The Electromagnetic Spectrum
Electromagnetic radiation is the flow of energy through free space or through a material medium. It is in the form of electric and magnetic fields which make up electromagnetic waves of different wavelengths and frequencies. Our eyes can only detect a small range of these wavelengths, which we call visible light. Visible light from a lamp in your house and radio waves from a radio station are two types of electromagnetic radiation. All of these wavelengths travel at the same speed in vacuum.
The electromagnetic spectrum is a continuous range of wavelengths. The types of radiation that occur in different parts of the spectrum have different uses and dangers, which depend on their wavelength and frequency.
Hotter temperatures correspond to greater energies (higher frequencies) and smaller wavelengths. Only extremely hot objects or particles moving at very high velocities can create high-energy radiation like X-rays and gamma-rays.
By looking at distant objects at different wavelengths/energies we can study the range of processes occurring in an object such as the Crab Nebula; this allows astronomers to then piece together what is happening in terms of its properties, history, composition etc.