Astronaut - An astronaut or cosmonaut is a person trained by a human spaceflight program to command, pilot, or serve as a crew member of a spacecraft. Astronom - An astronomer is a scientist who studies celestial bodies such as moons, planets, stars, nebulae, and galaxies); the physics, chemistry, and evolution of such objects; and phenomena that originate outside the atmosphere of Earth (such as supernovae explosions, gamma ray bursts, and cosmic background radiation).