The Astrophysics Spectator
The Astrophysics Spectator
The very different appearance of Sgr A* from the massive black holes at the centers of other galaxies has inspired novel theories for accretion disks that emit little light. By their nature, accretion disks convert gravitational potential energy into thermal energy as they rotate. The basic idea transfixing theorists working on Sgr A* is that this thermal energy in carried into the black hole before the disk can radiate it away. A variant of this idea is that the thermal energy is converted into the kinetic energy of a wind flowing away from the black hole. Either way, the accretion disk orbiting the central black hole is relatively dark.
Inevitably, reality will not precisely conform to either of these seductively-simple theories. With luck, it will resemble one of them closely enough to prove kinship. This application of simplicity to complexity is inevitable in astrophysics. The first theories are always physically and geometrically simple. They rely on simple physical processes to generate the radiation. They ignore chaos. With better observations of the central Galactic black hole, we will see the complexity, the turbulence, and the chaos. Astrophysicists understand this inevitable shortcoming of their theories. But the simple theories can get the gross behavior right, because they will get the conservation of energy and momentum right; a large part of astrophysics is simply understanding the flow of energy and momentum within a system.
This issue of the web site adds one final page to the current discussion of Sagittarius A*. The page describes two theories for advective accretion disks. Next time, a new subject will be discussed. What that subject will be has yet to be decided, although it won't be black holes, and it will be Galactic astronomy.
Next Issue: The next issue of The Astrophysics Spectator is scheduled for December 5.
Freddie Wilkinson
The Dark Accretion Disk of Sgr A*. The central Galactic black hole should be orbited by an accretion disk. If it exists, it is unusually dim, unlike the accretion disks seen orbiting other black hole candidates. Many theorists are pursing a theory that the accretion disk carries energy into the black hole before the energy is radiated away. Other theorists believe the energy is converted into the kinetic energy of a wind. In either theory, the accretion disk radiates only a tiny fraction of the power that it generates. (continue)
