1. Stellar Kinematics Astronomy 315 Professor Lee Carkner Lecture 17

2. Upcoming Extra Credit Events  Thursday, April 27, 7pm, Olin Auditorium  Andrew Knoll  “Are We Alone in the Universe?”  Thursday, May 4, 7pm, Olin Auditorium  Kjell Lundquist  “Stars Above, Stars Below and Stars Within: On Tycho Brahe, Uraniborg and a Garden Reconstruction”  Saturday, May 6, Planetarium  Planetarium Open House  Will receive 10 points extra credit on the observing project for each you go to (up to a maximum of 20 points)  See me there to sign in

3. Moving Stars   We don’t see the constellations change  By careful telescope observations we find that some stars do move a short distance on the sky   There are many other stars that do not show proper motion, but we can observe moving from Doppler shifts   Takes thousands of years to notice motion with your eyes

4. Why Do Stars Move?  Sometimes we see stars that are all in the same place or are moving together   Stellar motions are due to:  Inherited velocity   Gravity   Stars will stay bound in a cluster unless their initial velocities allow them to overcome the gravity of the rest of the cluster

5. T Associations  One cloud (or group of clouds) can form a group of stars   Association will appear together in the sky, but each star has its own velocity inherited from the birth cloud   These velocities may disperse the association after some time (~100 million years)

6. Clusters  Association: A group of stars that were born together but rapidly disperse   Open Cluster: A group of stars that is loosely bound (stars slowly escape)     Globular Cluster: Stars are very strongly bound 

7. The Rosette Nebula and Cluster

8. The Pleiades

9. Star Cluster and Dark Cloud

10. Globular Cluster 47 Tuc

11. Galactic Motions   All objects in the disk orbit the center of the galaxy   We then use this data to get the period (P in years) and semi-major axis (a in AU) and thus the mass (M in solar masses) M = a 3 /P 2

12. Rotation Curves  If we find the rotational speed for stars at different distances from the galactic center we can plot a rotation curve   What would we expect the rotation curve to look like?   If the galaxy is centrally condensed   What do we see?   Even past the point where there are almost no more stars!

13. Milky Way Rotation Curve

14. Dark Matter  A flat rotation curve indicates a large amount of mass outside of the inner galaxy   We can’t see this missing mass at any wavelength   What is the mass?   Dark matter is mass we cannot see directly, but we know it is there because we can see its gravitational effects  What is dark matter?

15. MACHO’s  Massive Compact Halo Objects  Cool compact objects, brown dwarfs or dim red dwarfs in the halo could provide the mass  Properties of MACHO’s   

16. Brown Dwarfs  What are brown dwarfs?   “Stars” that are not massive enough to have hydrogen fusion in their cores   No fusion makes them very faint   Since very low mass stars are common (red dwarfs), maybe very, very low mass brown dwarfs are even more common

17. The Brown Dwarf Gliese 229B

18. Finding MACHO’s  If MACHO’s are so faint, how can we find them?   Einstein’s General Theory of Relativity says that light is affected by gravity   A MACHO should be detectable as it bends light from a distant star behind it, making the star seem brighter

19. Einstein Ring

20. Gravitational Lensing

21. MACHO Lensing Event

22. MACHO Results  A MACHO passing in front of star should be a rare event   Have to observe many stars and observe them multiple times   Lensing results indicate than MACHOs have to be less than ~50% of dark matter

23. WIMPs  Weakly Interacting Massive Particles   Lensing experiments indicate that much of the dark matter must be diffuse, and regular dust and gas would emit at some wavelength  We look for WIMPs with particle accelerator experiments

24. Dark Matter and You  Dark matter accounts for 10-100 times as much matter as we can see   If dark matter is WIMPs, then a huge fraction of the universe is made up of strange subatomic particles  

25. Next Time  Read Chapter 18.1-18.5