One way you can see this yourself, is if you have a digital camera. Most digital camera CCDs are sensitive to infrared light.

Turn your camera on, and while looking at the screen at the back, point your TV remote at the camera and push some buttons. On most cameras you will see bright lights where the LEDs are when looking at the screen, but to your eye, the LEDs on the remote will be dark.

If you translate that to a photograph in infrared ONLY, you’d have two bright spots in a black field. That is like what you’re looking at in the photo above.

What happens is that the astronomer takes a photo with film/CCD/whatever that is sensitive to the particular kind of light desired. As you are aware, a human cannot see infrared or ultraviolet or x-rays… but film and other devices and materials can. Images made with devices seeing into those wavelengths would be greyscale because infrared and so on have no “colour” – you’d have an image of bright and dark spots and everything in between… just like a black and white photograph.

When the photograph is processed, false colour is added to bring out details. In the case above, I would conjecture that it was made red-scale, with white being the brightest spot in infrared, and so on down through yellow, red and brown to black which represents very dark spots in infrared. That would be an easy transformation to make.

Often images are also combined: the infrared image might be made red-scale , combined with a UV image that has been made bluescale and an X-ray image that has been made green-scale to give a false “true-colour” type of image that shows details that no human could see with the naked eye.