A1 20 Milky Way

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Miller's Astronomy 1 lecture notes on the Milky Way galaxy

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A1 20 Milky Way

  1. 1. Discovering our Milky Way LACC: §24.1, 3, 5 • Where are we in the Universe? • What are “spiral nebulae”? • What is our Milky Way galaxy like? An attempt to answer the “big questions”: where are we? how did we get here? Thursday, May 6, 2010 1
  2. 2. The Discovery of the Milky Way • 1755 Immanuel Kant speculates that there may exist "Island Universes" like our Milky Way. • 1785 William Herschel studies star counts along several hundred lines of sight in the galaxy. http://cass.ucsd.edu/physics/ph162/lect1.html Thursday, May 6, 2010 2
  3. 3. Harlow Shapley (1915) and Globular Clusters) Sketch based on Shapley's original data, uncorrected for interstellar absorption. The Sun is located at the center of the axes (looking roughly side-on), and the center of the Milky Way inferred by Shapley is marked by the red X. http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit4/milkyway.html Thursday, May 6, 2010 3
  4. 4. “Spiral Nebulae” • 1920 Shapley Curtis Debate Harlow Shapley took the position that the universe consisted only of our Galaxy, which was very large -- about 300,000 light-years in diameter. The spiral nebulae, while distant, were still part of our galaxy Heber Curtis argued for a smaller galaxy - about 30,000 light-years in diameter - that was one of a vast number of similar systems. The spiral nebulae, he said, were separate star systems similar to our own galaxy, and at great distances "from 500,000 to 10,000,000 light-years away" • 1923 Edwin Hubble discovers Cepheid Variable stars in Messier 31 - the Great Nebula in Andromeda, estimating its distance as nearly 0.3Mpc (modern value is about 0.7Mpc), well outside our Galaxy. http://ottawa-rasc.ca/features/marchHubble/index.html Thursday, May 6, 2010 4
  5. 5. Milky Way - Spiral Arms (via radio observations) http://www.astro.wisc.edu/goat/article/7/the-milky-way-using-real-data Thursday, May 6, 2010 5
  6. 6. The Size of the Milky Way http://zebu.uoregon.edu/~imamura/123/lecture-2/lecture-2.html Thursday, May 6, 2010 6
  7. 7. What is a parsec? An astronomical unit of length, equal to the distance at which the radius of the Earth's orbit subtends an angle of one arcsecond. The name is a contraction of "parallax- second." 1 parsec = 3.259 light-years = 206,265 AU = 30.83 trillion km = 19.16 trillion miles. The parsec is generally used by astronomers in preference to the light- year. For larger distances, the kiloparsec (kpc) = 1,000 pc or megaparsec (Mpc) = 1,000,000 pc are used. http://www.daviddarling.info/encyclopedia/P/parsec.html Thursday, May 6, 2010 7
  8. 8. The Size of the Milky Way This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version. Thursday, May 6, 2010 8
  9. 9. A recent survey of stars Milky Way - Bulge: conducted with the Spitzer Space Telescope is convincing astronomers that our Milky Way A Barred Spiral Galaxy is not just your ordinary spiral galaxy anymore. Looking out from within the Galaxy's disk, the true structure of the Milky Way is difficult to discern. However, the penetrating infrared census of about 30 million stars indicates that the Galaxy is distinguished by a very large central bar some 27,000 light- years long. In fact, from a vantage point that viewed our galaxy face-on, astronomers in distant galaxies would likely see a striking barred spiral galaxy suggested in this artist's illustration. While previous investigations have identified a small central barred structure, the new results indicate that the Milky Way's large bar would make about a 45 degree angle with a line joining the Sun and the Galaxy's center. DON'T PANIC ... astronomers still place the Sun beyond the central bar region, about a third of the way in from http://apod.nasa.gov/apod/ap050825.html the Milky Way's outer edge. Thursday, May 6, 2010 9
  10. 10. Milky Way: What Do We See? Our sun orbits every 225–250 million years https://sites.google.com/site/earthsplaceintheuniverse/ Thursday, May 6, 2010 10
  11. 11. Here's the Universe within 5000 The stars on the plot are all light years, our little arm of the thousands of times brighter Milky Way galaxy, the Orion Arm. than the Sun. The brightest star Virtually every star we can see here is Rho Cassiopeia (ρ Cas,) with the naked eye from Earth is some 100,000 times brighter within this distance. than the Sun. At 4000 light years away it is barely visible to the naked eye. Milky Way: What Do We See? https://sites.google.com/site/earthsplaceintheuniverse/ Thursday, May 6, 2010 11
  12. 12. Discovering our Milky Way LACC: §24.1, 3, 5 • Where are we in the Universe? We are about 2/3 away from the center of our galaxy’s core (Hershel and Shapley). Our galaxy is nowhere special. • What are “spiral nebulae”? They are other galaxies; Curtis suspected this (Shapley didn’t), Hubble proved it using a Cepheid variable • What is our Milky Way galaxy like? It is a barred- spiral galaxy about 30 kpc wide with a Bulge (barred), Disk (spiral arms), Halo (globular clusters) An attempt to answer the “big questions”: where are we? how did we get here? Thursday, May 6, 2010 12
  13. 13. LACC HW: Franknoi, Morrison, and Wolff, Voyages Through the Universe, 3rd ed. • Ch 24: Tutorial Quizzes accessible from: http:// www.brookscole.com/cgi-brookscole/course_products_bc.pl? fid=M20b&product_isbn_issn=9780495017899&discipline_number=19 Due at the beginning of next week’s first class period (unless there is a test that week, in which case it’s due the same period as the test). Be working on your Distance Ladders. Thursday, May 6, 2010 13
  14. 14. Our Milky Way Galaxy LACC: §24.1, 3, 5 • Formation and Evolution • Composition • Recent Discoveries An attempt to answer the “big questions”: where are we? how did we get here? Thursday, May 6, 2010 14
  15. 15. The Formation of the Milky Way A cloud of hydrogen/helium gas begins to form myriad stars. As this continues, the cloud may contract somewhat and the assemblage of stars begin to rotate around a common center.... With rotation, there is a tendency for the cloud to assume a more oblate ellipsoidal shape and begin to spin. The spinning produces strings of stars in at least several distinct arms. When well developed, the stars have organized into a spiral galaxy. http://rst.gsfc.nasa.gov/Sect20/A2.html http://www.youtube.com/watch? v=n0jRObc7_xo&feature=related Thursday, May 6, 2010 15
  16. 16. Spiral Arms are Density Waves http://abyss.uoregon.edu/~js/ast123/lectures/lec10.html Thursday, May 6, 2010 16
  17. 17. Spiral Arms are Density Waves http://abyss.uoregon.edu/~js/ast123/lectures/lec10.html Thursday, May 6, 2010 17
  18. 18. Composition of the Milky Way This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version. The components merge smoothly into each other with the stellar halo among the disk and the inner part of dark matter halo and the dark matter halo among the disk, stellar halo, and bulge, etc. The bulge is the elliptical-shaped center part of the Galaxy about 1000 to 2000 parsecs in radius. It had lots of star formation early on, so now it is made of tens of billions of old, metal-rich! stars. The disk is the thin pancake-shaped part about 400 parsecs thick and 15 to 20 thousand parsecs in radius with the Sun 8000 parsecs from the center. The disk contains over 98% of the dust and gas in the Galaxy and has a few hundred billion stars. Some stars continue to form so the disk has some young metal-rich stars. The gas and dust are found in a layer that is thinner than the star layer (the gas/dust layer is the thin dark line at the midplane of the disk in the picture above and the star layer is the thicker light band). The stellar halo is a roughly spherical distribution of hundreds of millions of old, metal-poor stars that has increasing concentration of stars toward the center of the galaxy. It is about 20 to 30 thousand parsecs in radius and it may contain small amount of hot gas, but the disk contains the vast majority. Most of the globular clusters are found in the halo and, like the halo stars, the number of them increases toward the galactic center. If the solar system was at the center, you would see approximately the same number of globular clusters in any direction you looked in the sky. Since the globulars are found bunched up in one part of the sky, i.e., they are swarming around some other point in the Galaxy, and we are not at the center. The dark matter halo is denser toward the center. It extends further out than the stellar halo. Thursday, May 6, 2010 18
  19. 19. Stellar Population: Population I Stars Population I includes the younger stars in the disk/plane of the galaxy. Because these stars formed recently, they have all be enriched in heavy elements produced in previous generations of stars. http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/milkywayparts.htm Thursday, May 6, 2010 19
  20. 20. Stellar Population: Population II Stars Population II is the older stars that tend to lie around the center and in globular clusters, and hence have orbits that take them well out of the disk/plane. Many of these stars were among the first to form, and hence they tend to be almost pure hydrogen and helium, not enriched by previous generations of stars because there were no previous generations. From Gene Smith, http:// casswww.ucsd.edu/public/tutorial/Galaxies.html http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/milkywayparts.htm Thursday, May 6, 2010 20
  21. 21. Milky Way Satellite Galaxies Not shown here is the Canis Major Dwarf Galaxy--discovered in 2003, it is 42 000 ly from our galactic center (and about 25 000 ly from us). http://8minutesold.com/?p=135 Thursday, May 6, 2010 21
  22. 22. The Galactic Core A panoramic X-ray view, covering a 900 by 400 light year swath, shows that the center of the Galaxy is a teeming and tumultuous place. There are supernova remnants: SNR 0.9-0.1, probably the X-ray Thread, and Sagittarius A East. There are many bright X-ray sources, which astronomers believe are binary systems—or pairs of orbiting objects—that contain a black hole or a neutron star (the 1E sources). There are hundreds of unnamed point-like sources that scientists think are solo neutron stars or white dwarfs, which all light up the region. In addition, the massive stars in the Arches and other star clusters (the DB sources) will soon explode to produce more supernovas, neutron stars, and black holes. Sgr A*, the supermassive black hole that marks the center of the Milky Way Galaxy. Sgr A* contains about 3 million times the mass of the Sun, and is gaining weight daily as it pulls in more material. http://xrtpub.harvard.edu/edu/gcenter/ Thursday, May 6, 2010 22
  23. 23. The Galactic Core: A Supermassive Black Hole This page was copied from Nick Strobel's Astronomy High-resolution infrared measurements of Notes. Go to his site at www.astronomynotes.com for the orbits of the stars at the center show the updated and corrected version. that a very compact mass---a super- massive black hole---with about 3.7 million solar masses lies at the center. The picture below (courtesy of Andrea Ghez and the UCLA Galactic Center Group) shows the orbits of the stars around the black hole from the years 1995 to 2006. At a distance of 8 kpc for the Sun, the 0.2 arc second scale bar in the figure corresponds to about 0.025 light years or 1600 AU. The object is too compact to be a dense cluster of stars---the Chandra X-ray Observatory's observations of X-ray bursts from the object place an upper limit of the diameter of the object of the size of the Earth's orbit. An expanding ring is also seen about 9000 light years from the center. Other galaxy cores have supermassive compact objects (the Andromeda Galaxy, M32, Sombrero Galaxy, M87, and many others). Thursday, May 6, 2010 23
  24. 24. Rotation Curves http://abyss.uoregon.edu/~js/ast122/lectures/lec25.html Thursday, May 6, 2010 24
  25. 25. Rotation Curves of Milky Way To determine the rotation curve of the Galaxy, stars are not used due to interstellar extinction. Instead, 21-cm maps of neutral hydrogen are used. When this is done, one finds that the rotation curve of the Galaxy stays flat out to large distances, instead of falling off as in the figure above. This means that the mass of the Galaxy increases with increasing distance from the center. The surprising thing is there is very little visible matter beyond the Sun's orbital distance from the center of the Galaxy. So the rotation curve of the Galaxy indicates a great deal of mass, but there is no light out there. We call this the dark matter problem, and states that the halo of our Galaxy is filled with a mysterious dark matter of unknown composition and type. http://abyss.uoregon.edu/~js/ast122/lectures/lec25.html Thursday, May 6, 2010 25
  26. 26. What is Dark Matter? Basically, according to careful observations, we know that up to 90% of the matter in galaxies must be in the form of "dark matter" to account for the dynamics we observe. On top of that, the dark matter appears to be distributed in a spherical halo around the Milky Way, while the luminous matter is located largely in the flat disk. http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970417c.html http://relativity.livingreviews.org/open?pubNo=lrr-2002-4&page=node9.html Thursday, May 6, 2010 26
  27. 27. Our Milky Way Galaxy LACC: §24.1, 3, 5 • Formation and Evolution: protogalactic cloud, globular clusters, population I and II stars • Composition: Bulge--population I, random orbits; Disk--population I, “normal” orbits; Halo-- population II, random orbits • Recent Discoveries: Canis Major Dwarf Galaxy, our nearest neighbor; Sagitarius A*, a supermassive black hole in the galactic center (ir and x-ray); dark matter halo (rotation curve) An attempt to answer the “big questions”: where are we? how did we get here? Thursday, May 6, 2010 27
  28. 28. LACC HW: Franknoi, Morrison, and Wolff, Voyages Through the Universe, 3rd ed. • Ch. 24, pp. 554-555: 11 (a--3 choices, b--1 choice, c--pick 2, d--pick 1, pick 2). • Ch 25: Tutorial Quizzes accessible from: http:// www.brookscole.com/cgi-brookscole/course_products_bc.pl? fid=M20b&product_isbn_issn=9780495017899&discipline_number=19 Due at the beginning of next week’s first class period (unless there is a test that week, in which case it’s due the same period as the test). Be working on your Distance Ladders. Thursday, May 6, 2010 28

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