Story of the nebula
A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases in a galaxy. The Persian astronomer, Abd al-Rahman al-Sufi, mentioned a true nebula for the first time in his book, Book of Fixed Stars (964). He said that there was a "little cloud" near the Andromeda galaxy. A nebula is usually made up of hydrogen gas and plasma. It may be the first stage of a star's cycle, but it may also be one of the last stages. Many nebulae or stars form from the gravitational collapse of gas in the interstellar medium or ISM. As the material contracts, massive stars may form in the center, and their ultraviolet radiation ionises the surrounding gas, making it visible at optical wavelengths. The size of these nebulae, known as H II regions, varies depending on the size of the original cloud of gas. These are sites where star formation occurs. The formed stars are sometimes known as a young, loose cluster. Some nebulae are formed as the result of supernova ex...