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.
Model organisms used to study the genetics of development
A Starburst Galaxy
Figure 7.1 shows the rapid decline in the trust of Americans in government since the 1960s. This de
In 1891 William Dean Howells championed the poetry of Emily Dickinson: “This poetry is as characteri
Leonardo da Vinci, Study of a Woman's Head or of the Angel of the Vergine delle Rocce.
Carons' study