Note:  Self-assess your work on the basis of 3 points total for this week.

1. Describe in your own words the various components in the Milky Way galaxy, and list the features associated with each component.

a. The central nuclear bulge is a slightly flattened spherically shaped cloud of stars about 4 kiloparsec (kpc) across - a little over 15% the overall extent of the luminous part of the galaxy. Constituents in this bulge are closely packed older stars, with little gas and dust. At the center of the bulge is a supermassive black hole with a mass of 2.6 million solar masses.
b. The "visible" halo of the galaxy is an approximately spherically shaped cloud of a few stars and around 200 globular clusters, each of which contains hundreds of thousands of old stars. This halo extends outward as far as the outer parts of the spiral arms. The stars and clusters in this halo orbit the center of the galaxy in randomly oriented elliptical (but nearly circular) orbits. Although these clusters contain many stars, their luminosity is relatively low because of their age - the brighter stars that formed with the cluster have all evolved off the H-R diagram main sequence either via supernova or formation of dim white dwarf stars.
c. The spiral arms, centered in a disk-type shape on the black hole at the middle of the bulge, contain almost all the luminous mass (stars) and gas and dust of the galaxy. Most of the gas and dust are in the spiral arms, as are all the younger massive (blue giant) stars. Star formation regions are all in the spiral arms. Most stars in the galaxy are somewhere in the disk that contains the spiral arms.  The structure of the disk spins about the center; the stars, gas, and dust spin even more rapidly in the same direction so that they pass through the spiral arms.
d. The galaxy also apparently contains a dark halo that is much larger than the visible part of the galaxy and contains 80-90% of the total mass of the galaxy in some as-yet mysterious form of "dark matter."  Dim compact objects such as black holes, brown dwarfs, etc., have been ruled out as this dark matter.

2. Describe how astronomers use Newton's and Kepler's laws of gravity to measure the mass distribution (amount of mass versus distance to the center) in our Galaxy.

In the early 1600's, Kepler first derived "rules" (based on observed planetary motion data) for motions of the planets around the Sun. Near the end of that century Isaac Newton introduced the theory or gravity, and he DERIVED Kepler's laws based on that theory. In an extension of the Newtonian version of Kepler's laws for spinning things like our galaxy, a star's speed in orbit around the galaxy should be consistent with the following equation:
TOTAL MASS INSIDE STAR'S ORBIT IN SOLAR MASSES = (ORBIT SIZE IN A.U.)³ DIVIDED BY (ORBITAL PERIOD IN YEARS)², or MASS_total = (R)³/(P)²
Doppler shift of absorption lines in the stellar spectra to determine star speeds.

3. Relate the evidence that is consistent with the hypothesis that almost all large galaxies (at least) have supermassive black holes (at least one) at (or near) galactic center. The APOD site has lots of examples of these kinds of observations - use the "search" function at the site to look for "supermassive black hole".

The evidence is two-fold. First, the center of most large galaxies is the source of intense, variable X-radiation, something you saw in past weeks to be a signal of very high temperatures (millions of Kelvins) associated with accretion disks around compact massive objects including black holes. Second, astronomers measure the speed of gas and stars near galactic center and use the generalized form of Kepler's laws (see question 2 above) to compute the mass of whatever it is at the galactic center. After examining alternate hypotheses for the Milky Way Galaxy, they conclude that the only explanation that makes sense is a supermassive (2.6 million solar mass) black hole (as opposed, say, to lots of extremely closely spaced stars).