McGraw-Hill Encyclopedia of Science and Technology: An International Reference Work. — McGraw-Hill, 1966. — С. 24. — 666 с. — «Examples of radio nebulae are the Network Nebula in Cygnus and NGC 443. Whereas the source of excitation in diffuse, variable, and planetary nebulae is the illuminating star or stars, radio nebulae apparently derive their luminosity from collisions with the surrounding interstellar medium. They emit nonthermal radio-frequency radiation».
