Binary galaxy . Two galaxies orbiting around each other and bound by their mutual gravitational attraction. Binary star . A stellar system which is composed of two stars orbiting one another about their common centre of mass. The two stars are held together by the force of their mutual gravity.
Binary stars are twins in the sense that they formed together out of the same interstellar cloud.
Since they may have formed with different masses to one another, however, they will evolve at different rates. As stars grow to become red giants, once nuclear fusion of helium begins in their cores, they can fill their Roche lobes. This means that matter can pass through the inner Lagrangian point and form an accretion disc around the companion star. This increases the mass of the companion and, thus, causes it to evolve faster. Roughly one half of the stars in the sky are binaries or multiple systems.