The Milky Way (continued)

1. The Milky Way (continued)

2. Here! Attendance Quiz Are you here today? (a) yes (b) no (c) Milky way? Mmmm…. Chocolate….

3. Reminder: No class next Monday, 5/30Have a great Memorial Day!

4. Final reminder about the study • You will receive an e-mail for the final two post-instruction inventories containing the links (URL) to follow later today • They are due next Tuesday at 9am (due to Memorial Day) • None of the inventories will be graded. However,you will get course credit (10 clicker points or about 2% of your total course grade) for completing all eight inventories • If you have completed only the pre- and post-instruction versions of the Light and Spectroscopy Concept Inventory (a total of two inventories) you will still receive 5 clicker points • You should not study for them, or use any outside resources. The idea is to measure how much the class has influenced what you know and think about science (astronomy in particular)

5. Today’s Topics • Structure of the Milky Way • Star formation and spiral arms • Stellar clusters • The galactic cycle (gas-stars-gas) • Observing galactic abundances • Ionization nebulae (Orion) • Advantages of far-infrared observations • Airborne observatories & radio telescopes

6. Star Formation and Spiral Arms • What are the hallmarks of star formation? • Hot, blue, and therefore young stars • Interstellar clouds, including gas and dust • In the spiral galaxy M51, the evidence for star formation appears along the spiral arms, suggesting that something about the spiral arms causes star formation

7. What are Spiral Arms? • Spiral arms look like they could be groups of stars and other material rotating to form a spiral structure • However, this is wrong, because the inner stars rotate so fast that physical spiral arms would wind up many times in the history of the Milky Way

8. Spiral Density Waves • Spiral arms are spiral density waves • Imagine a traffic jam behind a stalled truck • As cars approach the narrowing, they slow down and bunch up • As they pass the obstruction they speed up and spread out again • Now imagine you come back a few hours later and the truck has been moved a few hundred feet down the highway • The position of the pile-up (density wave) would have moved, but it would be different cars passing through • The “speed” of the wave (the truck being moved down the highway a few hundred feet every few hours) is much slower than the stars (cars)

9. Structure of Spiral Arms • Thus, one would expect to find molecular clouds being compressed as they slowed and entered the spiral arm, triggering new stars to form • Also, one would expect newly formed (blue) stars and (red) ionization nebulae behind the wave edge • This is exactly what you do find • This explains the fact that star-formation occurs preferentially in spiral arms

10. Stellar Clusters • Most stars form in clusters • There are two main types: • Open clusters • Young (1-10 million years - recent) • Sparse • Few thousand stars • 30 light years across • Globular clusters • Older (10-15 billion years) • Concentrated “balls” of stars • 10s-100s of thousands of stars • 60-150 light years across • Clusters are very important for studying stars since all the stars are at roughly the same distance and are all the same age!

11. Stellar Cluster Ages • Because all the stars in a cluster are at roughly the same distance, they can be plotted on an HR diagram, with apparent brightness on the y-axis • Then by looking for where along the main sequence the stars have turned off(the main-sequence turnoff point), the age of the cluster can be determined • Here are a number of clusters of different ages plotted together

12. “The Circle of Life”

13. Implications of the Star-gas-star Cycle • Stars form where there is gas and dust, i.e., inside molecular clouds • Places where gas (and therefore stars) are denser will have more star formation and therefore gas more enriched in heavy elements • Since the stars (and gas) are concentrated towards the center of our galaxy, we would expect the proportion heavy elements to be higher in the center and fall towards the edge • Do we see this? (more on this in a moment)

14. H+ Ionization Nebula H+ Young, massive star H+ H+ H+ H+ H+ H+ H+ H+ O+++ H+ H+ H+ H+ H+ S++ H+ H+ H+ H+ S+++ H+ N+ H+ N++ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ O+ + H+ H+ H+ H+ S+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ O+ H+ N+++ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ Interstellar Dust and Gas Cloud H+ H+ Observing Galactic Abundances • One way to observe abundances of the elements, is studying stars • Another place to observe abundances is in ionization nebulae around very young, massive stars (e.g., Orion nebula) • These objects are very bright, and can be seen over most of the galaxy • When very massive stars (O, B stars) form, they emit mostly UV light • Therefore, they ionize much of the gas around them • If there are heavy elements present in the cloud, they become ionized as well • When these ionized atoms reabsorb the electrons, they emit light (emission spectrum) • The amount of light indicates the amount of a given element

15. Problems with Visible Light Observations • Recall that even bright objects can be dimmed or obscured in visible light by interstellar dust • Even if you can see an object, its light is reddened, causing difficulty determining reliable abundances • One way around this is to observe far-infrared light emitted by some of the ionized atoms in an ionization nebula • The difficulty with this method is that such light is absorbed by our atmosphere

16. Research Studying Abundances • In order to observe the far-infrared light from an ionization nebula, we observed from an airplane • To measure the amount of hydrogen, to compare to the heavier elements, we observed radio waves from the nebulae using large telescope arrays • This is a picture of an undergraduate physics major who flew aboard the KAO with me a half dozen times over a two week period (flying from Hawaii) • He helped plan the observations, take and reduce the data, and was a co-author on the paper we published • He is currently a physics professor in Vermont Kuiper Airborne Observatory in Flight Australia Telescope Compact Array

18. Yours truly on-board the KAO

19. SOFIA: Stratospheric Observatory for Infrared Astronomy

20. Cutting the Cavity

21. 2.5-m Telescope in the Cavity

25. SOFIA: Stratospheric Observatory for Infrared Astronomy

26. Results from Our Research From Rudolph et al. 2006, Astrophysical Journal

27. Galaxies and Cosmology

28. Here! Attendance Quiz Are you here today? (a) yes (b) no (c) Cosmetology? Like hair and nails and makeup?

29. Today’s (2nd half) Topics • Large Scale Structure in the Universe • The Distance Ladder and Galactic Distances • Hubble’s Law • The Distances to the Galaxies • Redshifts and Hubble’s Law • Expansion of the Universe • The Age of the Universe

30. 2.5 Million ly Local Group The Andromeda Galaxy and the Milky Way are approaching each other at 119 km/s, and will collide and merge into a giant elliptical galaxy in approximately 5 billion years

31. Local Supercluster 52 Million ly

32. 100s-1000s of galaxies Giant elliptical galaxies 10s of Millions of ly Abell 1689 Abell 1060 Virgo cluster Galaxy Clusters

33. Large Scale Structure in the Universe 1 billion light years

34. The Distance Ladder • Radar ranging (few AU) • Parallax (AU  few 1000 light years) • Main sequence fitting (100s  10s of thousands of light years) • Cepheid variable stars (100s of thousands  100s of millions of light years) • Distant standards (Tully-Fisher, White Dwarf supernovae; 10s of millions to 10+ billion light years)

35. Standard Candles • With the exception of radar and parallax, all these methods rely on the concept of a standard candle • Recall, the apparent brightness of an object is related to its intrinsic brightness (luminosity) and distance by • Thus, if we know an object’s luminosity, and measure its apparentbrightness, we can find its distance Interactive Figure

36. Standard Candles - Examples • Main sequence stars (Interactive Fig.) • Spectral type  Luminosity • Calibrated for stars near enough to measure parallax • Many stars at same distance is better (clusters)  Main Sequence Fitting • Calibrated by cluster near enough to show parallax - Hyades • Cepheid variable stars (Inter. Fig. II) • Very bright stars which vary in luminosity in a regular way • A relation exists between variation period and the star’s luminosity • Calibrated using nearby Cepheids of known distance • White Dwarf (Type I) supernovae • All WD Sne cross the Chandrasekhar limit and explode in the same way; hence all have similar luminosities • Calibrated in nearby galaxies of known distance

37. Galactic Distances Quiz Cepheid variable stars are located in two different galaxies, A and B. Both stars have the same average apparent brightness. The star in galaxy A has a bright-dim-bright period of 10 days, while the one in galaxy B has a bright-dim-bright period of 30 days. Which of the two galaxies is at a greater distance from us? • Galaxy A • Galaxy B • They are located at the same distance. • There is insufficient information to tell.

38. Edwin Hubble and Galactic Distances • Before Hubble, the nature of so-called “spiral nebulae” was unknown • Other galaxies (“island universes”) • Gas clouds in our galaxy • The key missing piece of evidence was the distances to these objects • Hubble, using the Mt. Wilson 100” telescope in Pasadena (then the largest in the world) observed Cepheid variable stars in other galaxies • Using the period-luminosity relationship, he found that these objects were much further away than the size of the Milky Way, meaning they are independent galaxies

39. Redshifts and Hubble’s Law • It was known since the 1910s that the spectra of most spiral galaxies were redshifted, meaning they were moving away from us • Once Hubble found the distances to these galaxies he found something startling! • Rather than the motions of the galaxies being random, there is a relationship between redshift and the distance to the galaxy • The more distant the galaxy, the faster it recedes (Hubble’s Law)

40. Expansion of the Universe • If all the galaxies are moving apart, then earlier, they must have been closer together, i.e., the universe is expanding! • Since all the galaxies are moving away from the Milky Way, does this mean that we are at the center of the universe? • Our knowledge of the history of astronomy (Copernicus, Shapley) should make us skeptical of this idea • How else can we explain these facts? • If the entire universe is expanding then every galaxy sees all others receding, and if that expansion is uniform, then the rate of motion apart is proportional to distance (Hubble’s Law) • Note: because gravity holds them together, individual galaxies do not expand as the space they occupy expands Space Expansion + Int. Fig. 20.22

41. Cosmology Quiz I Consider three widely separated galaxies in an expanding universe. Imagine that you are located in galaxy 1 and observe that both galaxies 2 and 3 are moving away from you with a speed proportional to their distance from you. If you asked an observer in galaxy 3 to describe how galaxy 2 appears to move, what would he or she say? • “Galaxy 2 is not moving.” • “Galaxy 2 is moving toward galaxy 3.” • “Galaxy 2 is moving away from galaxy 3.”

42. Lecture Tutorial: Expansion of the Universe, pp. 133-134 • Work with one or more partners - not alone! • Get right to work - you have 10 minutes • Read the instructions and questions carefully. • Discuss the concepts and your answers with one another. Take time to understand it now!!!! • Come to a consensus answer you all agree on. • Write clear explanations for your answers. • If you get stuck or are not sure of your answer, ask another group. • If you get really stuck or don’t understand what the Lecture Tutorial is asking, ask me for help.

43. Cosmology Quiz I Consider three widely separated galaxies in an expanding universe. Imagine that you are located in galaxy 1 and observe that both galaxies 2 and 3 are moving away from you with a speed proportional to their distance from you. If you asked an observer in galaxy 3 to describe how galaxy 2 appears to move, what would he or she say? • “Galaxy 2 is not moving.” • “Galaxy 2 is moving toward galaxy 3.” • “Galaxy 2 is moving away from galaxy 3.”

44. Cosmology Quiz II According to modern ideas and observations, what can be said about the location of the center of our expanding universe? • The Earth is at the center • The Sun is at the center • The Milky Way Galaxy is at the center • The universe does not have a center