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, so I have given them yellow and orange colours. There are two images here, showing the system before and after their atmospheres have fully merged.