About the Milky Way

The Milky Way is a vast spiral, similar to our neighbor the Andromeda galaxy, shown in the photo. From our perch in one of the spiral arms, we can see the Milky Way as a great band of stars across the sky (provided we are far from an urban area).

The Andromeda galaxy, similar to our own Milky Way. (image credit: NRAO/NSF)

This photo clearly shows the dust lanes looking towards the center of the Milky Way galaxy. These dust lanes obscure the center of the galaxy in visible light. (photo credit: Dave Palmer)

Where is the center of the Milky Way? Astronomers answered this question by carefully mapping the distribution of stars, and by 1930 they placed the center in the constellation Sagittarius, near its border with Scorpio. Looking in this direction with an optical telescope—all that was then available—revealed nothing of the center, which is totally obscured in the visible spectrum by broad lanes of dust, as shown in the photo. The size of these dust particles is approximately the same as the wavelength of visible light; therefore, this light is strongly scattered. Telescopes using light with a larger wavelength were clearly needed, and they were not long in coming.

This radio image, made at a wavelength of 3.6 cm, shows the extremely compact radio source-- the bright spot--at the center of the Milky Way galaxy. The spiral pattern is made by ionized gas that is orbiting the object in the center. (image credit: NRAO/NSF)

Radio astronomy began in 1930 when electrical engineer Karl Jansky, investigating radio static, found a source of radio noise in Sagittarius. At much shorter wavelengths, infrared observing technology developed as well, and both the radio and infrared results showed a highly localized energy source at the same location.

Fast forwarding to 1971, Martin Rees and Donald Lynden-Bell, both of the University of Cambridge , hypothesized that a black hole at the center of the galaxy could explain the intense radio source observed there. The mechanism is the acceleration of charged particles spiraling into the black hole, since accelerated charges produce electromagnetic radiation. Then, in 1981, Mike Watson, of Leicester University , using the Einstein X-ray Observatory, found well-defined sources of x-rays very close to the galactic center. In fact, x-ray emission is predicted to occur very close to a black hole, due to heating of the accretion disk of matter in orbit about the black hole. However, the x-ray energy was much fainter than expected—a puzzle to this day.

In a quite different approach, infrared astronomers began in about 1980 to explore the dynamics of both gas and individual stars orbiting the presumed black hole. The idea was right out of introductory physics—the orbit and period of an object moving in a gravitational field determine the mass of the object whose gravity produces the orbit. (see sidebar)