Universe. If you could see it from afar. Chapter 22. Our Galaxy 8/17/2015. By reading this chapter, you will learn. Tenth Edition

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1 Roger Freedman Robert Geller William Kaufmann III Universe Tenth Edition Chapter 22 Our Galaxy By reading this chapter, you will learn 22 1 How astronomers discovered the solar system s location within the Milky Way Galaxy 22 2 The shape and size of our Galaxy 22 3 How the Milky Way s spiral structure was discovered 22 4 The evidence for the existence of dark matter in our Galaxy 22 5 What causes the Milky Way s spiral arms to form and persist 22 6 How astronomers discovered a supermassive black hole at the galactic center And a couple of other things too If you could see it from afar But how do we know? Techniques follow 1

2 Factinos: A barred spiral galaxy billion stars 100,000 LY across the disk* If the Solar System was a CD, the Earth would be the Milky Way Pop I (young, hot) stars Central bulge 15,000 LY thick Pop II (old, red) stars Halo out to 75 kpc ~ 470,000 LY across Pop II stars Sun located 28,000 LY from nucleus 1 Rotation takes ~ Myr *Your text puts this at closer to 160,000 ly. 18 th Century View William Hershel, while star gauging, saw that stars were not evenly distributed across the sky and imagined the Galaxy to be this disk shape, assuming: all stars have the same brightness the galaxy has a uniform density he can see to the edge 22 1: The Sun is located in the disk of our Galaxy about 8000 parsecs from the galactic center Our View of the Milky Way 2

3 Our Galaxy Seen Face on: Artist s Impressions Side View 3

4 View from above Latest Spitzer pictures show only two major arms, the Scutum Centaurus arm and the Perseus Arm These end on the central bar The other arms are minor, not ending on the central bar Our View of the Milky Way Finding the Center of the Galaxy 4

5 22 2: Observations at nonvisible wavelengths reveal the shape of the Galaxy The Infrared Milky Way Our Galaxy Schematic Edge on View (like before) Milky Way with its Gas Streams Why there is a discrepancy in the disk diameter 5

6 NGC 7331: A Near Twin of the Milky Way NGC 7331: A Near Twin of the Milky Way Note the different wavelength for new stars 22 3: Observations of cold hydrogen clouds and star forming regions reveal that our galaxy has spiral arms The Sky at 21 cm 6

7 Magnetic Interactions in the Neutral Hydrogen Atom Pioneer 11 Greeting Card Actual separation: 21cm A Map of Neutral Hydrogen in Our Galaxy 7

8 A Technique for Mapping Our Galaxy Getting the Size Right: Henrietta Leavitt and Cepheid Variables a straight line can readily be drawn... showing that there is a simple relation between the brightness of the variables and their periods... 5log 10 Period and Luminosity for Cepheid and RR LyraeVaraibles 8

9 RR LyraeVaribles in a Globular Cluster A Spiral Galaxy: using other galaxies to help us learn about our own Stellar Populations: Disk Versus Central Bulge 9

10 22 4: The rotation of our Galaxy reveals the presence of dark matter The Galaxy s Rotation Curve: the orbital motion of stars in the disk. Distinguishing Motion The Rotation of Our Galaxy 10

11 Star Orbits in the Milky Way How Planets Move How Stars Move Not like planets in a solar system Vera Rubin discovered that, rather than falling off with distance, the star s rotation (or velocity) curve flattens out. 11

12 Faster Rotation than Predicted First predicted by the irritable Fritz Zwicky 1933 Vera Rubin circa 1970 finds undue rotation in many galaxies, not just the Milky Way James Peebles and Jeremiah Ostriker showed mathematically that visible matter was insufficient to keep galaxies together, early 1970s At most, 20% is visible matter All else Dark Matter Rotation Measurement: Doppler Shift An Astounding Implication! The only way a velocity curve could have this shape would be if there was a lot more mass distributed throughout, above and below the disk of the Milky Way We can t see it, therefore it s called Dark Matter 4 times the amount of matter we can see Diffuse like a gas, not compact like dwarfs Cold (non radiating, slow moving) 12

13 The Galaxy and its Dark Matter Halo Dark Matter Candidates WIMPS Weakly Interactive Massive Particles Standard Model (next slide) To be found in the lab (maybe) MACHOS Massive Compact Halo Objects (maybe) Observed via gravitational microlensing Insufficient IR seen from the effect Not Black Holes Brown Dwarfs and other dark stars too dim to see These are of ordinary matter and will interact 13

14 DON T memorize this table! Supersymmetry Necessary for String Theory Labels: s and ino Symmetry as with quarks and leptons Supersymmetric w and z bosons Too massive to yet be discovered in colliders Good candidates because they are weakly interactive and slow (1000M proton ) Detecting Dark Matter WIMPS; perhaps supersymmetric bosinos 14

15 Detecting Dark Matter MACHOs Microlensing by Dark Matter in the Galactic Halo Microlensing by Dark Matter in the Galactic Halo 22 5: Spiral arms are caused by density waves that sweep around the Galaxy The Winding Dilemma 15

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17 Spiral arms in galaxies like the Milky Way cannot simply be assemblages of stars. If they were, the spiral arms would wind up and disappear in just a few hundred million years. New stars must be forming: where? Question.: Where are stars forming? Answer: Where you can see them! i.e. in the spiral arms e.g. where the density wave is 17

18 A Density Wave on the Highway Star Formation in the Density Wave Model Star Formation in the Whirlpool Galaxy 18

19 Stars in the Milky Way The movement of a star around the galactic nucleus is a complicated process; there s more than just orbital motion It s tugged gravitationally by various and varying gravitational forces From above and below From within and without 19

20 Now I am become Death, the Destroyer of Worlds The Shiva Hypothesis: Periodically the Solar System s Oort comet cloud is perturbed by proximity to nearby stars New comets streak into the inner system and may impact Earth, causing mass extinctions 20

21 It s just a hypothesis, but Oh crap! Younger Dryas Boundary hypothesis The Younger Dryas hypothesis contends that a cosmic impact occurred 12,900 years that caused the extinction of North American megafauna, the disappearance of a widely distributed Clovis culture, and resulted in the Dr. Ted Bunch, NAU / NASA development of farming in place of a hunter gatherer culture in humans. New evidence supporting Younger Dryas Boundary hypothesis exposed ; Paul Hamaker, Examiner.com 22 6: Infrared, radio, X ray and gamma ray observations are used to probe the galactic center The Galactic Center 21

22 At the Galactic Core The galactic rotation curve is not only a strange shape, it indicates a lot more mass in the nucleus of the galaxy, where stars are light weeks, not light years apart The following picture shows stars that are orbiting a massive yet unseen object The observed speeds indicate the mass of the object > 1 million M sun! Stars Orbiting Sagittarius A * Dr. Andrea Ghez, UCLA 22

23 More Evidence: Galactic Gamma Ray Bubbles Supermassive Black Hole SagA* (Sagittarius A star) Formed contemporaneously with the nuclear bulge Strong radio source Feeds off close by stars and gas first, then becomes less active as fuel diminishes Active galaxies 4X10 6 Solar Masses Less that Earth s R orbit Turns every 11 minutes The Energetic Center of the Galaxy 23

24 Still Hungry Consumed something ~ Mercury s mass 60 years ago (local time) Brightened 100,000X (in the x ray band) from the meal You can see the 3 year illumination trace Chandra X-Ray Telescope picture So many stars The Drake Equation Only a guess of a guess of a guess SETI Is Life Common? Stanley Miller in 1953 made precursor molecules in a simulated early Earth atmosphere That s what Astronomy 104 talks about 24

25 But are they/we broadcasting? The problem is broadcasting Our signals would be only 1/10 30 of a watt 10 pc away Narrowcasting would work better NASA has recently beamed Across the Universe to Polaris, 131 pc, to celebrate the anniversary of its Deep Space Network, with 400kW of power Key Ideas The Shape and Size of the Galaxy: Our Galaxy has a disk about 50 kpc (160,000 ly) in diameter and about 600 pc (2000 ly) thick, with a high concentration of interstellar dust and gas in the disk. The galactic center is surrounded by a large distribution of stars called the central bulge. This bulge is not perfectly symmetrical, but may have a bar or peanut shape. The disk of the Galaxy is surrounded by a spherical distribution of globular clusters and old stars, called the galactic halo. There are about 200 billion ( ) stars in the Galaxy s disk, central bulge, and halo. Key Ideas The Sun s Location in the Galaxy: Our Sun lies within the galactic disk, some 8000 pc (26,000 ly) from the center of the Galaxy. Interstellar dust obscures our view at visible wavelengths along lines of sight that lie in the plane of the galactic disk. As a result, the Sun s location in the Galaxy was unknown for many years. This dilemma was resolved by observing parts of the Galaxy outside the disk. The Sun orbits around the center of the Galaxy at a speed of about 790,000 km/h. It takes about 220 million years to complete one orbit. 25

26 Key Ideas The Rotation of the Galaxy and Dark Matter: From studies of the rotation of the Galaxy, astronomers estimate that the total mass of the Galaxy is about M. Only about 10% of this mass is in the form of visible stars, gas, and dust. The remaining 90% is in some nonvisible form, called dark matter, that extends beyond the edge of the luminous material in the Galaxy. Our Galaxy s dark matter may be a combination of MACHOs (dim, star sized objects), and is hypothesized to consist mostly of WIMPs (relatively massive subatomic particles). Key Ideas The Galaxy s Spiral Structure: OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms. Spiral arms can be traced from the positions of clouds of atomic hydrogen. These can be detected throughout the galactic disk by the 21 cm radio waves emitted by the spinflip transition in hydrogen. These emissions easily penetrate the intervening interstellar dust. Key Ideas Theories of Spiral Structure: There are two leading theories of spiral structure in galaxies. According to the density wave theory, spiral arms are created by density waves that sweep around the Galaxy. The gravitational field of this spiral pattern compresses the interstellar clouds through which it passes, thereby triggering the formation of the OB associations and H II regions that illuminate the spiral arms. According to the theory of self propagating star formation, spiral arms are caused by the birth of stars over an extended region in a galaxy. Differential rotation of the galaxy stretches the star forming region into an elongated arch of stars and nebulae. 26

27 Key Ideas The Galactic Nucleus:The innermost part of the Galaxy, or galactic nucleus, has been studied through its radio, infrared, and X ray emissions (which are able to pass through interstellar dust). A strong radio source called Sagittarius A* is located at the galactic center. This marks the position of a supermassive black hole with a mass of about M. 27