For this review exercise, you are given objects or descriptions of parts of the Milky Way Galaxy. You are to decide the most likely place that these objects reside, or what part of the Milky Way is being described: DISK, HALO, OR BULGE. It may be possible that more than one part of the Galaxy applies, and you will be able to check more than one box. In all cases, you should justify your answer with clear logic. The summary of the logic behind each of the answers is at the end of the exercise. It is really worth your while, from a learning standpoint, to come up with your own answer and logic before looking at the answers.

Where in the Galaxy would you find:

Active star forming regions	Disk Halo Bulge	answer Disk Bulge
Open clusters with young stars	Disk Halo Bulge	answer Disk
Low-mass, Sun-like stars with relatively large proportions of heavy elements	Disk Halo Bulge	answer Disk Bulge
Extremely crowded star fields just about everywhere	Disk Halo Bulge	answer Bulge
Supernovae and supernovae remnants	Disk Halo Bulge	answer Disk Bulge
Clearly defined spiral arms	Disk Halo Bulge	answer Disk
Neutron Stars	Disk Halo Bulge	answer Disk Bulge
White dwarfs	Disk Halo Bulge	answer Disk Bulge Halo
Earth	Disk Halo Bulge	answer Disk
Lots of dust and gas in the interstellar medium	Disk Halo Bulge	answer Disk Bulge
Globular clusters	Disk Halo Bulge	answer Halo
Most of the dark matter	Disk Halo Bulge	answer Halo
ONLY stars that have proportionally an extremely small amount of heavy elements	Disk Halo Bulge	answer Halo
Giant molecular clouds	Disk Halo Bulge	answer Disk Bulge
Planetary nebulae	Disk Halo Bulge	answer Disk Bulge Halo

Reasoning

In many of these cases, it is what is NOT going on in a given part of the Galaxy or what objects are NOT found there. The halo has no active star forming regions. The stars in the halo formed (we theorize) as the galaxy was collapsing, early on in its history, and thus divorced themselves from the rest of the collapse to the disk. It was the gas that was forced onto a fairly flat plane due to the conservation of angular momentum.

Open clusters with young stars are found mostly in the disk. The regions of the bulge are so dense with stars, that any new clusters are quickly torn apart by gravitational interactions.

Stars having a higher proportion of the heavy elements are found in the bulge and the disk, not the halo.

The bulge is packed full of stars.

Supernovae and the remnants come from massive stars, and the halo does not have any massive stars nor has any resided there for billions and billions of years.

It is the disk that has the spiral arms, or so our observations of the Galaxy using a wide range of wavelengths and energies tell us.

Neutron stars remain where supernovae were. See answer above.

White dwarfs are the dying remnants of Sun-like stars, no matter what the elemental abundances are, and so they are found everywhere in the Galaxy.

Earth resides in the disk, about 2/3 of the distance from the center to the edge. Our observations have provided evidence for this.

The halo totally lacks dust and gas (mysterious!). The disk and bulge have ample supplies.

Globular clusters spend the vast majority of their orbits in the halo. They follow Kepler's laws and have highly elliptical orbits, or so we think.

Most of the dark matter seems to reside in the halo. How do we know this if we cannot see it?

The halo has only "metal-poor" stars. Observations (spectroscopy) tells us this.

Giant molecular clouds are found where stars will form, and that is mostly in the disk, but the bulge takes part as well.

Planetary nebulae are found where Sun-like stars are dying, and Sun-like stars are found throughout the Galaxy. Different parts of the Galaxy will have different numbers of these nebulae, however.