1. A star appearing single, but in the telescope resolved into two.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• souble star,
• xouble star,
• couble star,
• fouble star,
• rouble star,
• eouble star,
• diuble star,
• dkuble star,
• dluble star,
• dpuble star,
• d0uble star,
• d9uble star,
• doyble star,
• dohble star,
• dojble star,
• doible star,
• do8ble star,
• do7ble star,
• douvle star,
• dounle star

The adjective form of the phrase "double star" is "double stellar".

The plural form of the word "double star" is "double stars".

The singular form of the word "double star" is "double star". The term itself does not change in singular form because it is already singular.