Wednesday March 3rd 1999
Announcements:

Lecture notes:

Class 23

"Dark Matter" - many/most astronomers think that greater than 90% of the Universe exists in a "dark" form that we cannot see at any wavelength.  Today I will explain why we think this.

First of all what if the sun emitted no light.  Would we know it is still there?  yes how?  gravitational influence.

Astronomers use this same trick over and over - detect the evidence of something we can't see by its gravitational influence
on things we can see.
- giant black hole in center of our Galaxy
- black holes in binary star systems.

Dark Matter in the Milky Way.

First lets do a rough calculation to estimate the amount of mass interior to orbit of sun.  Remember that sun orbits in disk of
Galaxy.  In first section of class we learned about Kepler's Laws in particular the 3rd law.
Ma + Mb = a³/p²
Ma Mb in solar masses
a in AU
p in years.

This also works roughly to figure out amount of mass interior to Suns orbit around center of Galaxy.

Mass interior to orbit of sun  +  Mass of sun = a³/p²
a = 8.5 kpc = 1.7 x10⁹ AU
p = 240 million years
Mass interior to orbit of sun = (1.7x10⁹)³/(240 x10⁶)² = 9x10¹⁰ M_(.)
= 100 billion M_(.)
This gives us a rough idea of the kinds of masses we're dealing with when we talk about our Galaxy.  However leaves out mass outside the orbit of our Sun.  Astronomers can do similar calculations to the one above for stars and gas more distant from center of Galaxy than our sun.

Astronomers think that total mass of our Galaxy is more like 1.5x10¹² M_(.) about 1500 billion M_(.)
They think there is a lot of dark matter outside the optically visible edge of our own Galaxy.  Greater than 90% of Galaxy's
mass.

- Dark Halo - extends about 7 times further out than optically visible stuff.  What is the dark matter?
-We don't know but do have some ideas about what it can/cannot be.
-cannot all be normal stars - we would see them - red dwarfs ruled out by HST.

"Jupiters" - brown dwarfs < .08 M_(.)  (microlensing events show some dark massive compact halo objects but probably not enough to explain the whole thing)  Low mass sub-stellar objects that never started fusion.

Black dwarfs or black holes - perhaps plausible but evolutionary problems - too many massive stars needed.

exotic subatomic particles - we are searching for them.

A mixture of above.

Dark matter in other galaxies.

We measure the masses of other galaxies in essentially the same way that we measure mass of Milky Way - by watching stars and gas move.

Need to introduce a new concept to explain this further - the Doppler shift.

Train or fire truck analogy - hear frequency higher when approaching and lower when going away.

approaching - higher frequency - shorter wavelength ( for light -bluer color)
receding - lower frequency - longer wavelength ( for light - red color)

penny dropper machine analogy - due to compression or expansion of wavelength.
- also works for light.  approaching objects are blue shifted receding objects are redshifted.

Can use to learn about galaxy rotation.  The part coming towards us is blueshifted and the part moving away is redshifted.  Use a spectral line to measure blueshift or redshift.

Can study rotation speed at different distances from the center then like before use Newton's Law of gravity to get mass.

Dark Matter in clusters of galaxies.
-velocities of galaxies is so high cluster would fly apart without more unseen mass.  Coma cluster is almost 90% dark
matter.