Many nebulae are formed during the gravitational collapse of diffuse gases in the interstellar medium (ISM).
Other nebulae may form as planetary nebulae (discussed below).
Dark nebulae are similar to diffuse nebulae, but they are not seen by their emitted or reflected light.
Planetary nebulae are nebulae that form from the gaseous shells ejected from low-mass red giants (toward the end of the star's life) when they transform into white dwarfs.
When a star has lost a sufficient amount of material, its temperature increases and the ultraviolet radiation it emits is capable of ionizing the surrounding nebula.
The expanding shell of gas forms what is called a supernova remnant, a special type of diffuse nebula.
A protoplanetary nebula (PPN) is an astronomical object that is at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch (LAGB) phase and the subsequent planetary nebula (PN) phase.
The name "planetary nebulae" was given because the astronomers who first observed these objects thought that they resembled the disks of planets.
Many nebulae (known as diffuse nebulae) have poorly defined boundaries; others (such as planetary nebulae) may be described as discrete objects with identifiable boundaries.
nebulae, nebulж, or nebulas; derived from the Latin word for "mist" or "cloud") is an interstellar cloud of dust, hydrogen gas, and plasma.
Originally, the term nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way.
The exact stage when a PPN becomes a planetary nebula (PN) is defined by the temperature of the central star.
Instead, they are observed as dark clouds in front of more distant stars or in front of emission nebulae.
Some nebulae are formed as a consequence of supernova explosions, which are the death throes of massive, short-lived stars.
Most nebulae can be described as diffuse nebulae, which means that they are extended and have no well-defined boundaries.
One of the best examples of this is the Crab Nebula, in Taurus.
Nebulae often contain star-forming regions, as observed in the Eagle Nebula, depicted in NASA's famous image known as the Pillars of Creation.