With our eyes alone we can see visible light from several thousand stars. Until the seventeenth century, few people even dreamed that there were more of them. It was then that telescopes revolutionized human understanding of the universe, showing for the first time how little of the cosmos we normally see. The process of discovery continues, as telescopes reveal new things in space nearly every day. We can think of the radiation emitted by objects out there as the medium of natural cosmic communication, and telescopes as the means by which we gather and read those cosmic messages.

We have also discovered that visible light is only a tiny fraction of the energy emitted by objects in space. Indeed, such phenomena as interstellar clouds of gas and dust, the bodies lying behind these clouds, newly forming stars, intergalactic gas clouds, and a variety of exotic objects, such as black holes and neutron stars, are nearly invisible to even our best optical telescopes. However, many of these objects strongly emit a variety of nonvisible radiations (namely radio waves, microwaves, infrared and ultraviolet radiations, X-rays, and gamma rays) that we now have the technology to detect. Today, we use telescopes on the ground, floating high in the atmosphere, orbiting Earth, or traveling elsewhere in the solar system to see these myriad “stealth” objects in space.