A nebula is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of a star’s cycle. Nebulae often form star-forming regions, such as in the Eagle Nebula. This nebula is depicted in one of NASA's most famous images, the "Pillars of Creation". In these regions the formations of gas, dust and other materials 'clump' together to form larger masses, which attract further matter, and eventually will become big enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects.
Some nebulae are formed as the result of supernova explosions. The material thrown off from the supernova explosion is ionized by the supernova remnant. But many nebulae originate from the gravitational collapse of gas in the interstellar medium. As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths. An example of this type of nebula is the Rosette Nebula or the Pelican Nebula. The size of these nebulae varies depending on the size of the original cloud of gas, and the number of stars formed can vary too. As the sites of star formation, the formed stars are sometimes known as a young, loose cluster.