Post on 22-Dec-2015
Stellar Kinematics
Astronomy 315Professor Lee
CarknerLecture 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 = a3/P2
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 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/4d2
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
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 MSun (84 MJupiter)
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 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
Next Time
Read Chapter 18.1-18.5