Double star
Also called: binary star, optical double, visual double, pair.
Two stars that appear close together in the sky — either a true orbiting pair (binary) or a chance alignment.
Many “single” stars split into two in a telescope. In a true binary the two stars orbit each other, some in a few years, some over millennia. An optical double is a chance line-up of stars at very different distances.
Doubles are perfect targets from light-polluted towns and on moonlit nights: they're bright, they show colour (Albireo's gold and blue is the classic), and they test your optics and the seeing. What matters is their separation, the brightness difference between the stars and their position angle.
At the eyepiece
Close pairs need steady air and enough power — roughly 240 divided by the separation in arcseconds. Unequal pairs need even more, to lift the faint star out of the bright one's glare.
In AstroPilot
Object pages give each pair's separation, magnitudes and position angle, the power that splits it, and whether your telescope can.