On a calm and dry night, when the sky is clear and moonless, a person standing in a field or on a hill miles from any source of light may be able to see with the unaided eye as many as three thousand stars. A star, like the Sun at the center of the solar system, is a hot, roughly spherical ball of gas that emits light and other forms of electromagnetic radiation as a result of nuclear fusion reactions in its core. (Nuclear fusion is the merging of two hydrogen nuclei into one helium nucleus, accompanied by a tremendous release of energy.) One of the fundamental objects in the universe, stars are composed mostly of hydrogen, the simplest and lightest of all chemical elements.

A large collection of stars, gas, dust, nebulae (clouds of dust and gas), and empty space all...