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