Stellar Kinematics Astronomy 315 Professor Lee Carkner Lecture 18.
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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!
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 10 30 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 BB 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?
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
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
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 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