''There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from gas that is already in the
interstellar medium while others are produced by
stars. Examples of the former case are
giant molecular clouds, the coldest, densest phase of interstellar gas, which can form by the cooling and condensation of more diffuse gas. Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its
stellar evolution.
Star-forming regions are a class of emission nebula associated with giant molecular clouds. These form as a molecular cloud collapses under its own weight, proceeding stars. Massive stars may form in the center, and their
ultraviolet radiation ionizes the surrounding gas, making it visible at optical
wavelengths. The region of ionized hydrogen surrounding the massive stars is known as an
H II region while the shells of neutral hydrogen surrounding the H II region are known as
photodissociation region. Examples of star-forming regions are the
Orion Nebula, the
Rosette Nebula and the
Omega Nebula. Feedback from star-formation, in the form of supernova explosions of massive stars, stellar winds or ultraviolet radiation from massive stars, or outflows from low-mass stars may disrupt the cloud, destroying the nebula after several million years.
Other nebulae form as the result of
supernova explosions; the death throes of massive, short-lived stars. The materials thrown off from the
supernova explosion are then ionized by the energy and the compact object that its core produces. One of the best examples of this is the
Crab Nebula, in
Taurus. The supernova event was recorded in the year 1054 and is labelled
SN 1054. The compact object that was created after the explosion lies in the center of the Crab Nebula and its core is now a
neutron star.
Still other nebulae form as
planetary nebulae. This is the final stage of a low-mass star's life, like
Earth's
Sun.
Stars with a
mass up to 8–10 solar masses evolve into
red giantsand slowly lose their outer layers during pulsations in their atmospheres. When a star has lost enough material, its temperature increases and the
ultraviolet radiation it emits can
ionize the surrounding nebula that it has thrown off. Our Sun will produce a planetary nebula and its core will remain behind in the form of
white dwarf.'' Nebula - Wiki