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1 Exam #3 Average: 80.1% Median: 83.8% High: 100% Scores available on Blackboard If you d like to see/discuss your exam, come to my office hours, or make an appointment.

2 Exam #3 The Sun is made of A) all hydrogen B) all hydrogen and helium C) mostly hydrogen and helium, with 1-2% heavier elements it made in its core D) mostly hydrogen and helium, with 1-2% heavier elements it was created with The Sun is only making helium right now, the heavier elements were already present in the original solar nebula.

3 Exam #3 The main sequence on the Hertzsprung-Russell (H-R diagram represents a A) stellar age sequence B) heavy element sequence C) stellar mass sequence D) all of the above

4 A Chance to Replace an Exam The Final Exam (Exam 4) will be held during Finals Week at the scheduled time, but will just be a regular 40-question exam as before, and will only cover the final five chapters of the book. Syllabus mentions a 5 th Comprehensive Exam following the Final Exam for students who have missed an exam due to illness, etc. Everyone can take the 5 th Comprehensive (cumulative) exam to replace a low exam grade among the four regular exams. Comprehensive Exam is 40 questions, but covers the entire course, and is be taken immediately after Exam 4. There is no harm to your grade if you do poorly on the Comprehensive Exam, it can only help you (if you do well).

5 Homework #8 Due Wednesday, April 18, 11:59PM Covers Chapters 15 and 16 Estimated time to complete: 40 minutes Read chapters, review notes before starting

6 You Can Still Do Homeworks Late You can complete old homeworks late for up to 50% credit (even Homework #1). Particularly important to finish homeworks if you plan to take Comprehensive 5 th Exam. Also get a license for your clickers!

7 One Last Chance For Extra Credit There will be a telescope viewing night on the roof of Gallalee Hall on April 27. Get an extra day s worth of clicker points for free by attending. More details as the date approaches.

8 Chapter 15 Our Galaxy

9 What does our galaxy look like? Galaxy: a collection of stars (100 billion for the Milky Way galaxy), gas, dust, etc. bound together by gravity as a single entity. There are ~100 billion galaxies in our observable Universe.

10 The disk of the Milky Way Galaxy appears in our sky as a faint band of light. We re are seeing our Galaxy from the inside out.

11 If it weren t for gas/dust, we would see the center of our Galaxy here Dusty gas clouds obscure our view because they absorb visible light. This gas and dust is the interstellar medium (ISM) that makes new star systems.

12 William and Caroline Herschel performed one of the first all-sky mapping in the 1700s.

13 If a modest sized telescope is good a big one is even better. 18 Newtonian-style telescope 47 telescope (note eyepiece location)

14 Herschel s Picture of the Milky Way Sun is nearly in the center surrounded by a flattened disk of stars (irregular) according to Herschel s (wrong) idea. How did he determine distances??

15 The Kapteyn Universe (really, just the Milky Way) Jocobus Kapteyn (1908) Sun near the center again in this older model. But this is wrong! As usual, humans erroneously put themselves at the center of everything.

16 If it weren t for gas/dust, we would see the center of our Galaxy here Error came in not accounting for gas/dust absorbing the light from more distant stars makes it look like we are surrounded by same number of stars in all directions. Think of standing in a pumpkin patch on a very foggy day it would appear to you that you are at the center of the pumpkin patch no matter where you stood in the patch.

17 If you removed all the gas/dust from our Galaxy, how would the night sky look? A) much the same as it does now B) like it does now, but with many more stars in all directions C) a much higher concentration of stars along the Milky Way plane and direction of the bulge

18 If you removed all the gas/dust from our Galaxy, how would the night sky look? A) much the same as it does now B) like it does now, but with many more stars in all directions C) a much higher concentration of stars along the Milky Way plane and direction of the bulge We would be able to see the disk and bulge of our Galaxy much more easily, and it would not appear as if we are in the center of the stellar distribution.

19 Modern View of Our Galaxy Our galaxy as it would appear edge-on. Primary features: disk, bulge, halo, globular clusters

20 If we could view the Milky Way from above the disk, we would see its spiral arms.

21 Components of the Milky Way Disk: 100,000 light year wide, 1000 light year thick spiral structure of old and young stars (and intermediate-age stars like our Sun), gas, and dust (recent star formation), open clusters of stars Bulge: 10,000 light year radius sphere of old stars at the center of the galaxy, little gas or dust (no star formation) Halo: ~50,000 light year radius sphere of old stars surrounding the disk and bulge, little gas or dust (no star formation) Globular Star Clusters: compact collection of 100,000 to 1,000,000 old (12-13 billion year) stars orbiting Galaxy in the halo each a few tens of light years in radius, roughly 150 total clusters in the Milky Way

22 Where in the Milky Way Galaxy does the Sun/ Earth reside? A) in a globular cluster of our galaxy B) in the bulge of our galaxy C) in the halo of our galaxy D) in the disk of our galaxy

23 Where in the Milky Way Galaxy does the Sun/ Earth reside? A) in a globular cluster of our galaxy B) in the bulge of our galaxy C) in the halo of our galaxy D) in the disk of our galaxy

24 How do stars orbit in our galaxy?

25 Merry-go-round orbits Stars in the disk all orbit in a circle around the galactic center in the same direction with a little up-and-down motion (orange orbits).

26 Up and down like a merrygo-round Stars in the disk (like our Sun) all orbit in the same circular direction with a little up-and-down motion. Sun orbits once every ~230 million years.

27 Bulge/halo stars orbit like bees around a beehive Orbits of stars in the bulge and halo have random, 3-D orientations (red orbits in the bulge, and aqua orbits in the halo).

28 Mass of the Milky Way To measure the mass of the Milky Way, what quantities do we need? We need period and distance to use Newton s Law for orbits using gravity. Distance is known, but period is not measurable because it is so long (230 million years!). We can measure the Sun s velocity, and turn that into a period.

29 Orbital Velocity Law M r = r v 2 G The orbital speed (v) and distance (r) of an object on a circular orbit around the galaxy tell us the mass (M r ) within that orbit. This is Newton s version of Kepler s 3 rd Law (in an alternate form)!

30 Sun s orbital motion (radius and velocity) tells us mass within Sun s orbit (orange circle): 100 billion M Sun Mass of entire Milky Way is even larger.

31 How is gas recycled in our galaxy?

32 Star gas star cycle Continuously recycles gas from old stars into new star systems.

33 Retina Nebula Let s start at the end of a star s lifetime. Lower-mass stars return gas to interstellar space through stellar winds and planetary nebulae.

34 High-mass stars have strong stellar winds that blow bubbles of hot gas more gas contributed to the galaxy

35 X-rays from hot gas in supernova remnants reveal newly made heavy elements. Supernova shock waves sweep up existing interstellar material like a snow-plow, mixing with the newly-made elements.

36 Multiple supernovae create huge hot bubbles that can blow out of disk. Gas clouds cooling in the halo can rain back down on disk galactic fountain.

37 Cool atomic hydrogen gas (neutral hydrogen atoms) forms as hot gas cools, allowing electrons to join with protons. cooling Cold molecular clouds form next, after gas cools enough to allow atoms to combine into molecules (mostly H 2 two hydrogen atoms bound together).

38 Gravity forms stars out of the gas in cold, very dense molecular clouds. Radiation from newly formed stars erodes away these starforming clouds.

39 Stars live, die, eject gas into environment, completing the star gas star cycle (wash, rinse, repeat)

40 Summary of Galactic Recycling Gas Cools Stars make new elements by fusion. Dying stars expel gas and new elements, producing hot bubbles (~1 million K). Hot gas cools, allowing atomic hydrogen clouds to form (~100 10,000 K). Further cooling permits molecules to form, making molecular clouds (~30 K). Gravity forms new stars (and planets) in molecular clouds. Ongoing process makes generation after generation of stars

41 Every successive generation of stars in a given location of the Galaxy should. A) be more heavy element-rich than the last B) be more heavy element-poor than the last C) following the Big Bang, have about the same level of heavy elements from generation to generation D) contain only hydrogen and helium

42 Every successive generation of stars in a given location of the Galaxy should. A) be more heavy element-rich than the last B) be more heavy element-poor than the last C) following the Big Bang, have about the same level of heavy elements from generation to generation D) contain only hydrogen and helium As more supernova occur, more heavy elements get incorporated into future generations of stars more heavy element-rich (enrichment is a one-way street increasing)

43 We observe the star gas star cycle operating in Milky Way s disk using many different wavelengths of light.

44 Bulge of Milky Way Infrared Visible Infrared light reveals stars whose visible light is blocked by gas clouds.

45 X-rays X-rays are observed from hot gas above and below the Milky Way s disk (from galactic fountains).

46 Radio (21cm) Radio waves emitted by cool neutral atomic hydrogen show where gas has cooled and settled into disk.

47 Radio (CO) Radio waves from carbon monoxide (CO) show locations of molecular (star-forming) clouds.

48 IR (dust) Long-wavelength infrared emission shows where young stars are heating dust grains.

49 Where do stars tend to form in our Galaxy?

50 Orion star-forming cloud Ionization nebulae are found around short-lived high-mass stars, signifying active star formation.

51 Halo (and also bulge): No ionization nebulae, no blue stars no star formation Disk: Ionization nebulae, blue stars star formation

52 Spiral Arms Whirlpool Galaxy Much of star formation in disk happens in spiral arms. Ionization nebulae blue stars gas clouds Little/no star formation in bulge, halo, or globular clusters.

53 Suppose you observe an O main sequence star. Where is it likely to be located? A) in the Milky Way bulge B) in the Milky Way halo C) in the Milky Way disk D) it might be found in any of these locations

54 Suppose you observe an O main sequence star. Where is it likely to be located? A) in the Milky Way bulge B) in the Milky Way halo C) in the Milky Way disk D) it might be found in any of these locations New star formation only occurs in the disk of the galaxy (preferably in a spiral arm), so any current massive star like an O star must have been formed in the disk

55 What do halo stars tell us about our galaxy s history?

56 Halo Stars: 0.02% 0.2% heavy elements (O, Fe, ), only old stars Disk Stars: 1 2% heavy elements, stars of all ages

57 Halo Stars: 0.02% 0.2% heavy elements (O, Fe, ), only old stars Halo stars formed first, then stopped. Disk Stars: 1 2% heavy elements, stars of all ages

58 Halo Stars: 0.02% 0.2% heavy elements (O, Fe, ), only old stars Halo stars formed first, then stopped. Disk Stars: 1 2% heavy elements, stars of all ages Disk stars formed later, and kept forming.

59 How did our galaxy form?

60 Our galaxy probably formed from a giant protogalactic gas cloud, much like in star formation but on a MUCH larger scale

61 Halo stars formed first as gravity caused the cloud to contract (few heavy elements in their initial composition).

62 The remaining gas settled into a spinning disk before it could make stars.

63 Stars continuously form in the disk as the galaxy grows older. Stars continue to form in the disk even today. This describes formation of spiral galaxies like the Milky Way only!

64 Warning: This model is oversimplified. Stars continuously form in the disk as the galaxy grows older. Stars continue to form in the disk even today. This describes formation of spiral galaxies like the Milky Way only!

65 A Nice Tidy Picture? This framework paints a simple picture that the oldest (and therefore most heavy element-poor) stars in the halo should be in the outskirts of the halo. Observation shows this not to be the case. Need to revise this simple picture somewhat.

66 Detailed studies: Halo stars formed in clumps that later merged. This can distribute heavy element-rich stars to the outskirts of the halo.

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