A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Most nebula are of vast size, even hundreds of light years in diameter. Although denser than the space surrounding them, most of these are far less dense than any vacuum created in an Earthen environment – a nebular cloud the size of the Earth would have a total mass of only a few kilograms. These are often star-forming regions, such as in the “Pillars of Creation” in the Eagle Nebula. 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 massive enough to form stars. The remaining materials are then believed to form planets and other planetary system objects Our Sun is expected to spawn a planetary nebula about 12 billion years after its formation.
Some these are formed as the result of supernova explosions, the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are ionized by the energy and the compact object that it can produce nebula or stars form from the gravitational collapse of gas in the interstellar medium or ISM. 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. Examples of these types of nebulae are the Rosette Nebula and the Pelican Nebula. The size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas. New stars are formed in the nebula.The formed stars are sometimes known as a young, loose cluster.