1.02 The stellar nursery

Interstellar gas clouds

Stars are born inside enormous interstellar gas clouds (called nebula). These clouds contain predominantly hydrogen and some helium produce during the birth of the universe. They also contain about 1 or 2 % of heavier materials produced by previous generations of stars. (The very first stars that formed in the universe were obviously formed from just hydrogen and helium). Many stars are formed from a single massive cloud which may have dimensions as large as tens of light years.

orion nebula

The cloud may have existed in a fairly stable state for billions of years with the gravitational attraction between the gas molecules balanced by their kinetic energy. A shock wave then upsets the equilibrium of the system causing areas within the cloud to collapse. This shock wave could be the result of a distant supernova or perhaps the collision of two clouds. The increased gravitational attraction of the denser regions within the cloud will then pull in even more material. Each of these regions has the potential to develop into a star or a cluster of stars. As material falls inwards it is accelerated due to gravity which increases its kinetic energy. This kinetic energy is then converted into heat as the particles collide with each other as they fall inwards.


As the material from the dense region of cloud falls inwards it begins to spiral as a result of the gas particles initial motion. If enough of this rotational energy can be dissipated as heat as the material falls inwards it will eventually form a rotating sphere of hot gas called a protostar. It takes about a million years for a protostar to form. A large region of the nebula may fragment as its collapses due to its rotational motion, this can lead to the formation of a star cluster as each fragment then condenses into an individual protostar.

As the protostar contracts the temperature at the core rises to extreme levels and the gas becomes compressed to incredible densities. Eventually after about 50 million years the temperature and density become so great that nuclear fusion occurs within the core, the protostar "ignites" and a new star is born. The birth of a new star is obscured from our vision by the surrounding dense cloud, however we can detect its presence from infra red imaging.