Universe as all of space and all the matter and energy within it. Cosmology is the study of the overall structure and history of the universe. The universe contains two related entities: matter and energy. Matter takes up space and can be felt. The amount of matter in an object is its mass. Density refers to the amount of mass occupying a given volume of space. The mass of an object determines its weight. Energy is the ability to do work on another object using heat, light, magnetism or gravity. Stars are an immense ball of incandescent gas that emits intense heat and light. Solar system is the sun and all the other objects with it, such as planets, stars, meteorites, etc. A planet is an object that orbits a star. Terrestrial planets consist of a shell of rock surrounding a ball of metallic iron alloy.