1. ASTR 113 – 003 Spring 2006 Lecture 09 March 29, 2006 Review (Ch4-5): the Foundation Galaxy (Ch 25-27) Cosmology (Ch28-29) Introduction To Modern Astronomy II Star (Ch18-24) 1.Sun, Our star (Ch18) 2.Nature of Stars (Ch19) 3.Birth of Stars (Ch20) 4.After Main Sequence (Ch21) 5.Death of Stars (Ch22) 6.Neutron Stars (Ch23) 7.Black Holes (Ch24) Extraterrestrial Life (Ch30) 1.Our Galaxy (Ch25) 2.Galaxies (Ch26) 3.Active Galaxies (Ch27) 1.Evolution of Universe (Ch28) 2.Early Universe (Ch29)

2. Our Galaxy Chapter Twenty-Five ASTR 113 – 003 Spring 2006 Lecture 09 March 29, 2006

3. Guiding Questions 1.What is our Galaxy? How do astronomers know where we are located within it? 2.What is the shape and size of our Galaxy? 3.How do we know that our Galaxy has spiral arms? 4.What is most of the Galaxy made of? Is it stars, gas, dust, or something else? 5.What is the nature of the spiral arms? 6.What lies at the very center of our Galaxy?

4. Short Movies Galaxy: The Milky Way Galaxies Note: the movies can only run in-class Galaxy: The Milky Way Galaxies

5. Milky way, the band of light, is caused by the shining of numerous stars in our own disk-shaped galaxy Milky way galaxy has about 200 billion stars The dark streaks across the milky way is due to the presence of Interstellar dust within the disk, which causes interstellar extinction Interstellar extinction makes distant objects dim or dissapper In visible light, we could only see the nearby star Milky Way

6. The Sun’s location in the Galaxy? Counting the stars in all directions, with the largest number in the direction of the center It was thought that the Sun was in the center of the galaxy, according to Herschel’s map that showed roughly equal number of stars in all direction However, this bias is caused by interstellar extinction Herschel’s Map (1785)

7. The Sun’s location in the Galaxy? Counting the number of globular clusters A globular cluster is a spherical distribution of roughly 1 million stars packed in a small size; it is bright Many globular clusters exist in the halo outside the obscuring disk

8. The distance of globular cluster is determined using the pulsating variable stars, Cepheid variables and RR Lyrare variables. There is a well known period-luminosity relation for variables The Sun’s location in the Galaxy?

9. Globular clusters forms a huge spherical distribution centered not on the Earth but rather about a point in the direction of Sagittarius Our Sun lies within the galactic disk, some 8000 pc (26,000 ly) from the center of the Galaxy The Sun’s location in the Galaxy?

10. The shape of the Galaxy Observations at non-visible wavelengths reveal the shape of the galaxy The longer the wavelength, the less the amount of interstellar extinction, because the dust scattering is less efficient in longer wavelength Infrared wavelength can see farther into the plan of the Milky way Radio wave can see through the entire disk of the Milky way

11. The shape of the Galaxy Far-infrared light (~ 50 μm) is directly from the black-body emission of the interstellar dust (~ 50 Kelvin) Near-infrared light (~ 2 μm) is from emission of cool stars, including red giants

12. There are three geometric components of our Galaxy 1.A Disk 2.A central bulge 3.A Halo The shape of the Galaxy

13. 1.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 Contains metal-rich (Population 1) stars The Sun, at 8 kpc, orbits around the center of the Galaxy at a speed of about 220 km/s It takes about 220 million years to complete one orbit The shape of the Galaxy

14. 2.Central Bulge –The galactic center is surrounded by a large distribution of stars called the central bulge –About 2 kpc (6500 ly) in diameter –Contain a mixture of Population 1 and Population II stars –This bulge is not perfectly symmetrical, but may have a bar or peanut shape The shape of the Galaxy

15. 3.Halo –The disk of the Galaxy is surrounded by the galactic halo –Halo is composed almost exclusively of old, metal poor (Population II) stars –These old stars are in globular clusters or isolated The shape of the Galaxy

16. The shape of the galaxy NGC 4565 If we could view our Galaxy edge-on from a great distance, it would look like this galaxy (NGC 4565, 15 million pc (50 million ly away from us).

17. ASTR 113 – 003 Spring 2006 Lecture 10 April 5, 2006 Review (Ch4-5): the Foundation Galaxy (Ch 25-27) Cosmology (Ch28-29) Introduction To Modern Astronomy II Star (Ch18-24) 1.Sun, Our star (Ch18) 2.Nature of Stars (Ch19) 3.Birth of Stars (Ch20) 4.After Main Sequence (Ch21) 5.Death of Stars (Ch22) 6.Neutron Stars (Ch23) 7.Black Holes (Ch24) Extraterrestrial Life (Ch30) 1.Our Galaxy (Ch25) 2.Galaxies (Ch26) 3.Active Galaxies (Ch27) 1.Evolution of Universe (Ch28) 2.Early Universe (Ch29)

18. There are three geometric components of our Galaxy 1.A Disk: ~ 50 kpc 2.A central bulge: ~ 2 kpc 3A Halo The shape of the Galaxy

19. The disk is not uniform The disk has several spiral arms The Sun is located just outside the Orion arm A spiral arm is a concentration of stars, dust and gas The Structure of the Disk

20. Observation at radio wavelength at 21 cm The 21-cm radio emission is from neutral hydrogen When the spin of the binding electron flips, it emits a tiny amount of energy, the radio photon at wavelength 21-cm Radio observations can see through the whole galaxy, because interstellar dust does not scatter radio waves

21. The Map of Galaxy at 21-cm

22. 21-cm observation maps the 2-D disk Along one line of sight, hydrogen gas at different distances has different velocity with respect to the observer Doppler shift reveals the velocity, thus the distance

23. Spiral structure of the disk The 21-cm observations map the distribution of neutral hydrogen in the disk, revealing the spiral structure

24. What are in the Spiral Arm? Observations indicate spiral arms are concentration of 1.Young, hot and luminous O and B stars (in blue color) 2.HII regions, or emission nebulae (in red color) surrounding O,B stars 3.Neutral hydrogen gas (HI) that gives rise to star formation Spiral arms are star-forming region

25. The Cause of Spiral arms Spiral arms are not caused by winding rotation of assemblages of stars If they were, spiral arms would “wind up” too much and disappear in a few hundred million years

26. Spiral arms are caused by density waves spiral arms are created by density waves that sweep around the Galaxy A density wave is a slow-moving region where stars, gas, and dust are more densely packed than in the rest of the galaxy A density wave is like a cosmic traffic jam

27. Spiral arms are caused by density waves Interstellar matter or gas moves around the galactic center rapidly It slows down and is compressed as it goes through the slow-moving spiral arms The high density of gas in the spiral arms triggers star formation

28. Dark Matter and Rotation Curve Kepler’s third law: period square = radius cube Modified Kepler’s Law (in Newton’s form): the total mass of matter can be inferred from the period and distance of an orbiter Or it can be inferred from the distance and velocity of an orbiter E.g., The sun is 8 kpc from the center of the Galaxy with a rotation speed of 220 km/s, the inferred mass of Galaxy within the Sun’s orbit is about 10 12 M  By studying the rotation speed with distance, mass distribution in the galaxy can be inferred

29. Dark Matter and Rotation Curve The orbital speed of stars and gas is almost uniform throughout the Galaxy

30. Dark Matter and Rotation Curve The orbital motion is not a solid body; solid body rotation implies faster speed in larger distance

31. Dark Matter and Rotation Curve Rotation curve is the distribution of the orbital speed of stars and gas with respect to the distance from galactic center

32. Dark Matter and Rotation Curve The orbital motion is not the Kepler orbits like in the solar system; Kepler orbit means slower velocity at larger distance

33. Dark Matter and Rotation Curve If the mass of Galaxy is from the matter that emits light, the orbital speed would decline at larger distance But observations show a flat rotation curve at large distance It implies that most of the mass in the Galaxy is in the form of dark matter, emitting no light at all Stars, gas and dust account for only about 10% of the Galaxy’s total mass

34. The innermost part of the Galaxy, or galactic nucleus, is located in Sagittarius, and 8 kpc from the Sun The center is close to Sagittarius A, a strong radio source The Center of the Galaxy

35. Sagittarius A is one of the strongest radio source in the sky On either side of Sagittarius A, show lobes of hot gas as seen in X-ray

36. The motion of star S2 is 5000 km/s, or 2% of speed of light This speed infers a central mass of 3.7 × 10 6 M , indicating the presence of a supermassive black hole at the center The Center of the Galaxy

37. Key Words central bulge (of a galaxy) dark matter density wave disk (of a galaxy) far-infrared flocculent spiral galaxy galactic nucleus galaxy globular cluster grand-design spiral galaxy H I halo (of a galaxy) high-velocity star interstellar extinction Local Bubble magnetic resonance imaging (MRI) massive compact halo object (MACHO) microlensing Milky Way Galaxy near-infrared rotation curve RR Lyrae variable Sagittarius A* self-propagating star formation spin (of a particle) spin-flip transition spiral arm 21-cm radio emission weakly interacting massive particle (WIMP) winding dilemma