1. Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the month- November 3, 2008 Dr. Uwe Trittmann Welcome to

2. Today’s Topics Galaxies – Island Universes The Night Sky in November

3. Galaxies – Island Universes A historic tour of the discovery of the dwindling significance of humans in the universe: From the center of the universe towards the edge of in an average galaxy amongst 100 billion others

4. How do we know where we are? “Obviously” we are living on a flat Earth at the center of the universe, as a quick look tells us: –The stars, Sun, Moon and planets rotate us –There is no apparent curvature of the ground –The Milky Way is a band that surrounds us –There are no signs for any movement of the Earth (like wind, or forces throwing us off)

5. Science to the Rescue How do we avoid these wrong conclusions? –The data were sound –The interpretation not  Further observations are necessary to decide! Do we have to question everything? –Yes, in principle. –The signature of genius is to ask the right question, not necessarily to answer them.

6. Exploring our own Island Universe: The Milky Way A galaxy is a huge collection of stars, gas, dust, neutron stars, and black holes, isolated from others and held together by gravity

7. Our view of the Milky Way Appears as a milky band of light across the sky A small telescope reveals that it is composed of many stars (Galileo again!) Our knowledge of the Milky Way comes from a combination of observation and comparison to other galaxies

8. How do we know? Question: How can we say anything about our Milky Way, if we cannot see it from outside? Obviously a bogus picture of our milky way!

9. Enter the Genius William Herschel (XVIII century) Simple model: –Assumed all stars have the same absolute brightness –Counts stars as a function of apparent magnitude –Brighter stars closer to us; fainter stars further away –Cut off in brightness corresponds to a cut off at a certain distance. Conclusion: there are no stars beyond a certain distance

10. Herschel’s Findings Stars thinned out very fast at right angles to Milky Way In the plane of the Milky Way the thinning was slower and depended upon the direction in which he looked Flaws: –Observations made only in visible spectrum –Did not take into account absorption by interstellar gas and dust

11. Discovering other Island Universes Data: Lots of nebulous spots known in the nightsky Questions: What are they? All the same? Different things? Need more observations!  Build bigger telescopes

12. Famous Telescopes - Herschel Herschel detected Uranus (1781) (Uranus is visible with the unaided eye)

13. Famous Telescopes – Lord Ross 72 inch Reflector built during potato famine in Ireland Largest Telescope until Mt Wilson (1917)

14. The first nebula discovered to have spiral structure: M51

15. Lord Rosse, 1845

16. HST: M51 Spiral Galaxy

17. M99 is a spiral, too! Q: do we live in a spiral? Q: Are we in the center of the spiral? Philosophical answer: probably not!

18. Enter: next genius Harlow Shapley used variable stars, e.g. RR Lyrae stars, to map the distribution of globular clusters in the galaxy Found a spherical distribution about 30 kpc (30,000 pc) across –This is the true size of the galaxy Sun is (naturally!) not at the center – it’s about 26,000 ly out

19. Standing on the shoulders of Giants Shapley used methods developed by others to measure the distance to globulars Cepheid variables show luminosity-period correlations discovered by Henrietta Leavitt Shapley single-handedly increase the size of the universe tenfold!

20. Cepheids Henrietta Leavitt (1908) discovers the period-luminosity relationship for Cepheid variables Period thus tells us luminosity, which then tells us the distance Since Cepheids are brighter than RR Lyrae, they can be used to measure out to further distances

21. Properties of Cepheids or: How do I know if this star is a Cepheid? Period of pulsation: a few days Luminosity: 200-20000 suns Radius: 10-100 solar radii

22. Properties of RR Lyrae Stars or: How do I know if this star is of RR Lyrae type? Period of pulsation: less than a day Luminosity: 100 suns Radius: 5 solar radii

23. Cepheids and RR Lyrae: Yard-Sticks Normal stars undergoing a phase of instability Cepheids are more massive and brighter than RR Lyrae Note: all RR Lyrae have the same luminosity Apparent brightness thus tells us the distance to them! –Recall: B  L/d 2

24. Scientists are humans - Humans err Shapley was so convinced by his own findings that he held the opinion that our galaxy IS the universe He even won the great debate (1920) against Heber Curtis who correctly held the opposing view See: http://www.aip.org/history/cosmology/ideas/island.htm

25. Hubble supersedes Shapley Edwin Hubble identified single stars in the Andromeda nebula (“turning” it into a galaxy) Measured the distance to Andromeda to be 1 million Ly (modern value: 2.2 mill. Ly) Conclusion: it is 20 times more distant than the milky way’s radius  Extragalacticity!  Shapley’s theory falsified!

26. New Tools – New Discoveries Hubble in prime focus of Einstein visits Mt Wilson Mt Palomar. Hubble detected the Expansion of the Universe  “Proof” of Einstein’s General Relativity Theory

27. Structure of the Galaxy

28. Q: How many galaxies are there? Hubble Deep Field Project –100 hour exposures over 10 days –Covered an area of the sky about 1/100 the size of the full moon Probably about 100 billion galaxies visible to us!

30. About 1,500 galaxies in this patch alone Angular size ~ 2 minutes of arc

31. Other Galaxies there are ~ 100 billion galaxies in the observable Universe measure distances to other galaxies using the period- luminosity relationship for Cepheid variables Type I supernovae also used to measure distances –Predictable luminosity – a standard candle Other galaxies are quite distant –Andromeda (M31), a nearby (spiral) galaxy, is 2 million light-years away and comparable in size to Milky Way “Island universes” in their own right

32. Q: How does our galaxy look like from the outside? Probably like others, so observe them!

33. Hubble Classification Scheme Edwin Hubble (~1924) grouped galaxies into four basic types: –Spiral –Barred spiral –Elliptical –Irregular There are sub-categories as well

34. Spirals (S) All have disks, bulges, and halos Type Sa: large bulge, tightly wrapped, almost circular spiral arms Type Sb: smaller bulge, more open spiral arms Type Sc: smallest bulge, loose, poorly defined spiral arms

35. Barred Spirals (SB) Possess an elongated “bar” of stars and interstellar mater passing through the center

36. Elliptical (E) No spiral arms or clear internal structure Essentially all halo Vary in size from “giant” to “dwarf” Further classified according to how circular they are (E0–E7)

37. S0/SB0 Intermediate between E7 and Sa Ellipticals with a bulge and thin disk, but no spiral arms

38. Q: How do we know we live in a Spiral Galaxy? After correcting for absorption by dust, it is possible to plot location of O- and B- (hot young stars) which tend to be concentrated in the spiral arms Radio frequency observations reveal the distribution of hydrogen (atomic) and molecular clouds Evidence for –galactic bulge –spiral arms

39. Rotation of the Galaxy Stars near the center rotate faster; those near the edges rotate slower (Kepler) The Sun revolves at about 250 km/sec around the center Takes 200-250 million years to orbit the galaxy – a “galactic year”

40. How do spiral arms persist?  Why don’t the “curl up”?

41. “Spiral Density Waves” A spiral compression wave (a shock wave) moves through the Galaxy Triggers star formation in the spiral arms Explains why we see many young hot stars in the spiral arms

42. Shock Waves

43. The Mass of the Galaxy Can be determined using Kepler’s 3 rd Law –Solar System: the orbital velocities of planets determined by mass of Sun –Galaxy: orbital velocities of stars are determined by total mass of the galaxy contained within that star’s orbit Two key results: –large mass contained in a very small volume at center of our Galaxy –Much of the mass of the Galaxy is not observed consists neither of stars, nor of gas or dust extends far beyond visible part of our galaxy (“dark halo”)

44. The Missing Mass Problem Dark Matter is dark at all wavelengths, not just visible light The Universe as a whole consists of up to 25% of Dark Matter!  Strange! What is it? –Brown dwarfs? –Black dwarfs? –Black holes? –Neutrinos? –Other exotic subatomic particles? Actually: Most of the universe (70%) consists of Dark Energy  Even stranger!

45. Missing Mass Problem

46. Galaxy Masses Rotation curves of spiral galaxies comparable to milky way Masses vary greatly

47. The Night Sky in November The sun is past autumn equinox -> longer nights! Autumn constellations are coming up: Cassiopeia, Pegasus, Perseus, Andromeda, Pisces  lots of open star clusters!

48. Moon Phases Today (Waxing Crescent) 11 / 5 (First Quarter Moon) 11 / 13 (Full Moon) 11/19 (Last Quarter Moon) 11/ 27 (New Moon)

49. Today at Noon Sun at meridian, i.e. exactly south

50. 10 PM Typical observing hour, early November Uranus Neptune

51. Star Maps Celestial North Pole – everything turns around this point Zenith – the point right above you & the middle of the map 40º 90º

52. West The summer triangle lingers on …

53. Due North Big Dipper points to the north pole

54. High up – the Autumn Constellations W of Cassiopeia Big Square of Pegasus Andromeda Galaxy

55. “PR” Foto Actual look

56. South- East Perseus, Auriga & Taurus with Plejades and the Double Cluster

57. South- West – 2006 Planets –Uranus –Neptune Zodiac: –Capricorn –Aquarius

58. South- West – 2007 Planets –Uranus –Neptune Zodiac: –Capricorn –Aquarius

59. South – 2008 Planets –Uranus –Neptune Zodiac: –Capricorn –Aquarius

60. Mark your Calendars! Next Starry Monday: February 2, 2009, 7 pm (this is a Monday ) Observing at Prairie Oaks Metro Park: –Friday, January 30, 7:00 pm "The Journey to Palomar""The Journey to Palomar" airs on PBS on November 10, 2008 Web pages: –http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.)http://www.otterbein.edu/dept/PHYS/weitkamp.asp –http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)http://www.otterbein.edu/dept/PHYS/

61. Mark your Calendars II Physics Coffee is every Wednesday, 3:30 pm Open to the public, everyone welcome! Location: across the hall, Science 244 Free coffee, cookies, etc.

62. Famous Telescopes – Mt Palomar 5 Meter Telescope – Huge and heavy mirror On Mt. Palomar in California

63. Visiting Mount Palomar November 2005

64. Model of the 200” Telescope

65. Transport of the Mirror ~ 1945