Physics 110: Homework #8 Solutions

8.1

O stars emit more ultraviolet radiation than B stars. You wouldn't expect strong 21-cm emission because there is little atomic hydrogen in an H II region.

8.2

Interstellar dust prevented Herschel and Kapteyn from seeing far enough to learn the true shape and size of the Galaxy.

8.3

More matter is required to account for the speed of rotation of the Galaxy than can be found in visible forms such as stars and nebulae. The supporting evidence is the flat rotation curve of the Milky Way which implies that the Galaxy's mass is increasing out to radii of 50 kpc.

8.4

The spiral arms of the Galaxy near the Sun are revealed by the distribution of young star clusters and HII regions. Elsewhere clouds of molecular gas more-or less trace the spiral arms. The arguments in favor of the density-wave origin for spiral structure are (1) the arms can't be ``material arms'' because of the winding dilemma, (2) stars tend to slow down as they approach the arm and speed up as they leave it, consistent with a wave phenomenon, (3) the compression of gas that occurs at spiral arms in the density-wave theory naturally explains the enhancement of star formation there.

8.5

Rapidly orbiting gas near the center shows that there are several million solar masses of material within the inner 0.1 pc of the Galactic center. In addition infrared observations show that stars near the Galactic center are orbiting Sgr A at speeds of more than 1500 km/s, implying a compact object near the center with a mass solar masses.

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