Bohdan Paczyński or Bohdan Paczynski was a Polish astronomer notable for his theories and work in the fields of stellar evolution, accretion discs, and gamma ray bursts. He is the recipient of the Eddington Medal (1987), the Henry Draper Medal (1997), the Gold Medal of the Royal Astronomical Society (1999), and the Order of Polonia Restituta (2007).
Bohdan Paczyński
An accretion disk is a structure formed by diffuse material in orbital motion around a massive central body. The central body is most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other forces induce instabilities causing orbiting material in the disk to spiral inward toward the central body. Gravitational and frictional forces compress and raise the temperature of the material, causing the emission of electromagnetic radiation. The frequency range of that radiation depends on the central object's mass. Accretion disks of young stars and protostars radiate in the infrared; those around neutron stars and black holes in the X-ray part of the spectrum. The study of oscillation modes in accretion disks is referred to as diskoseismology.
The hot accretion disc of a black hole, showing the relativistic effects imposed on light when it is emitted in regions subject to extreme gravitation. This image is the result of NASA simulations and shows a view from outside the horizon of a Schwarzschild black hole.
Artist's view of a star with accretion disk
Artist's conception of a black hole drawing matter from a nearby star, forming an accretion disk
HH-30, a Herbig–Haro object surrounded by an accretion disk
