complete tidal disruption (and subsequent merger with the parent galaxy) of a dwarf satellite galaxy. Effects on bodies within a galaxy Tidal effects are also present within a galaxy, where their gradients are likely to be steepest. This can have consequences for the formation of stars and planetary systems. Typically, a star's gravity will dominate within its own system, with only the passage of other stars substantially affecting dynamics. However, at the outer reaches of the system, the star's gravity is weak and galactic tides may be significant. In the Solar System, the theoretical Oort cloud, source of most long-period comets, lies in this transitional region. The Oort cloud is a vast shell surrounding the Solar System, possibly over a light-year in radius. Across such a vast distance, the gradient of the Milky Way's gravitational field plays a far more noticeable role. Because of this gradient, galactic tides may then deform an otherwise spherical Oort cloud, stretching the cloud in the direction of the galactic centre and compressing it along the other two axes, just as the Earth distends in response to the gravity of the Moon. The Sun's gravity is sufficiently weak at such a distance that these small galactic perturbations are enough to dislodge some planetesimals from such distant orbits, sending them towards the Sun and planets by significantly reducing their perihelia. Such a body, being composed of a rock and ice mixture, would become a comet when subjected to the increased solar radiation present in the inner Solar System. It has been suggested that the galactic tide may also contribute to the formation of an Oort cloud, by increasing the perihelia of planetesimals with large aphelia. This shows that the effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system. However, cumulatively, the effect can be quite significant; up to 90% of all comets originating from an Oort cloud may be the result of the galactic tide. See also Oort cloud Roche limit Satellite galaxy Dwarf galaxy Interacting galaxy Tidal force References Extragalactic astronomy Oort cloud Tides