A nebula is an interstellar cloud of dust particles and gases in space. There are two general types of nebulae: diffuse nebulae and planetary nebulae. Both types are also called gaseous nebulae. Diffuse nebulae are the larger of the two types. Some diffuse nebulae contain enough dust and gases to form as many as 100,000 stars the size of the sun.
A diffuse nebula may occur near an extremely hot, bright star. The intense ultraviolet light from the star energizes the gas atoms of the nebula and enables the mass to emit light. A diffuse nebula of this kind is called an emission nebula and often referred to as H II regions.
A diffuse nebula also may occur near a cool star. In this case, the ultraviolet light from the star is too weak to make the nebula's gas atoms give off light. But the dust particles in the diffuse nebula reflect the starlight. Astronomers refer to this kind of diffuse nebula as a reflection nebula. Reflection nebulae are usually blue because the scattering is more efficient for blue light than red.
If a diffuse nebula occurs in an area that has no nearby stars, it neither emits nor reflects enough light to be visible. In fact, its dust particles blot out the light from the stars behind them. Astronomers call such a diffuse nebula a dark nebula. In the inner regions of dark nebulae important events take place, such as the formation of stars and masers.
Planetary nebulae are ball-like clouds of dust and gases that surround certain stars. They form when a star begins to collapse and throw off the outer layers of its atmosphere. When viewed through a small telescope, this type of nebula appears to have a flat, rounded surface like that of a planet. Planetary nebulae include some of the most stunning sights in the universe, contorted into various shapes by magnetic fields and the orbital motion of binary systems.
A protoplanetary nebula is an astronomical object which is at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch phase and the subsequent planetary nebula phase. A protoplanetary nebula emits strong infrared radiation, and is a kind of reflection nebula. The exact point when a protoplanetary nebula becomes a planetary nebula is defined by the temperature of the central star.
A supernova is a stellar explosion and occurs when a high-mass star reaches the end of its life. When nuclear fusion ceases in the core of the star, the star collapses inward on itself. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode. The expanding shell of gas form a supernova remnant, a special type of diffuse nebula. Although much of the optical and X-ray emission from supernova remnants originates from ionized gas, a substantial amount of the radio emission is a form of non-thermal emission called synchrotron emission. This emission originates from high-velocity electrons oscillating within magnetic fields.
Star-forming nebulae contain a combination of raw materials that makes star birth possible. Star formation, also referred to as a stellar nursery, can be seen throughout the Milky Way, but it is principally evident in the spiral arms and toward the galactic center. In these regions, the interstellar matter is dense enough for molecular clouds to exist. When stars are born, these clouds are illuminated from within to become emission nebulae, some of the most beautiful sights in the Milky Way.
It is believed that the creation of newborn stars occurs exclusively within molecular clouds. The dense part of these molecular clouds collapse into a ball of plasma to form a star. Remnants from the star-forming nebulae will surround the stars, and the stellar winds produced by the new stars can, in turn, cause these remnants to collapse. If the clouds are part of a larger complex, this can become a great stellar nursery. Massive stars have relatively short lives, and they can be born, live, and die as a supernova while their less-massive siblings are still forming. The shock wave from the supernova may plow through nearby interstellar matter, triggering yet more star birth.