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The Shape of the Universe

The Shape of the Universe. This logo denotes A102 appropriate. The Progress of Distance Measurements (in Earth Radii). 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

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The Shape of the Universe

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  1. The Shape of the Universe This logo denotes A102 appropriate

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

  3. 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.

  4. 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.

  5. If you could see it from afar…

  6. Where the Sun is:

  7. Factinos: • 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

  8. Side View

  9. View from above

  10. Diagram Sparke/ Gallagher

  11. 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

  12. Consistently 0.5% of Galactic Mass Galaxies were found to rotate at speeds that could no be accounted for by stars alone. (See the Doppler shift?) A SMBH accounts for some of the speed, s, but not all.

  13. Not seeing

  14. Seeing!

  15. Companions: LMC, GMC

  16. Hubris • We always tend to put ourselves in the middle of things • Geocentrism • Heliocentrism • Watch how this theme takes centuries to undo

  17. Nebulous Ideas • We’ve seen early ideas about the shape of the Universe

  18. Descartes’ Vortices circa 1670

  19. 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 celestial objects became visible • What were they? • How far away? • Are they all basically the same?

  20. How would you differentiate these?

  21. Thomas Wright (circa 1750) • Theologian; thought of the Universe to be made up of spheres, but not with the Earth at the center • We are in a thin shell with Providence within and the Outer Darkness without, so when you die… • Looking along the shell you’d see the Milky Way

  22. Immanuel Kant • ~1755 received a text version of Wright’s idea • From that he postulated that: • The Galaxy (loosely, Greek for Milky Way) was a flattened ring of stars that included our Sun • With Providence still at the center • Also, 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

  23. 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

  24. 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 54 trillion miles or 9 light years. • Thus, according to Herschel, the Milky Way is 6,400 light years in diameter and 1300 light years thick, roughly 10% the value of modern estimates.

  25. 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

  26. Nebulous Nebulae • Herschel, as did Messier, catalogued many (2500) nebulae, faint fuzzy patches, some of which resolved into stars and some of which did not • He tried to distinguish between star systems and ‘true nebulosity’, a luminous fluid • 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

  27. From all this incomplete data, three big questions emerge: • What is the shape of the Galaxy? • What is its size? • 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?

  28. 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.

  29. 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

  30. 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

  31. 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

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

  33. 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

  34. 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 AFine *Girl, Kiss Me *or Guy

  35. 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

  36. 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

  37. 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

  38. 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?

  39. 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 • Coupled with apparent magnitude, simple math then gives the distance

  40. 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%

  41. 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

  42. The Inverse Square Law

  43. 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).

  44. 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)

  45. 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!

  46. Harlow Shapley (left) Mt. Wilson Observatory/ Heber D. Curtis  Lick Observatory

  47. The galaxy is approximately 300,000 light years in diameter and the sun is located far from the center. The spiral nebulae are associated with the galaxy, although outside the main body.  The nature of the spirals is not known, but is probably some combination of gas and faint stars. The Milky Way and its “halo” of globular clusters and spiral nebulae is all there is to the universe. The galaxy is approximately 30,000 light years in diameter and the sun is located near the center. The spiral nebulae are “island universes”, i.e., other galaxies comparable in size to the Milky Way. The universe contains a large, indeterminate, number of galaxies spread out over a large, indeterminate volume of space. Positions HC HS

  48. 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

  49. 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

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