Protostar

The starting point in the birth of a star is known as protostar. A protostar is a big celestial body formed by compression of a giant molecular cloud. A protostar has the ability to develop into a star in the future. The phase where the process of formation of a protostar begins, is known as protostellar phase.This is an early stage in the formation of a star, which lasts for around 100,000 years. This phase ends with the formation a T Tauri star (which is a class of stars).

Stages of Development of a Protostar

The space between the stars within a galaxy is termed as interstellar space. This space is filled with molecules of gases and dust particles. However, the density of this cloud of dust and gas is not uniform. Those regions which have a higher density are called molecular clouds. A molecular cloud comprises a large quantity of hydrogen, around 25 percent of helium and a small quantity of heavier elements. When the size and density of a molecular cloud increases to a certain level, then the condition is just right for the development of a protostar.

Normally a balance exists in a molecular cloud, between the compressive pressure of gravitational energy and the internal thermal pressure in the opposite direction. However, if any kind of major disturbance occurs, then the equilibrium gets affected. Such disturbances may happen due to massive stellar explosions called supernova or collisions between two galaxies. They cause a lot of instability within a molecular cloud. Due to this instability, a section of the molecular cloud may begin to contract under its own gravitational force and form separate clusters of thick, dark clouds of dust and gas. These clusters are called Bok globules.

As the contraction process of a globule continues, there is a drastic increase in its density. This high density converts the gravitational energy into heat energy and results in a tremendous rise in the temperature of the globule. This goes on, until gradually a stage is reached, when the thermal pressure and the pressure due to gravity become equal. It is in this stage that the protostar is formed at the core of the globule. The protostar is always surrounded by a ring of clouds called proplanetary disk.

A protostar can become a star, if it can sustain the state of equilibrium between the pulling force of its gravity and the pushing/outward thrust of the heated gases. Maintaining this balance is quite tough, as there are many other reactions also going on within a protostar. All these reactions make it a very unstable entity.

The existence of a protostar continues, till the phenomenon of contraction continues within it. This could last for anything between 10-15 million years. When it stops, the growth of the protostar ceases and the disk encircling it, is destroyed by nuclear radiation. The protostar changes to a pre-main sequence star. If the mass of the star is small, it would be called a T Tauri star, and if bigger, then it would be called a Herbig Ae/Be star.