Double Stars- Distinguish between optical double stars and binary stars
Many of the stars in the sky are binary stars. This means they are actually two stars that orbit around a common centre of orbit. Some stars like Castor in the constellation Gemini have more than one binary pair and so there are at least four stars, two sets of binaries that orbit each other.
Some stars look close to one another in the sky but are not related; they can be millions of kilometres, even light years apart. A further star may be brighter than the closer star but much further away. These are called optical double stars.
Stars that orbit around a common centre
Optical double stars
Stars that look close together from Earth but are actually not related to each other
Paul A. Heckert Multiple and Binary Star Systems