For planets, the point of the orbit closest to the Sun is the perihelion, and the point farthest from the Sun is the aphelion. If the bodies appear near each other as seen from Earth, they will rise and set at the same time. Greatest elongation is the greatest apparent angular distance from the Sun, when a planet is most favorably suited for observation. Mercury can be seen with the naked eye only at about this time. An occultation of a planet or star is an eclipse of it by some other body, usually the Moon.
Stars are the basic units of population in the universe.
Our Sun is the nearest star. Stars are composed of intensely hot gasses, deriving their energy from nuclear reactions going on in their interiors. Galaxies are immense systems containing billions of stars. The few exceptions are other galaxies. The spiral galaxy nearest to ours is in the constellation Andromeda.