MagnitudeThe brightness scale of objects, running backwards: the smaller the number, the brighter the object. Five magnitudes make a factor of 100, one magnitude a factor of about 2.5. Vega is 0, Sirius −1.5, Venus up to −4.6; the eye reaches 6 under a dark sky.
Double starTwo stars very close together on the sky. Some are true pairs bound by gravity (binaries); others only line up by chance (optical doubles). Albireo in Cygnus shows a lovely orange and blue contrast from 30×; Mizar and Alcor split with the naked eye.
SupernovaThe explosion of a star, which can shine for a few weeks as brightly as a whole galaxy. Many are found every year in distant galaxies, sometimes by amateurs, at magnitude 12 to 16. The last one seen in our own Galaxy was in 1604.