Until the 20th Century, astronomers learned virtually all they knew about sources in the sky from only the tiny fraction of electromagnetic radiation which is visible to the eye. However, as astronomers have discovered how to collect radiation outside this part of the spectrum , they have been able to learn much more about the Universe.
Many objects reveal different aspects of their behavior at different wavelengths .
Other objects are completely invisible at one wavelength yet show up clearly at another. The following section explains a little about what is revealed by observing at each wavelength, and includes an example image of the Crab nebula to illustrate how one source varies its appearance from one wavelength to another. Scientists now believe the Crab nebula is the remains of a star which suffered a supernova explosion. The core of the star collapsed and formed a neutron star which released a tremendous amount of energy, sufficient to blast the surface layers of the star into space. The expelled gases have formed the Crab nebula which is still expanding.