Answer to Question 1
Cepheid variables have a known relationship between their luminosity (absolute magnitude) and their pulsation period. An astronomer can easily observe the pulsation period of the variable star to determine its absolute magnitude and compare this with the apparent magnitude to determine the distance to the star. Cepheid variables are very luminous. This allows them to be seen over very long distances.
Cepheid variables are present in very distant globular clusters, so the distance to these clusters can be determined by studying the Cepheids. The distribution of globular clusters is a key to determining the center of the galaxy, since globular clusters orbit around this center.
Answer to Question 2
Tides are caused by small differences in gravitational forces. For example, Earth's gravity attracts your body downward with a force equal to your weight. The Moon is less massive and more distant, so it attracts your body with a force that is a tiny percent of your weight. You don't notice that little force, but Earth's oceans respond visibly.
The elliptical galaxy Centaurus A at visible, far-infrared and x-ray wavelengths.
Biology deals with the study of the many varieties of living organisms.
Important Motor Regions of the Human Brain
Figure 7.1 shows the rapid decline in the trust of Americans in government since the 1960s. This de
Historian James Merrell notes several errors in Benjamin West’s famous 1771 painting, William Penn’s
Michelangelo, Study for the Libyan Sibyl.