Issue 4.17

The Astrophysics Spectator

October 31, 2007

The rare bright objects of our Galaxy, whether they are stars or black holes, are bright because they generate tremendous power that is efficiently converted into electromagnetic radiation.  The common dim objects of our Galaxy are dim because they generate little power, not because they are unable to create radiation; these objects are also very efficient at converting the power they generate into electromagnetic radiation.  The central Galactic black hole, Sgr A*, is unusual in that it radiates very little light despite pulling the winds of surrounding stars onto itself, which should enable it to generate considerable power.

Astrophysicists developing theories for the central Galactic black hole are pursuing two ideas to explain its dimness.  The first is that the captured winds flow radially towards the black hole, so that they arrive at the black hole before emitting a significant amount of light.  This type of accretion occurs if the winds carry only a small amount of angular momentum.  Such accretion should describe well the outer regions of the central Galactic black hole.  The second idea is that the winds flow into an accretion disk, and virtually all of the thermal energy within the disk is carried by the free atomic nuclei of the disk, so that the temperature of the free atomic nuclei is much greater than that of the free electrons.  Because electrons generate electromagnetic radiation much more readily than protons and helium nuclei, this segregation of energy away from the electrons slows the cooling of the disk.  Eventually, the energy is either carried into the black hole by the gas or expelled from the black hole in a jet or wind.  This dark accretion disk is a plausible description of the regions close to the black hole, because the captured stellar winds should carry enough angular momentum to cause a disk to form close to the black hole.

This issue of the web site adds a page that describes the radial accretion onto Sgr A*.  The next issue will describe the accretion disk theory.  After that, I'll move to a new topic, which may overjoy regular readers who are sick of reading about Sgr A*.

Next Issue:  The next issue of The Astrophysics Spectator is scheduled for November 14, 2007.

Milky Way Galaxy

Radial Accretion onto Sgr A*. The winds captured by the central Galactic black hole must fall to the event horizon in a way that prevents the generation of light.  One way to achieve this condition is for the gas to flow radially to the black hole. If the gas carries very little angular momentum, it can free-fall to the black hole faster than it radiatively cools.  The outer regions of Sgr A* are most likely described by this simple accretion theory.  The theory should break down close to the black hole, where the angular momentum of the gas should cause the gas to form a disk around the black hole.  (continue)