The classification scheme described above, while helpful, seems to imply that a nebula is constant and unchanging, existing in one state forever. This is not the case. The various bright and dark nebulae actually represent different stages in stellar evolution. Let's examine this evolutionary process to understand how nebulae act as a cradle of star formation.
Dark Nebulae: Seeds Are Planted
We already know that nebulae are low-density clouds. We also know, intuitively, that stars are very dense objects. If a nebula is to act as a birthplace to stars, then its building-block materials -- dust particles and hydrogen and helium gas -- must be pulled together and compressed into a relatively small "ball" of matter. As it turns out, this condensation process occurs in various regions throughout dark nebulae (reflection nebulae, as well, which are really nothing more than dark nebulae that reflect the light of nearby stars).
Gravity is the force that drives condensation. As a ball of dust and gas contracts under its own gravity, it begins to shrink and its core begins collapsing faster and faster. This causes the core to heat up and to rotate. At this stage, the condensed material is called a protostar. One nebula may have many protostars, each of which is destined to be an individual stellar system.
Some protostars have less mass than our sun. They're so small that they can't initiate the thermonuclear reactions so typical of stars. Even still, these objects may glow dimly because the force of gravity causes them to continue shrinking, which releases energy in the process. Astronomers label these objects brown dwarves as a way to describe their small size and relatively insignificant power output.
Other protostars are bigger, many times more massive than our own sun. These large protostars continue to contract, but instead of producing heat through contraction alone, they begin to convert hydrogen into helium in a process known as thermonuclear fusion. At this point, the protostar phase is over and a true star begins to form. Around it's a whirling cloud of residual dust and gas -- the very material that can build, over billions of years, a system of planets and moons.
Emission Nebulae: A Star Is Born
When a protostar becomes a self-radiating object, fueled by its own thermonuclear reactions, it becomes a true star. If it's massive enough, a star can ionize the nebular material, producing an area of fluorescence around it. The dark nebula, now glowing, becomes an emission nebula.
A single emission nebula can be filled with numerous newborn stars. A good example is the Cone Nebula, in Monoceros the Unicorn, an area of active star formation. The Cone Nebula is part of an enormous cloud of hydrogen gas that cradles many brand-new stars, which vary drastically in brightness because many are still cloaked in cloud and dust. The brightest star associated with the Cone Nebula is S Monocerotos.
Courtesy NASA and STScI
Nebulae may also mark the site of a star's demise. On the next page, we'll look at how that can happen.