Chapter 14 The Milky Way Galaxy

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1 Chapter 14 The Milky Way Galaxy

2 Spiral Galaxy M81 - similar to our Milky Way Galaxy

3 Our Parent Galaxy A galaxy is a giant collection of stellar and interstellar matter held together by gravity Billions are known Our galaxy is the Milky Way Galaxy or the Galaxy (capital G) The disk contains most of Galaxy s luminous stars and interstellar matter (and sun)

4 The Milky Way Line of sight perpendicular to disk - see few stars Line of sight within disk - see many stars merging into continuous blur Band of light is the Milky Way

5 Figure 14.1 Galactic Plane

6 Spiral Galaxies Our Galaxy is a spiral galaxy consisting of: Galactic disk with spiral arms Galactic bulge at center Galactic halo (roughly spherical)

7 Figure 14.2a - Andromeda galaxy similar to our own kpc away

8 Figure 14.2b,c - Spiral Galaxies a) M101, b) NGC 4565

9 Measuring the Milky Way Herschel in late 1700 s counted stars Found Galaxy is disk shaped Judged size wrong - didn t know about obscuring gas and dust

10 Figure 14.3 Herschel s Galaxy Model

11 Variable Stars Systematically studied near start of 20th century Not eclipsing binaries or novae Pulsating variables of two types: RR Lyrae variables Cepheid variables Recognizable by shape of light curve Are post-main sequence stars temporarily unstable

12 Figure Variable Stars a) RR Lyrae, b) & c) Cepheid

13 RR Lyrae variables Periods from 0.5 to 1 day Lower mass horizontal branch (H-R diagram)

14 Cepheid variables Periods from 1 to 100 days High mass stars

15 Figure 14.5 Variable Stars on the H-R Diagram

16 Period-Luminosity relationship All RR Lyrae stars roughly 100X luminosity of sun (averaged over a cycle) Close correlation between luminosity and pulsation period for Cepheid variables Discovered by Henrietta Leavitt in 1908

17 Discovery 14-1 Early Computers

18 Figure 14.6 Period-Luminosity Plot

19 Variable as yardstick Apparent brightness proportional to luminosity/distance 2 Stellar or spectroscopic parallax distance of nearby variables luminosity For distant variables period luminosity distance

20 Distance measurements Radar to about 1 AU Stellar parallax to about 200 pc Spectroscopic parallax to about 10 kpc Variable stars to about 25 Mpc

21 Figure 14.7 Variable Stars on Distance Ladder

22 Globular cluster distribution Early 1900 s Harlow Shapley used RR Lyrae s to map globular clusters - he found: Globulars many kpc s from sun Globulars spherically distributed Center is 8 kpc from sun Globular clusters in Galactic Halo

23 Figure 14.8 Globular Cluster Distribution

24 Evolving ideas of our place in the universe Earth at center Sun at center of solar system Sun at center of Galaxy Sun not at center of Galaxy Galaxy bigger than previously thought

25 Figure 14.9 Stellar Populations in Our Galaxy

26 Mapping our Galaxy Optical observations useful for halo For disk, visible wavelengths obscured by dust and gas Measure disk with 21 cm radio wavelength Galactic diameter 30 kpc Disk thickness 300 pc Centers of disk and halo roughly coincide

27 Galactic bulge Use infrared wavelengths to image 6 kpc in plane of disk by 4 kpc perpendicular to plane of disk Football shaped - elongated along disk High gas density at center Vigorous star formation

28 Figure Infrared View of the Milky Way Galaxy

29 Stellar Populations Halo and bulge redder, disk bluer Disk has all the bright blue stars, open star clusters and star-forming regions Cooler redder stars distributed in disk, bulge and halo

30 Halo No gas or dust Stars are old - 10 billion years No star formation Stars less abundant in heavy elements Referred to as Population II stars

31 Disk Active star formation O and B supergiants give it bluish color Younger stars contain heavier elements Young disk stars called Population I

32 Orbital motion Galactic disk rotates differentially At radius of 8 kpc (sun) orbital speed is 220 km/s and period is 225 million years Globular clusters, stars in halo and bulge orbit in random orientation about center

33 Figure Orbital Motion in the Galactic Disk

34 Figure Stellar Orbits in Our Galaxy

35 Milky Way Formation Formed from several smaller systems Irregular shape initially Stars formed throughout Gas and dust fell to galactic plane, forming spinning disk Halo stars left behind Star formation ceased in halo New stars form in the disk

36 Figure Milky Way Galaxy Formation

37 Table 14.1 Overall Properties of the Galactic Disk, Halo, and Bulge

38 Galactic Spiral Arms Radio studies show spiral arms in Galaxy Typical of other spiral galaxies Young O and B stars and recently formed open clusters found in spiral arms Arms are regions of star formation

39 Figure Gas in the Galactic Disk

40 Figure Milky Way Spiral Structure

41 Problem Differential rotation can t fully explain spiral arms Spiral arms would wrap up over many rotations

42 Figure Differential Galactic Rotation

43 Spiral arm explanation Spiral density waves moving through disk Region of compression rotates more slowly than stars and gas Stars formed in spiral arms Material through arm consists of High density dust and gas Dust lane Emission nebulae and young O, B stars Older stars

44 Figure Density-Wave Theory

45 Density wave Think of work crew slowly moving along freeway Traffic jam (increased density of cars) forms around work crew Cars enter at back and leave at front Jam (spiral arm) moves slower than cars (gas, dust and stars)

46 Discovery 14-2 Density Waves

47 Alternative explanation Formation of stars drives waves (rather than other way around) Self-propagating star formation Can t fully explain galaxy-wide spiral arms

48 Figure Self-Propagating Star Formation

49 Mass of Milky Way Galaxy Using Kepler s third law and sun s orbit, get 1 X M Assumes all mass at center (not true) Ignores mass outside of sun s orbit

50 Figure 14.19a Weighing the Galaxy

51 Rotation curve Plot rotation speed vs distance from center Within 15 kpc radius (visible edge of galaxy) 2 X M Not Keplerian beyond 15 kpc Invisible matter beyond 15 kpc

52 Figure 14.19b Weighing the Galaxy

53 Dark matter 6 X M lies within 50 kpc 2 X M lies within 15 kpc (visible edge of galaxy) Dark halo out to 50 kpc So 2X as much invisible dark matter as visible Not visible at any wavelength - only detected gravitationally

54 What is dark matter? Possibilities: Brown dwarfs and white dwarfs Exotic subatomic particles WIMPs - Weakly Interacting Massive Particles MACHOs - Massive Compact Halo Objects

55 Search for dark matter Gravitational lensing of light around a faint massive object such as brown dwarf or white dwarf Rare - use automated telescopes and computer processing to find

56 Figure Gravitational Lensing

57 Galactic center Bulge and especially nucleus should be densely populated with stars Interstellar medium blocks the view

58 Figure Galactic Center

59 Infrared and radio observations Can penetrate though interstellar matter At center, 50,000 stars per cubic pc Million times more than at sun s location Clouds rich in dust Ring of molecular gas Bright radio source Sagittarius A Sgr A* at center (supermassive black hole)

60 Figure Galactic Center Close-Up

61 Sgr A* VLBI measurements show less than 10 A.U. across W output - more than a million times the sun 3 million M Event horizon less than 0.05 A.U.

62 Figure Orbits Near the Galactic Center

63 Figure 14.24a-c Galactic Center Zoom

64 Figure 14.24d-f Galactic Center Zoom

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