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.
A laser-guided observation of the Milky Way Galaxy at the Paranal Observatory in Chile in 2010.
A Starburst Galaxy
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Berko’s Language Study
The areas in which reading-associated increases in activity were observed in the fMRI study of ...
Sensitivity study for facial difference