luminous than the Sun while Vega and Altair were more luminous. Only Alpha Centuri had a luminosity nearly the same as the sun. William Huggins His application of spectroscopy comes to have a critical second use towards the second half of the 19th century: spectroscopic parallax The categorization of stars turns out to reveal their distance Annie Jump Cannon (1863-1941) Attended Wellesley – Studied physics and astronomy and learned to make spectroscopic measurements 1896: she became a member of the group of women hired by Harvard College Observatory director Edward Pickering to reduce data and carry out astronomical calculations Edward Pickering Directed the Harvard College Observatory for forty-two years Instituted the Henry Draper *Memorial Catalog in 1884 as a long-term project – Obtain optical spectra of as many stars as possible – Index and classify the stars by their spectra. * Well-known amateur Astronomer Pickering’s “Harem” AKA “computers” AJC First System of Ordering Pickering and Williamina Fleming classified stars in the 1890s based on the strength of H lines – A for strongest H lines – B for H plus He – C for more He, etc Cannon’s Improvement By 1901she had looked at the spectra of ¼ million stars (!) and rearranged them according to temperature, eliminating redundancy and adding subdivisions Oh Be A Fine *Girl, Kiss Me *or Guy Cannon’s Canons Published catalogs of variable stars (including 300 she discovered) First recipient of an honorary doctorate from Oxford – She earned a B.S. from Wellesley First woman elected an officer of the American Astronomical Society Curator of Astronomical Photographs at Harvard William C. Bond Astronomer at Harvard Cecilia Payne (1900-1979) Studied with AJC Her book, Stellar Atmospheres, “undoubtedly the most brilliant Ph.D. thesis ever written in astronomy.” She used the new quantum mechanical understanding of atomic structure, elaborating on Planck’s model, to show how and why the spectral lines of the various elements varied with respect to spectral type But the rub… In the chapter entitled “The Relative Abundance of the Elements”, CP could not account for the fact that, even though H and He are most abundant in stars, they are rare on Earth “If . . . the earth originated from the surface layers of the sun, the percentage composition of the whole earth should resemble the composition of the solar (and therefore of a typical stellar) atmosphere. . . . Considering the possibility of atomic segregation both in the earth and in the star, it appears likely that the earth’s crust is representative of the stellar atmosphere.” This ultimately meant that the theory of solar system formation of the time was incorrect Henry Norris Russell (1877-1922), and Ejnar Hertzsprung (1873-1967) Working independently they derived (perfected) this diagram in the early 20th C. – See the classifications? Astrometrical CSI Once a star's spectrum is identified, the star can be correctly placed on the H- R diagram. Knowing a star's proper location on the H-R diagram makes it possible to determine its intrinsic brightness – Absolute magnitude The inverse square law then gives the distance, out to 10,000 pc Henrietta Leavitt and Cepheid Variables Another of Pickering’s group “a straight line can readily be drawn ... showing that there is a simple relation between the brightness of the variables and their periods...” Actually, all stars are variable – The Sun varies 0.07% Cepheids vary greatly, and their output is closely correlated with their period Using the inverse square law, Cepheids are also good distance markers – Standard Candles The Inverse Square Law With this technique she estimated the Small Magellanic Cloud to be about 50,000 parsecs away -- making it one of the most distant objects known at the time (1912). Other observations For some astronomers, the spirals and nebulae were new planetary systems in formation For others, they exemplified what a complex star system like our own Milky Way might look like if we could see it from a great distance. The Milky Way star system Cornelius Easton (1900) The Stage is Set Better stellar distancing and new observations add fuel to what would become known as The Great Debate: – Is the Universe just the Milky Way? Or – Is our galaxy just one of many island universes? Two camps emerge, one out of the Mt. Wilson (LA) observatory, the other out of the Lick observatory (SJ) Civil but intense – And neither side had it exactly right! Harlow Shapley (left) Mt. Wilson Observatory / Heber D. Curtis Lick Observatory HS Positions HC The galaxy is approximately The galaxy is approximately 300,000 light years in 30,000 light years in diameter diameter and the sun is and the sun is located near the located far from the center. center. The spiral nebulae are The spiral nebulae are “island associated with the galaxy, universes”, i.e., other galaxies although outside the main comparable in size to the Milky body. The nature of the Way. spirals is not known, but is The universe contains a large, probably some combination indeterminate, number of of gas and faint stars. galaxies spread out over a The Milky Way and its “halo” large, indeterminate volume of of globular clusters and spiral space. nebulae is all there is to the universe. In 1915, Shapley used Cepheids contained in globular clusters to estimate the distance to each cluster He calculated that M13 is about 30,000 pc away In 1916, Adriaan van Maanen (1884-1946) announced he had photographic proof of rotation in face-on spiral nebulae: T* = 85,000 years He calculated that if this spiral were millions of light years away and comparable to the size of the Milky Way, a point on the edge of this galaxy would be traveling at a speed greater than the speed of light M33 Actual T = ~200 million years Shapley to van Maanen, 1916 “Congratulations on the nebulous results! Between us we have put a crimp in the island universes, it seems, -- you by bringing the spirals in [i.e., closer] and I by pushing the Galaxy out. We are indeed clever, we are” Additionally; In 1885 there was a nova in the Andromeda galaxy that almost reached naked eye visibility. If M31 is really a million light years away, to appear that bright the peak luminosity of this single star would have to equal the combined luminosity of billions of suns, something inconceivable (at the time) Shapley’s three basic reasons for rejecting the theory that the spiral nebulae are “island universes” The spiral nebulae are distributed uniformly about the Milky Way with a very distinct “zone of avoidance” containing no spiral nebulae running exactly along the galactic equator. – Why would randomly distributed galaxies at vast distances always avoid only this specific region of the sky? Spectroscopic studies showed conclusively that many nebulae are gaseous – Even “stellar” nebulae have a gas component, suggesting that there’s a continuum from gas to stars The enormous size of the Milky Way argues against the island universe theory Shapley’s Picture of the Universe On the other hand 1917: George Ritchey photographed a nova in the spiral nebula, NGC 6946. – NGC 6946 must be more than just a glowing cloud of gas and dust. – This was seen in 2004 Must be a huge, distant collection of stars Curtis argued: A large number of novae have been identified in spiral nebulae. This makes sense if spiral nebulae are island universes, but hard to explain if spirals are part of the Milky Way. Why should this region of the Milky Way have such a high concentration of novae? Why not in the galactic plane? If the spirals are as close as Shapely claims (20,000 light years for M31), then the large number of novae observed there have very high absolute magnitudes (very low absolute luminosity), therefore much dimmer than novae observed in the Milky Way. Why should these objects and these objects alone have extraordinarily dim novae? On the other hand, if the spiral distance is more like 5 times Shapley’s estimate, then the absolute magnitudes correlate well with the absolute magnitudes of local Milky Way novae. If we assume the spirals are galaxies like the Milky Way, we can use their observed properties as analogies for our own galaxy. Many edge-on spirals, for example, show dark bands of obscuring material along the major axis of their disks. We see similar regions of obscuring matter in the Milky Way. This offers an explanation for why spirals are never seen in the Milky Way (the so- called zone of avoidance) – they are blocked from view. Jacobus Cornelius Kapteyn (1851-1922) Worked on the photographic plates of David Gill for 4 years to produce a catalog of 455,000 Southern stars Plan of Selected Areas – Hoped to organize Astronomers all over the world for a massive star catalog Founded the productive Dutch school of Astronomers Found evidence that proper motion was not random – Two streams oppositely directed – Led to discovery of galactic rotation Hale met Kapteyn at a conference in 1904 and invited him to become a research associate at the Mount Wilson Observatory Kapteyn launched a plan for a major study of the distribution of stars in the Galaxy, using counts of stars in different directions "First attempt at a theory of the arrangement and motion of the sidereal system" was published in George Ellery Hale 1922 His sidereal models worked for high galactic latitudes but failed in the disk because he didn’t account for extinction Kapteyn’s Island Universe 1920 Harlow Shapley Shapley Curtis presented the case Distance to Cepheids in how do we for his model at a globular globular know that clusters? clusters are Cepheids in meeting of the the same as globular those nearby clusters are National Academy of like those that are nearby? Sciences What are the rotation of novae have spirals? spirals proves been Heber Curtis argued they are close observed in they must be some of these for Kapteyn's "Island true nebulae nebulae they must be Universe" model collections of stars like our own system The tie-breaker In 1923 Edwin Hubble (1899-1953) used the new 100-inch telescope at the Mt. Wilson observatory to photograph M31. For the first time ever, individual stars could be distinguished on the photographic plate Var! Hubble scratched “N” on the plate, first believing the Cepheid was a nova His discovery of a Cepheid in M31 made it possible to apply Henrietta Leavitt's period- luminosity method and determine that M31 is located at the remarkable distance of 1 million light-years (300,000 parsecs) For most Astronomers this settled the issue of the spiral nebulae and the island universe theory, and also gave a clue as to how really big the cosmos is Who was right about what HS HC The galaxy is approximately The galaxy is approximately 300,000 light years in 30,000 light years in diameter diameter and the sun is and the sun is located near the located far from the center. center. The spiral nebulae are The spiral nebulae are “island associated with the galaxy, universes”, i.e., other galaxies although outside the main comparable in size to the Milky body. The nature of the Way. spirals is not known, but is The universe contains a large, probably some combination indeterminate, number of of gas and faint stars. galaxies spread out over a The Milky Way and its “halo” large, indeterminate volume of of globular clusters and spiral space. nebulae is all there is to the universe. But now new questions arose Why are globular clusters so different from other stars in a galaxy? What is the significance of the different shapes and structures of these distant star systems? Do galaxies evolve? Why do distant galaxies appear to be moving away from us? Why do distant galaxies appear to be moving away from us? A whole other can of worms A whole other PPT!
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