When our eyes look at the heavens we see the visible light from stars and other objects in the Universe. Thousands of years ago astronomers in Greece and other ancient cultures already built a detailed understanding of the night sky. Many names and concepts then developed are still in use today. However, our human eyes are actually not very sensitive and modern astronomers use sophisticated telescopes to study the Universe.

The telescopes used by astronomers do not just study visible light. While visible light is the type of 'electromagnetic radiation' that our eyes can see, there are many other types of such radiation. Different types of radiation are characterized by different wavelengths. If the wavelength is much shorter than that of visible light we speak about X-rays. We encounter X-rays often in our daily lives, for example at the hospital or during security screening. If the wavelength is much larger than that of visible light we speak about radio waves. We encounter radio waves often in our daily lives, for example in radios and cell phones.

The black holes in the Universe do not emit any detectable type of light. However, astronomers can still find them and learn a lot about them. They do this by measuring the visible light, X-rays and radio waves emitted by material in the immediate environment of a black hole. For example, when a normal star orbits around a black hole we can measure the speed of the star by studying the visible light that it emits. Knowledge of this speed can be combined with the laws of gravity to prove that the star is in fact orbiting a black hole, instead of something else. It also yields the mass of the black hole. Alternatively, when gas orbits around a black hole it tends to get very hot because of friction. It then starts emitting X-rays and radio waves. So black holes can also often be found and studied by looking for bright sources of X-rays and radio waves in the sky.

There are many other types of electromagnetic radiation as well. Radiation that has even smaller wavelengths (and even higher energies) than X-rays is called gamma-rays. Radiation with wavelengths between those of X-rays and visible light is called ultraviolet light. We encounter ultraviolet light in our daily lives for example in fluorescent lamps. Ultraviolet telescopes allow astronomers to study things such as the composition of the gas that exists between stars. Electromagnetic radiation with wavelengths between those of radio waves and visible light is called infrared light. We encounter infrared light in our daily lives for example in heat lamps and night-vision cameras. Infrared telescopes allow astronomers to study things such the formation of stars.

The following links provide more information about the types of radiation that are most important for studies of black holes, and about the telescopes that astronomers use to study them: