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Explain why the Doppler shift tells us only about the motion directly along the line of sight between a light source and an observer, but not about motion across the celestial sphere.

Why can radio astronomers observe at any time of day or night whereas optical astronomers are mostly limited to observing at night?

What are the vernal and autumnal equinoxes? What are the summer and winter solstices? How these four points related to the ecliptic and the celestial equator?

If more massive stars evolve and die before less massive ones, why do some black-hole candidates have lower masses than their stellar companions?

What prevents thermonuclear reactions from occurring at the center of a white dwarf? If no thermonuclear reactions take place in its core, why doesn’t the star collapse?