A "double star" refers to a system of two stars that are gravitationally bound to each other, orbiting around a common center of mass. These stars can be seen as separate points of light in the night sky, and they may have different colors, sizes, and brightness levels. Some double stars are close together, making them appear as a single star to the naked eye, while others are more widely separated. Astronomers study double stars to learn more about stellar properties and evolution. By observing their orbits and brightness, scientists can determine their masses and distances. Notable examples of double stars include Alpha Centauri, the closest star system to Earth, and Sirius, the brightest star in the night sky.