Imitating Master Yoda

It took the development of more advanced radio telescopes in ensuing decades to begin to unravel the mystery of the Milky Way’s core. In the mid-1970s, radio images revealed three distinct non-stellar objects in the core. Two, which looked like hazy, cloudlike patches, were dubbed Sagittarius East and Sagittarius West (after Sagittarius, the archer, the constellation in which the core is situated in Earth’s night sky). The third object, a pointlike, very powerful radio-wave source lying in the galaxy’s very center, received the name Sagittarius A* (pronounced A-star).

For a long time, astronomers were puzzled by Sagittarius A*. It is clearly too energetic and hot to be an ordinary star. Indeed, studies reveal that it is hotter than any other object in the Milky Way. In the 1980s and early 1990s, more sophisticated images of the core were taken using infrared telescopes, which can see through most of the layers of gases and dust. These showed huge filaments of gases swirling around Sagittarius A*. Even more detail was revealed in 1997 by German astronomers Andrea Eckart and Reinhard Genzel, who announced that they had mapped the frenzied motions of the seventy stars closest to the core’s central object. According to Zimmerman:

They found that many of the stars were streaking across the sky at tremendous speeds, and that the closer to Sagittarius A* the stars were, the faster they moved. Stars at distances of more than half a light-year traveled at less than 100 miles per second. Closer in, the speeds increased to more than 500 miles per second, and the closest star to Sagittarius A*, dubbed S1, also had the fastest velocity, estimated at almost 900 miles per second. Furthermore, Eckart and Genzel found that the 100 nearest stars seemed to be moving in a generally clockwise direction, opposite to the rotation of the rest of the galaxy. This suggests that they were part of a large torus [doughnut-shaped structure] of stars orbiting a single invisible point. At the center of this whirling collection of stars was the radio source Sagittarius A*, which unlike any other star in the sky has no apparent proper [visible] motion.

Members of the scientific community are now nearly unanimous in their belief that Sagittarius A* is a supermassive black hole. As for just how massive it is, numerous estimates appeared in the 1990s, the most common being 2.6 million solar masses. In October 2002, however, the results of a study by Rainer Schodel, of Germany’s Max Planck Institute for Extraterrestrial Physics, showed a larger mass for the giant black hole 3.7 million times that of the Sun.

To measure the mass of Sagittarius A*, the scientists observed the speeds at which matter is orbiting it and determined how massive the central object would have to be to produce these movements. “In the same way that Master Yoda and his disciples [in the Star Wars series] saw through an attempt to wipe a planet from the Jedi archives [by detecting the telltale signs of the planet’s gravity],” William Keel quips, “astronomers can discern the existence of this object.”