Stellar Kinematics Astronomy 315 Professor Lee Carkner Lecture 18

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

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

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

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)

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

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

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!

Milky Way Rotation Curve

Mass to Light Ratio  Mass (M in M sun )  From Kepler’s Third Law: M = a 3 /P 2   Convert to solar masses M sun = 2 X kg  Light (L in L sun )  From the inverse square law: F = L/4  d 2   Convert to solar luminosities L sun = 3.8X10 26 W  We then define the Mass-to-Light ratio as M/L  BB  Compares the total mass of the galaxy to the visible stars

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?

MACHO’s  Massive Compact Halo Objects   Properties of MACHO’s    “Normal” matter

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

The Brown Dwarf Gliese 229B

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

Gravitational Lensing

MACHO Lensing Event

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

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

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

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

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

WMAP Haze

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

Dark Matter and You  Dark matter accounts for 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 

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