Spiral density waves

 The spiral arms are thought to be density waves - that is,
regions of higher-than-average density in the disk that move
around the galaxy at speeds much slower than those of the stars and
gas clouds.  Therefore, the arms are made of constantly changing
material, as stars and gas catch up to an arm, enter it, slow down
somewhat, move through it, and emerge on the other side.  This
process is exactly analogous to cars moving through a traffic jam.
The traffic jam doesn't move along with the cars; it stays near
the site of whatever caused it (an accident, for example).
 There are actually lots of stars and gas in between the
spiral arms of a galaxy - they just don't give off as much light.
The reason for this is that the arms are where star formation
occurs.  As a gas cloud enters a spiral arm, it encounters a
region of higher density, which compresses the cloud.  This
compression may trigger gravitational collapse, leading to the
formation of stars.  Some of these stars are massive, blue, and
very luminous, which is what causes the characteristic
appearance of spiral arms.  Since it takes millions of years to move
through an arm, these stars will end their lives as supernovae
before even leaving the arm in which they formed, making the
arms brighten further and possibly more stars to form.  Eventually,
the remaining gas and the lower-mass, dimmer stars leave the
arm and continue their journey around the galaxy.

The Galactic Nucleus

When you look at the center of the Galaxy with an optical telescope you can't see anything because of all the dust in the way. In order to see through the dust you have to look at infrared and radio wavelengths because these waves are long enough that they don't get scattered by the dust.  Radio images of the center of the Galaxy show a rotating disk of molecular gas about 5 parsecs across.  This is an accretion disk, where gas and dust being pulled towards the center piles up.  Just inside this disk is a spiral of hot ionized gas which is falling in towards the middle and converging at the dynamical
center of the Galaxy. Near where the spiral converges there is an extraordinarily bright spot of radio emission called Sgr A*. Astronomers think this is a supermassive black hole.  By measuring the velocities of stars and gas orbiting Sgr A* we can measure its mass and we find that it has to be at least 1 million solar masses. The central mass has to be contained in a region about 10 AU and the only way to put a million solar masses in a region 10 AU across is if it is
a black hole.  As we study the centers of other galaxies, it looks like supermassive black holes might be pretty common in the centers of galaxies on the whole.  Black holes in the centers of galaxies contain a million to a billion solar masses.

Stellar populations

 We divide stars up into 2 groups, based largely on their
time of birth.  The characteristics of each group are as follows:
Pop I: young stars (1 billion years), have circular orbits in the
disk of the galaxy, color tends to be blue, they are metal-rich.
Pop II: old stars (10 billion years), found in the bulge and halo
of our galaxy, orbits may be highly elliptical and inclined to plane
of galaxy, generally red, and metal-poor.
Of course, many stars formed in between 1 and 10 billion years ago
and hence there is a continuum of stellar properties, not a sharp
division between Pops I and II.  The Sun is classified as a Population
I star despite its age, because it is metal-rich and has a circular
orbit in the disk.
 Astronomers have recently begun to search for Population III
stars, which are supposed to be the leftovers of the very first
stars formed in the galaxy (so they have characteristics even more
extreme than thos of Pop II stars).  However, Pop III stars tend to be
difficult to identify for several reasons: they are low-mass, and
therefore dim, their orbits take them relatively far away from
the Sun's position in the galaxy, and their outer layers may be
contaminated by already-processed nuclear material from their cores,
making them appear spectroscopically similar to more metal-rich
stars.

Properties and Structure of the Milky Way

 Mass (in stars): ~10^11 Msun
 Luminosity: ~10^11 Lsun
 # of stars: ~10^11
 diameter of disk: 25 kpc
 diameter of bulge: 3 kpc
 diameter of halo: at least 50 kpc
 thickness of disk: ~500 pc
 distance from Sun to center of galaxy: 8.5 kpc
 age: 10-12 billion years (11-13 billion for globular clusters)

 Disk: Pop I stars, blue, spiral arms, open clusters, lots
  of gas and dust
 Bulge: Pop II stars, red, little gas or dust
 Halo: Pop II stars (Pop III?), globular clusters, dark matter

Formation of the Milky Way

 We are only now learning about how galaxies form and
their early evolution, because the galaxies we can see that are in
this stage of their existence are very far away and hence difficult
to examine in detail.  Nevertheless, we believe that we know
roughly how galaxies form.  They collapse out of large gas clouds,
similar to the way we learned that small gas clouds collapse
to form stars.  The globular clusters around a galaxy form first,
perhaps a billion years earlier than the galaxy itself.  The Milky
Way probably formed between 10 and 12 billion years ago - this
estimate is based on the ages of the oldest stars in globular
clusters.  When we look at distant galaxies, we see that they
are smaller and more numerous than galaxies are today, implying
that galaxy mergers are an important part of the formation process.