Stellar Kinematics Astronomy 315 Professor Lee Carkner Lecture 17.
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Stellar Kinematics Astronomy 315 Professor Lee Carkner Lecture 17
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
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
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
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) Globular Cluster: Stars are very strongly bound
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!
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?
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
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
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
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
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
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