Astronomers use electromagnetic radiation (fancy word for light) to study astronomical objects. Light can come in many varieties. There is visible light, which we see with our eyes, but there is also other types of light that human eyes are not sensitive to. For example, when you go to the hospital to have an x-ray, your body is exposed to x-ray radiation. Night vision cameras allow you to see infrared radiation, which is essentially the heat given off by different objects. The image below shows you all the different types of light, what wavelength and frequency they have, and compares the size of the wavelengths to a common object.
It was not long ago that astronomers realized that outer space glows in more than just visible light. With special cameras mounted on telescopes, astronomers can see how objects look at any light wavelength they choose. And boy were they surprised at the results! Below is an image of the cartwheel galaxy. On the right hand side is the galaxy in the x-ray, Ultraviolet (UV), visible, and infrared (IR) wavelengths. The big picture is a combination of all these pictures.
As you can see, the galaxy looks very different in each of the four different types of light. You can't really tell that the object is a galaxy in the x-ray, but in the visible and IR, three galaxies are visible! This use of the whole electromagnetic spectrum allows astronomers to study outer space in new ways and discover things that were "not visible" just 30 or so years ago!
