Astronomy is the study of celestial objects in space. Celestial objects are structures bound together by gravity in space. Examples of celestial objects are things like the sun, moon, earth, and meteors.
Stars are celestial bodies that are made up of plasma. The sun is an example of a star.
The earth has a greater mass than the moon, therefore the moon is held in an orbit around the earth by gravity. All objects with mass produce a force of attraction called gravity. The earth and moon orbit the sun because they are attracted to the sun by its mass. The sun is 93 million miles from the earth, yet the sun's mass is so great that the earth and moon orbit the sun.