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Stellar Kinematics

Learn about the movement of stars and galaxies in this lecture by Professor Lee Carkner. Topics include proper motion, stellar clusters, galactic motions, rotation curves, dark matter, brown dwarfs, and WIMPs. Extra credit opportunity at the planetarium open house on April 28th.

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Stellar Kinematics

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  1. Stellar Kinematics Astronomy 315 Professor Lee Carkner Lecture 18

  2. Extra Credit • Planetarium open house • Saturday April 28, 8:30-10 pm • Sign in at event • (Disregard previous extra credit slide)

  3. Moving Stars • We don’t see the constellations change • Called proper motion • 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? • In a cluster • 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) • Hard to distinguish from an association • Globular Cluster: Stars are very strongly bound • Seen in the halo

  7. 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 = a3/P2

  8. 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!

  9. Milky Way Rotation Curve

  10. Mass to Light Ratio • Mass (M in Msun) • From Kepler’s Third Law: M = a3/P2 • Convert to solar masses Msun = 2 X 1030 kg • Light (L in Lsun) • From the inverse square law: F = L/4pd2 • Convert to solar luminosities Lsun = 3.8X1026 W • We then define the Mass-to-Light ratio as M/L • B • Compares the total mass of the galaxy to the visible stars

  11. Dark Matter • Stars are moving fairly rapidly even very far from the galactic center where we don’t see much material • Adding up the mass of all the stars leaves us short • 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?

  12. MACHO’s • Massive Compact Halo Objects • Properties of MACHO’s • “Normal” matter

  13. Brown Dwarfs • What are brown dwarfs? • “Stars” that are not massive enough to have hydrogen fusion in their cores • Mass < 0.08 MSun (84 MJupiter) • Since very low mass stars are common (red dwarfs), maybe very, very low mass brown dwarfs are even more common

  14. The Brown Dwarf Gliese 229B

  15. Finding MACHO’s • Gravitational lensing • 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

  16. Gravitational Lensing

  17. MACHO Lensing Event

  18. MACHO Results • The event will also be quite short (duration ~ weeks) • Need automated telescopes and software • Lensing results indicate than MACHOs have to be less than ~25% of dark matter

  19. WIMPs • Sub-atomic particles that are hard to detect since they don’t interact with anything (except via gravity) • How do we find WIMPs

  20. WIMP Interactions • Normal matter interacts via the electron clouds • WIMPs don’t interact with the electron clouds • Can detect the vibration of the system from the WIMP hit

  21. WIMP Detections • Problems: • Or the thermal vibrations will overwhelm the WIMP induced vibrations • So no other things (like cosmic rays or alpha particles) hit the detector

  22. WIMP’s in Space • But, • They might produce other particles that can be • Can look for excess emission in microwave observations

  23. WMAP Haze

  24. Dark Matter Checklist • Galaxies are rotating as if they contain much more mass than we can see • Due to? • Faint stars – • Dust or gas – • Compact objects and planets – • Strange particles – should show up in very sensitive detectors

  25. 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 • It is possible that the universe is dominated by WIMPs and “normal” matter is rare

  26. Next Time • Read Chapter 18.1-18.5

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