Monster at the Center

The central part of our Galaxy (first identified by Harlow Shapley) is literally invisible. It cannot be observed at optical frequencies, but radio and infrared frequency observations have told us much about this hidden realm.

Some radio observations have identified a ring of molecular material orbiting the Galactic center at a distance of perhaps 8 or so parsecs (25 light-years). The rotational velocity of this ring tells us that there are several million solar masses of material located within its radius. Other radio observations (sensitive to hot gas) show that within this ring is a small amount of ionized material, nowhere near a few million solar masses.

Recent infrared observations show that the Galactic center region contains a great many stars, closely packed. But they do not account for all of the required mass.

The last clue: Radio astronomers have picked up strong radio emission from a source in the Galactic center region that appears to be very small—perhaps as small as a solar system. Called Sagittarius A* (pronounced "A-star"), this source may give the location of a black hole of several million solar masses.

Astro Byte

Our part of the Galactic disk orbits the Galactic center at over 136 miles per second (220 km/s). It takes about 225 million years for our region to complete one orbit around the Galactic center. Our solar system has orbited the Galactic center some 15-20 times since it formed. One-quarter Galactic orbit ago, dinosaurs roamed the earth.

Stars near the Galactic center are in orbit around a very small, very highmass object. Is there a monster at the center? It appears increasingly likely that a black hole lurks at the galactic center. These images show two clusters that are within 100 light years of the center of the galaxy.

(Image from D. Figer, STScl, and NASA)

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