Black Holes: Exploring the Universe

Text courtesy of NASA

Different types of telescopes detect different types, or wavelengths, of light--from microwaves to visible light (what our eyes can detect) to high energy X-ray photons and gamma rays. Each wavelength band is important for composing a full view of the Universe. Optical telescopes, such as Hubble Space Telescope, generate brilliant images of stars and galaxies, both near and far. Hubble also measures the sizes, distances and compositions of celestial objects. Radio telescopes, however, were the first to detect quasars, extremely distant galaxies that emit incredible amounts of radio energy. Radio telescopes also provided the first evidence of planets outside our Solar System. Infrared telescopes have identified dust clouds between stars and galaxies, as well as “nurseries” for possible star formation. High energy gamma-ray telescopes have discovered the most energetic explosion since the Big Bang. Thus, the entire electromagnetic spectrum is important for a complete picture, because different objects emit the bulk of their radiation at different wavelengths. Black holes are best detectable at high energies—that is, in X-rays and gamma rays.

Color composite of the supernova remnant E0102-72: X-ray (blue), optical (green), and radio (red). Hubble, Chandra, Australia Telescope Compact Array.. Courtesy of NASA

An X-ray telescope collects photons created naturally in some of the most violent and energetic events in the Universe: such as supernova explosions and the rapid flow of gas under the extreme gravitational pull of a black hole. This mechanism for collecting photons is quite different from taking an “X-ray” of a broken bone. Unlike a medical X-ray machine, the X-ray telescope doesn't generate X-ray photons. Rather, it collects these tiny packets of high energy that form at extremely high temperatures. X-ray detectors on satellites are like the film in the doctor's X-ray machine.

The Sun produces some X-rays, particularly during a solar flare. However, X-ray telescopes focus far beyond our Solar System and can study black holes, stellar explosions, galaxies that release great amounts of X-rays from their centers, and the pervasive, but optically invisible, hot gas that dominates space between stars and galaxies.