Merrimack College Astronomy Fall 2016 Ralph P. Pass

Transcription

1 Merrimack College Astronomy 1101 Fall 2016 Ralph P. Pass 1

2 Field Trip Three Sunday, November 13, 2016 Leave at 9am from Cascia You provide you lunch Return at 4pm. 2

3 Spectra Lines (both types) Blackbody curve Recall Peak of Spectra is determined by the temperature 3

4 Proper Motion Show example Do Stars Move? 4

5 Longest Baseline From Earth 5

6 So, Lets Try To Measure Distance Could randomly select stars and try to measure parallax Could select candidate stars based on proper motion 6

7 Blip along the way 1729 they discovered a shift But it was the wrong shift! ALL the stars in a given direction showed the same shift! Unlikely that all stars would be at the same distance, Aristotle notwithstanding 7

8 Walking in the Rain 8

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10 Proof! This was proof that the Earth orbits the Sun! 10

11 Finding Candidate Stars Astronomers could tell that stars moved in relation to each other on the celestial sphere (Proper Motion) Assume that the faster moving stars are closer Limit the search! 11

12 Frederick Bessel 1838 measured the first distance to a star using parallax, 702,000 AU Need a simpler distance scale Light year = distance light travels in a year 63,240AU 11.1 ly A parallax shift of 2 arc seconds corresponds to a distance of one parsec. It is about 3.26ly 61 Cygni (large proper motion discovered by Piazzi in 1792) nicknamed the flying star Parallax was 100 times smaller than aberration! 12

13 Limits on Parallax From the ground this works to about 100ly From space this works to about 1000ly Beyond that?? 13

14 Summary Stars Parallax, use of light year to measure distance Used proper motion to select candidates Two new distance units: light-year (ly) and parsec A parallax shift of 2 arc seconds corresponds to a distance of one parsec. It is about 3.26ly 14

15 Using Telescopes Stars Measuring Star Movement and looking for Parallax Assign brightness to 'stars now visible' describe the energy being received 15

16 Luminosity and Brightness Luminosity (Energy per time) is one component determining brightness A second component is distance The same luminosity at a further distance appears fainter 16

17 Light and Distance Next measure of a star: how much energy does it produce? Called Luminosity It is the product of Luminosity per area (Stefan- Boltzmann) times area Stefan-Boltzmann says luminosity per area varies as the surface temperature to the 4th power Assumes black body radiation! Electromagnetic energy per unit time (watts per second) 17

18 Inverse Square Many physical processes obey the inverse square law: Doubling the distance cuts the observable parameter by a factor of 4 Changing the distance by a factor of N reduces the observable parameter by a factor of N 2 Examples: Gravity 18

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20 So, 20

21 It would be nice if. All stars were equally luminous We could then determine their distance Not dependent on Parallax! Could work to a much larger distance Unfortunately, not all stars are equally luminous 21

22 However. If we could establish a standard candle (that is, a way to know the luminosity) then we could determine distance from a star s apparent brightness More to come on this 22

23 Turning the Problem Around Suppose you knew that a bunch of stars was at more or less the same distance, then differences in brightness are a direct measure of differences in luminosity 23

24 Magnitudes What we see is APPARENT MAGNITUDE Function of energy production and distance Astronomers define ABSOLUTE MAGNITUDE to be the magnitude of a star if its distance was 10 parsec (about 32.6 ly) Chicken and Egg problem! 24

25 Magnitudes Magnitudes can be negative! Is a star that is magnitude -1.4 brighter or fainter than a star that is magnitude 1? Is a star that is magnitude 0 brighter or fainter than a star that is magnitude 1? 25

26 Discovering the Milky Way Naked eye observations could not distinguish distances and people thought the unmoving stars were on a sphere, meaning all at the same distance And so it was until Thomas Wright, Immanuel Kant, and William Herschel! 26

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30 Herschel Tried to quantify the Milky Way Assumed all stars were sun like, so brightness variations were distance related Concluded it was 2500pc (7500 lyr) in diameter, sun near the center, thickness of 500pc (1500 lyr) Millstone 30

31 Herschel s Millstone 31

32 1900 s, Kapteyn s Universe 32

33 Distribution of Open Clusters 33

34 Distribution of Globular 34

35 Harlow Shapley Open clusters are closer Globular Clusters are further away (variable star data) Concluded Sun was about 2/3ths of the way from the center to the edge 100,000 light years in diameter 35

36 Current Thoughts on Milky Way Diameter about 100,000 lyr Center fluctuates brightness on the order of one minute 36

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38 Center of the Milky Way 38

39 Center of the Milky Way 39

40 Center of the Milky Way 40

41 Center of the Milky Way 41

42 Center of the Milky Way 42

43 Center of the Milky Way 43

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45 SO-2 16 yr period 5 light day size orbit Some Math 45

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47 Some Math Earth is 8 Light Minutes from the Sun (on average) Earth Period is 1 year Remember Kepler s Third Law (as enhanced by Newton) a 3 / p 2 = GM M mass of sun, m mass of the object at the center of the galaxy then 8*8*8/1/1 = 512 = GM 5 light days = 7200 light minutes so 7200 * 7200 * 7200 / 16 / 16 = 1,458,000,000 = Gm Whence, m = 2,850,000M is the mass of the object at the center of the Milky way 47

48 Gravity bends light Einstein Gravity could be strong enough to stop light from leaving (black hole) 48

49 Karl Schwarzschild Showed that given a mass there was a size such that if the mass was confined to that space, then it was a black hole (1915) R = 2GM/c 2 M in kilograms c in meters per second 49

50 Some values G = 6.67 x c = 3 x 10 8 m/sec Sun, M = 2 x kg Schwarzschild radius? Earth, M = 6 x kg Schwarzschild radius? 50

51 Answers Sun = 2950 meters (about a mile and a half) Earth = 8.8 mm (about a third of an inch) 51

52 How about the object at the center of the Milky Way? 3 million times more massive than the Sun If it was a black hole, radius is 3 million times larger than the radius of the Sun sized black hole or 9 million km or 30 light seconds 52

53 So We have placed an upper bound of twice this size on the object at the center of the Milky Way Most likely it is a black hole 53

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55 Outline Stars Stars Magnitude Distance 55

56 Okay, things to think about Why study the sun? Why study other stars? 56

57 Questions about stars How far are they? If we needed to find another home, how much of a journey is it? How big are they? Are they stable? Young? No sense going to a star that would die shortly after we got there Do they have planets? Need some solid ground Are they habitable? 57

58 Kinds of questions to ask Temperature Chemical makeup (type) Mass Size Age Remaining life time 58

59 Temperature and Composition Wien s Law If we can observe the spectrum of a star then we can determine its peak radiation This determines the temperature of its photosphere (where photons, the carriers of electromagnetic radiation are free to escape into space) 59

60 Composition From spectral lines in the spectrum Instrument to measure spectra does one star at a time. This would take a long time to do many stars! 60

61 Spectra Wide Fields Edward Charles Pickering of Harvard Observatory put the prism that causes the spectra in front of the telescope instead of behind the telescope. Coupled with photography this allowed multiple spectra per image rather than one per exposure Studies at Harvard Observatory were carried out by Mrs. W. P. Fleming, Miss A. C. Maury, and Miss A. J. Cannon. They were known as the Harvard Ladies (or Pickering s Harem) They looked at spectra and categorized each by a letter 61

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63 The Harem 63

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65 Spectra Star Temperature After studying 200,000 spectra, the Harvard Ladies found that 99% of the stars fell into six categories. These six categories were characterized by temperatures and elements: Hotter stars showed Hydrogen and Helium, cooler stars showed more elements, with heavier metals appearing as the temperature fell Peak intensity of spectra was a direct measure of temperature 65

66 Sample Spectra 66

67 Spectra Star Categories OBAFGKMLT O white-blue, very hot (20,000C) M red, very cool (3,000C) 67

68 Spectra Star Categories OBAFGKMLT O white-blue, very hot (20,000C) M red, very cool (3,000C) O, be a fine girl/guy, kiss me later tonight The Sun is a G2 star 68

69 Bonus Point Opportunity Due Create your own phrase (worth 1) Those judged to be good are worth an additional 1 69