Starry Monday at Otterbein

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

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

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/d2

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 3rd 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