The space between the stars within a galaxy is not empty, but contains expanses of gas and dust collectively known as the interstellar medium. This is generally tenuous, but in some regions of a galaxy it can collect to form nebulae, and where a nebula like this becomes dense enough, the material within it will be drawn together by gravity to form the beginnings of new stars.
The development of star-forming regions like this will depend on the structure of the galaxy; they are rare in elliptical galaxies, but more commonly found in galaxies of spiral or irregular form. At times galaxies can interact with one another, causing unsually dense patterns of material that result in so-called starburst galaxies, rich with star-forming regions.
The density of a given cloud might be increased by any of various factors, most commonly by merging with another drift of interstellar material. Once the density becomes high enough, the internal gravity of the particles within the region will start to overcome their kinetic forces, causing areas of the nebula to gradually accrete, and so become more and more dense, creating discrete regions known as globules.
These globules represent the first stage of star formation. Within many of them, matter continues to accumulate to greater and greater densities in the core, until it starts to generate sufficient heat energy to prevent further collapse. At this stage, the object is referred to as a protostar. Internal pressures within the protostar cause hydrogen in its core to begin to undergo fusion, at which point a true star is born.
Where material had formerly been drawn inward toward the zone of increasing density, the new star now produces radiation that forces away most of the nebulous material surrounding it. Any matter that survives from the star's formation will, by conservation of angular momentum, enter an orbit around the new star following its plane of rotation. Gravitational attraction within this plane causes material to continue to accumulate into larger, denser regions, and so the star may eventually acquire a planetary system.
Stars rarely form independently within a star-forming region. Instead, they more usually appear throughout the region, eventually forming an open cluster of related young stars. A clear example of this can be seen within the Rosette Nebula in Monoceros. Hundreds of newly-formed stars comprise the Rosette Cluster at the heart of the nebula, and their combined radiation pressure has cleared their surroundings, creating a 'pocket' within the diffuse surrounding cloud. The result is a lobed but roughly ring-shaped nebula around a dense grouping of stars, a formation that gives the Rosette its name.
Prominent Star-forming Regions
The Rosette Nebula is only one of many examples of star-forming regions within the Milky Way and beyond. The most important such nebula from the perspective of the Solar System, however, ceased to exist several billion years ago. This has been called the 'Solar Nebula', the molecular cloud out of which the Sun and its planets condensed, more than 4,500 million years ago.
The Pillars of Creation, in a detail of an image from the ESO showing the Pillars within the surrounding Eagle Nebula.
A particular well-known star-forming region is that formed by the 'Pillars of Creation', three light-years-long columns of material within the Eagle Nebula in Serpens. These were the subject of an early image from the Hubble Space Telescope, showing the structure of the region is extreme detail. Many other well-known nebulae are also home to star-forming regions, including the Orion Nebula, the Eta Carinae Nebula and the extensive Sadr Region in central Cygnus.
The closest star-forming regions to the Solar System are rather less prominent than these well-known examples. They are the Corona Australis Molecular Cloud and the Rho Ophiuchi Cloud Complex. Each of these lies less than five hundred light years from the Sun, and each is characterised by dark clouds of material obscuring much of their star formation from view, at least in optical wavelengths.
These cloud complexes are each tens of light years across, but star-forming regions can be far more extensive even than this. The nebula W51 in Aquila, some 17,000 light years from the Sun, is a particularly significant example, a highly active stellar nursery measuring some 350 light years from side to side.