Wednesday February 23rd 1999
Announcements:

Lecture notes:

Class 20

Now that we've gone through all the needed terminology and background we can start talking about how people first worked
out the size and shape of our Galaxy.

One of the first methods people used was to count stars in various directions.  William Herschel in 1785 - Milky Way map.  proposed disk of stars.  4x longer than thick - we are at center.

Jacobus Kapetyn - spent 40 years counting stars on photographic plates.  1920-22 said that disklike shape with sun near center.

But unfortunately both Herschel and Kapetyn models for the Galaxy were drastically wrong.  There was a problem they did not properly understand.  Space contains obscuring dust that makes it so that we cannot see stars throughout our Galaxy.  Dust makes a star's light fainter.  Also blue light is absorbed more than red light so dust makes a star look redder.

Since Herschel and Kapetyn could not properly see all the stars they could not properly map the Milky Way.

Rough Analogy - standing near 1 end of Beaver stadium in thick ground fog at night.  100 Watt light bulbs scattered throughout the field on the ground.  If you counted light bulbs you might wrongly conclude that you live in the center of a distribution of light bulbs.

Another astronomer named Harlow Shapley thought there was something wrong with the Herschel/Kapetyn model for the Galaxy. He noticed that globular clusters were not uniformly spread about  the sky but instead generally clustered in direction of the constellation Sagittarius.  Suggested we are not in center but are offset.

Shapley claimed that the globular clusters outlined the Galaxy.  So if we could find distances of globular clusters we could find
size of the Galaxy.  Shapley worked with Henrietta Leavitt - used Cepheid variables and RR Lyrae stars spectroscopic parallax to find the distances to globular clusters.  Shapley argued that the Galaxy was much bigger than claimed by Herschel/Kapetyn.

Subsequent studies showed that was right.  Shapley's method worked because glob clusters are luminous so you can see them far away and that glob clusters are above the plane of the disk of our  Galaxy so not as much dust.

Very Important discovery!  Similar to what Copernicus did.  Earth not at center of Universe for Copernicus.  Sun not at center of Galaxy for Shapley.

Modern measurements have improved even more our knowledge of the size of our Galaxy.  We are about 8.5 kpc from the center and go around the center in a circular orbit at about 220 km/s.  Takes 240 million years for one galactic orbit.

Typical radius of optical stars in our galaxy disk is 13 kpc Disk has spiral structure  Can see in radio maps of H gas  and obtical mabs of O and B stars.

Roughly how old is our Galaxy?  Must be as old as the oldest stars - about 15 billion years.  Spiral arms are not due to
rotation - too many twists.  Stars in disk are metal rich (2-3%) since enriched by all the supernovae that have occurred.

Differential rotation - inner is faster.  Our Galaxy also has other components

Nuclear bulge - dense cloud of stars surrounding the center of our galaxy.   2 kpc radius - relatively little gas dust star
formation.  most of stars are old cool low metals.

Halo - spherical cloud of thinly scattered stars and globular clusters.  Only about 2% as many stars as the disk.  Very little
gas and dust.  Old cool low metal stars (<.8% metals) also dark matter