Transcript Stellar Kinematics

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
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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)
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Hard to distinguish from an association
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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/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
Dark Matter

Stars are moving fairly rapidly even very far from the
galactic center where we don’t see much material
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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
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Sub-atomic particles that are hard to
detect since they don’t interact with
anything (except via gravity)
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How do we find WIMPs
WIMP Interactions
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Normal matter interacts via the
electron clouds
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WIMPs don’t interact with the electron
clouds
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Can detect the vibration of the system
from the WIMP hit
WIMP Detections
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Problems:
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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