Show the last entries for a time period: week two weeks three weeks month Stars An enormous luminous ball of gas that at some stage of its life produces energy by converting hydrogen into helium. Stars are either classified by their evolutionary stage or in terms of their spectra - a measure of the star's temperature. Star clusters the groups of stars connected by gravitational forces and common origin. Nebulae celestial objects composed mainly of dust and gas Interstellar medium Core of the galaxy catapults stars our way - a New Scientist article Analysis of a stream of stars called Hercules implies that the central bar of the Milky Way acts like a propeller and throws matter outwards.