W UMa (W Ursae Majoris) is the prototype of a class of interacting binary stars in which each member reaches out to touch the other. The stars are so close to one another that the gravity of each visibly distorts the other into a balloon shape. This kind of system is also known as a contact binary. The two stars are typically of spectral classes F, G or K. This version of the image shows the stars after their atmospheres have become merged, so that both stars share a common atmosphere.