1.)Double stars can develope independent from eachother and spin around each other because of their gravitational fields. Those double stars are called wide double stars.

2.)When a cloud compresses the velocity of gravitation increases (law of preservation of rotational impulse). The fast rotation flatens the cloud of gas and matter can split off in the equatorial area. This way a double star forms. The two stars can be of the same size of different. This sort of double star is called narrow double star.