What is a contact binary?

A contact binary is defined specifically as a celestial object composed of two connected parts, typically stars or other astronomical bodies that are in close proximity to each other. In a contact binary system, the surfaces of the two components touch, and they share a common envelope of material. This interaction can lead to interesting physical phenomena such as tidal forces affecting each body, as well as the potential for mass transfer between the two.

The defining characteristic of contact binaries is their physical connection and interaction, which distinguishes them from other types of binary systems where the components are separated by a significant distance. This aspect is crucial in the study of binary star evolution and in understanding how such systems can impact stellar lifecycles and the dynamics of galaxies.