# 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](https://astropilot.space/glossary/seeing). What matters is their [separation](https://astropilot.space/glossary/separation), the brightness difference between the stars and their [position angle](https://astropilot.space/glossary/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.

**Examples:** [albireo](https://astropilot.space/object/albireo.md), [mizar](https://astropilot.space/object/mizar.md), [epsilon-lyrae](https://astropilot.space/object/epsilon-lyrae.md), [castor](https://astropilot.space/object/castor.md)

**Related:** [Separation](https://astropilot.space/glossary/separation.md), [Position angle](https://astropilot.space/glossary/position-angle.md), [Resolution](https://astropilot.space/glossary/resolution.md), [Multiple star](https://astropilot.space/glossary/multiple-star.md), [Star colour](https://astropilot.space/glossary/star-colour.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/double-star