The maximum extent of the region in which the Sun's gravitational field is dominant, the Hill sphere, may extend to 230,000 astronomical units (3.6 light-years) as calculated in the 1960s. But any comet currently more than about 150,000 AU (2 ly) from the Sun can be considered lost to the interstellar medium. The nearest known star is Proxima Centauri at 269,000 AU (4.25 ly), followed by Alpha Centauri at about 4.35 light years.