The Milky Way Galaxy

1. The Milky Way Galaxy Astronomy 315 Professor Lee Carkner Lecture 17

2. Upcoming Extra Credit • Sigma Xi Science Café • 6:30 pm, Thursday, April 21 • New Hall 013, St. Ambrose University • More info at: sigmaxijd.org • Planetarium Open House • 8:30-10:00, Saturday, May 7 • More info at: http://helios.augustana.edu/astronomy/ • Sign in at event for extra credit

3. Via Lactia • The band forms a complete ring • We seem to be in the center of a disk of stars • Clouds of gas and dust block our view so it is hard to see beyond our local region

4. Our View of the Milky Way

5. The Milky Way from Outside

6. Discovering The Galaxy • In the early part of the century Harlow Shapley found the distance to globular clusters using Cepheid variables • Globular clusters • Unlike stars in the disk, we can see distant globulars • Cepheid variables • If we can find luminosity and flux we can get distance (F = L/4pd2)

7. The Center • Shapley found the distance to the globulars and plotted their positions • He found: • We are not at the center of the galaxy and the disk extends out much further than we can easily see

8. Changing Views of the Galaxy

9. How Do We Learn About The Milky Way? • Optical observations • Radio observations • Infrared observations • Observing other galaxies • Since we are in the middle of the Milky Way we can’t get an overview of it

10. Structure of the Milky Way • Disk • Nucleus • Halo • Spherical distribution of old stars and globular clusters around disk and bulge

11. Edge-on and Face-on

12. The Disk • The disk is very thin • Younger stars and star forming regions near the center, older stars above and below • Disk exhibits differential rotation (inner parts rotating faster than outer)

13. Differential Galactic Rotation

14. Mass and Orbits • Finding the properties of a star’s orbit allows us to find the mass internal to the orbit M = a3/P2 • M = mass (in solar masses) • P = • a = • You must use the correct units!

15. Spiral Structure • We know that other galaxies have spiral structure, but it is harder to see the Milky Way’s • We find spiral arms by tracing: • They are not uniformly distributed but are found in a loose spiral structure • How do spiral arms form?

16. Local Spiral Arms

17. Density Waves • Spiral arms are like traffic jams • This can trigger star formation in the arms • The clouds eventually move out the other side • The spiral arm material changes, only the pattern stays the same

18. Density Wave

19. At the Core • The nucleus is the hardest part of the galaxy to observe due to all the gas and dust • One, Sagittarius A* (Sgr A*), may be the center of the galactic core • What is it?

20. Properties of the Core • Stars near the core are very close together and moving very fast • Sgr A* is emitting enormous amounts of energy • A black hole

21. Massive Black Holes • Sgr A* does not move and may have jets and an accretion disk • Can get mass from orbit of star S2: • Period = • Semimajor axis = • Formed from stars and clouds of material falling into the center • Our own is very hard to observe

22. The Halo • The halo are stars orbiting in a large sphere around the galaxy • The halo is composed of old stars • Halo stars -- Population II -- metal poor • Disk stars -- Population I -- metal rich • Halo stars formed formed early from relatively unprocessed material

23. Globular Clusters • Size: • Shape: • Contents: • Globulars are in elliptical orbits around the galactic center • Unlike open clusters in the disk, globular clusters are very tightly gravitationally bound

24. History of the Milky Way • How did the galaxy form? • Basic theory has Milky way forming from smaller protogalaxies • Gas stripped out to form disk • Remains of protogalaxies form halo

25. Next Time • Quiz #2 • Covers lectures 10-16