The Astrophysics Spectator
The Astrophysics Spectator
The central Galactic black hole, Sgr A*, is not dim because of a lack of fuel. Enough gas surrounds it that it could generate 100 million times the power of the Sun. This rate of power generation would make the central black hole the most luminous object in the galaxy. The puzzle is that the central black hole generates only about 1,000 times the power of the Sun.
The direction astrophysicists are taking in solving this conundrum is to make the black hole an inefficient generator of radiation. This inverts the more typical problem encountered in astrophysics, which is how to efficiently generate radiation from an astrophysical system. Astrophysicists have encountered this latter problem so often, that its solution is always the starting point when theories for new sources are developed.
The natural theory for gas falling onto the central Galactic black hole is that the gas forms an accretion disk around the black hole. This seems likely because the gas should carry some angular momentum, which keeps it from falling straight into the black hole. The gas orbits the black hole and slowly drifts inward as energy is dissipated and angular momentum is transported to the outer edges of the accretion disk. This mechanism is an extremely effective method of converting gravitational potential energy into radiation. It is the mechanism thought to operate in the active galactic nuclei in distant galaxies. What is clear is that this mechanism is not at work in the center of our own Galaxy.
In this issue of the web site, a page that discusses the capture of gas by Sgr A* from nearby massive stars is added to the “Milky Way Galaxy” path. This page discusses why the massive stars expel strong winds, and how much wind the central black hole should capture.
Next Issue: The next issue of The Astrophysics Spectator is scheduled for October 17.
Freddie Wilkinson
The Capture of Winds by Sgr A*. A couple of dozen high-mass stars orbit within a parsec of the central Galaxy black hole. These stars are blowing their outer layers away in light-driven high-velocity winds. Sgr A* should capture some of these winds, which in principle should supply Sgr A* with enough gas to light it up close to its Eddington limit—the effective upper limit on the radiative power generated by the black hole. That Sgr A* is in fact a dim source suggests that the black hole is unable to efficiently convert gravitational potential energy into radiation. (continue)
