Stars

Stars are born in the interstellar medium by the gravitational collapse of gas and dust within interstellar molecular clouds which have mass many times greater than the mass of a single star. Starbirth results from Gravitational Instability, followed by
Gravitational Collapse.
Most stars in the sky that appear to be single are actually made up of two or more objects revolving around each other. Double stars are called "double" even when three or more stars are present. In some double-star systems, the stars periodically block each other as they orbit, making the total brightness we see var…