Simply their frequency or wavelength, which then relates to the amount of energy they can carry 'per photon'. Astronomers observe celestial objects at different wavelengths to build up a picture of the kinds of phenomena happening in them. It is kind of like peeling an onion. X-ray emission usually means that the object contains plasma heated to 100s or thousands of degrees.
Visible light comes from gases heated to only a few thousand degrees like the surface of a star.
Infrared light also comes from even cooler bodies, or dust clouds surrounding a hot body. Finally, radio waves usually mean that the object contains magnetic fields in which electrons are moving at nearly the speed of light to emit radio 'synchrotron' radiation. When you see a single object in all these different ways, you can very accurately tell what the object is, what it is doing, and many other things. Return to the Ask the Space Scientist main page.