"dying stars spew gas and dust back into space, and this material gives birth to new generations of stars. but after enough generations, all the nuclear fuel in the universe is eventually exhausted, and the formation of luminous stars must come to an end. so even if the universe were infinitely old as well as infinitely large, it would not contain enough fuel to keep the stars shining forever and to fill up all of space with starlight. and so the night sky is dark."