Disks

Abstract

Accretion Disks abound in the Universe: (i) in galaxies,both spiral and elliptical, (ii) around forming stars,(iii) around planets,and (iv) around mature and old, degenerate stars. They are the result of mass transfer, or mass accretion at large angular-momentum excess. In them, due to non-negligible friction, matter not locked up in bound objects spirals inward at grain-mass dependent rates, allowing for chemical segregation.Accretion onto compact stars leads to large energy releases at high temperatures, both during and after spiral-in, giving rise in particular to the binary X-ray sources. Mass densities in the centers of galactic disks can reach stellar-interior values and hence allow for nuclear burning.