If we watch the sky on a clear night, with the naked eye we can see about 3000 stars that are dozens or even hundreds of light-years away. With a big telescope we can see millions of galaxies that are even thousands of millions of light-years away.
However, there is much more out there that we do not see. Visible light is only a small part of the spectrum of electromagnetic radiation. Most of the huge range of light rays arriving from the Universe is invisible to the human eye, and a large amount is blocked by the Earth's atmosphere. Yet astronomers have learned how to look at these invisible rays. With big radio telescopes on Earth and clever satellites in space, they have dramatically extended our knowledge about the Universe. There will certainly be many more discoveries in modern multiwavelength astronomy.
With INTEGRAL astronomers are gaining a new understanding of objects and processes that emit the most powerful electromagnetic radiation. The gamma-rays detected by INTEGRAL are a million times more energetic than visible light. The shorter the wavelength, the higher is the energy of the radiation. Red light has a wavelength of 700 nanometres (0.000 000 7 metres), violet light of 400 nanometres. INTEGRAL studies radiation with a wavelength from 0.2 to only 0.0008 nanometres.
The energy of gamma-rays is usually given in electronvolt or eV, a unit of energy used in particle physics. 1 eV is the change in energy of an electron when it is moved through a difference of potential of 1 volt. Visible light has an energy of 2 to 3 eV, the energy of X-rays is in the range of thousands of eV, that is, kilo-electronvolt or keV. INTEGRAL studies radiation in the energy range from 15 keV to 10 MeV (10 mega-electronvolt or 10 000 000 eV).