Visible light waves, however, reveal only a fraction of the big picture. Just after World War II, developments in radio technology enabled scientists to measure energetic radio waves emitted by the Milky Way, other galaxies, and nebulas. Advances in electronics and solid-state physicsm
the 1960s and 1970s made radio astronomy one of the most powerful means of surveying the cosmos. It has been especially effective in probing distant galaxies, particularly their active centers, which are thought to harbor supermassive black holes.
Astronomers who were eager to explore the universe at other invisible wavelengths had to get above the distorting effects of Earth's atmosphere. Since the late 1950s, Earth-orbiting satellites have allowed them to do this.
On the low-energy end, infrared wavelengths reveal the warm-to-cold universe. With infrared cameras, star clusters still shrouded in dense dust can be "seen" despite being visually obscured, as can the heart of our own galaxy. On the more energetic end of the spectrum, ultraviolet and X-ray wavelengths show the hot, active universe. Here, astronomers study novae and supernovae, quasars and the cores of active galaxies, and the disks of gas around black holes and neutron stars.
All-sky surveys from space in the 1970s and 1980s revealed thousands of new objects, including new classes of infrared-bright galaxies, high-energy binary stars, black-hole candidates, and enigmatic objects that emit bursts of gamma-ray energy.
Nothing better illustrates how rapidly the frontiers are advancing than the stream of discoveries from the Hubble Space Telescope. Its detailed imaging and spectroscopic observations are of incalculable value.
The twentieth century has also seen amazing leaps in the study of the universe from space itself. The dream of venturing into space is an old one. As far back as AD 160, a Greek named Lucian of Samosata wrote a story about traveling to the Moon. In the early seventeenth century, a journey to the Moon and speculations about weightlessness figured prominently in a story called Somnium ("Dream") by the German astronomer Johannes Kepler. Imagine what such visionaries might say about the Apollo missions to the Moon, the Viking landers on Mars, or the Voyager probes to the Solar System's outer planets.