But to astrophysicists, X-rays are a window recently opened on the cosmos. Until the last half of this century, astronomers were restricted to looking at the universe with optical telescopes gathering images based on visible light, much like those that create a photograph. But there are many more ways of looking at the universe than with visible light.
Visible light is only a narrow band in the electromagnetic spectrum.
Most of the spectrum cannot be seen with the naked eye. With sophisticated instruments, researchers now have new sets of eyes to view radio, infrared, ultraviolet, X-ray and gamma ray radiation. With the exception of radio waves and parts of the infrared spectrum, the Earth's atmosphere blocks most of this radiation. Thus, the only way to gather the information for studying the universe via these other forms of radiation is to send satellites aloft, outfitted with special instruments to detect what human eyes cannot. The majority of the brightest X-ray sources vary in brightness, often by large amounts.