The Astrophysics Spectator



Interactive Pages


Other Pages



Contact Information

TAS Icon Send e-mail to the editor.

RSS Channel

TAS Icon The Astrophysics Spectator Channel

In Association with

Issue 4.09

The Astrophysics Spectator

June 13, 2007

In this issue, a page discussing the Sagittarius A complex (Sgr A), the very center of our Milky Way Galaxy, is added to the web site. As befits a region dominated by what appears to be a massive black hole, what we find is somewhat unexpected.

Some astronomers like to refer to our own Galaxy as a moderately-active Seyfert galaxy (Seyfert galaxies are distant galaxies with a brilliant cores). The theory behind the galaxies with active nuclei (AGN) such as Seyfert galaxy is that a large, thick disk of gas orbiting a massive black hole is radiating away its gravitational potential energy as electromagnetic waves. Some of the potential energy goes into a wind that is driven away from the black hole at close to the speed of light; this wind appears to us as a narrow jet extending many kiloparsecs from the core of a galaxy. The cartoon image this brings to mind is of a doughnut with a stick passing through its hole. The expectation of astronomers is that the same process, but generating much less power, is occurring at the center of our own Galaxy.

But what is striking to the author about the center of the Milky Way, and particularly about the Sgr A complex, is how weakly it calls to mind the black-hole theory for active galactic nuclei. The massive black hole candidate, Sgr A*, is a bright radio source, but other objects surrounding this source stand out as strongly. Throughout the Galactic center one sees filaments of radio emission extending perpendicular to the Galactic equator; these filaments are thought to be areas where a magnetized wind is moving away from the galactic plane. More striking is the prevalence of supernova remnants. The Galactic center is dotted with supernova remnants, one of which encloses the central black hole; this last supernova remnant is part of the Sagittarius A complex, and it is called Sagittarius A East. It calls to mind an old, defunct theory for AGNs that attributes the high power output to a high supernova rate. Sgr A East won't revive that theory, but the remnant is a nice irony. Sagittarius A West, which is actually inside Sagittarius A East, is the most unusual of the Galactic center objects. Not only is it the home of Sgr A*, it is the point around which the stars of the Galactic center are clustered; no other place in the Galaxy has such a high density of stars. Unexpectedly, a large number of these stars are big and young. One seldom thinks of millions of stars shoved into a 1 parsec sphere when one thinks about active galaxies, but that is the single most striking feature of our own local ?mini-AGN.?

Next Issue: The next issue of The Astrophysics Spectator is scheduled for June 27.

Jim Brainerd

Milky Way Galaxy

The Sagittarius A Complex. The Sagittarius A Complex is a collection of objects extending several parsecs out from the center of the Galaxy. The very center of this complex is the bright radio source Sgr A*, which appears to be a 3.6 million solar mass black hole. Millions of stars cluster around this black hole. Spirals of gas and a supernova remnant complete the complex. (continue)


The Black Hole at Our Galactic Center. The page has been updated to reflect the use of 7.62 kpc for the distance to the Galactic center and the use of 3.6 million solar masses for the mass of Sgr A*. Also added is a line about the observational upper limit on the radius of the black hole. (continue)

Properties of the Milky Way Galaxy. Additional references to some of the values in the table are added to the page. (continue)

Ad image for The Astrophysics Spectator.