Question

How do random motion and gravitational collapse combine to convert giant molecular clouds into open star clusters over a long period of time?

Answer 1

There are three basic stages of star formation-

(i) Molecular cloud/ Dark Nebula

(ii) Cloud fragments

(iii) Protostars

Stars form by relatively denser gas and dust than we really see in our environment. These interestellar gas and dust are called as molecular clouds. These regions are extemely cold about 10 to 20 kelvin, temperature just above the absolute temperature. At such low temperature gas atoms combine with each other to form molecules. CO and H₂ are the most common molecules in interestellar gas clouds.Further deep cold temperature causes to clump these molecules. These regions are dense enough so that light can't pass through. that is why these cloud clumps are known as dark nebula.

Now in the second phase, star formation really begins, when these clumps starts combining under their own gravity.These cores typically have masses around 10⁴ solar masses in the form of gas and dust. The cores are denser than the outer cloud, so they collapse first. As the cores collapse they fragment into clumps around 0.1 parsecs in size and 10 to 50 solar masses in mass. These clumps then form into protostars and the whole process takes about 10 millions years.

As the protostar forms, loose gas falls into its center. The infalling gas releases kinetic energy in the form of heat and the temperature and pressure in the center of the protostar goes up. As its temperature approaches thousands of degrees, it becomes a IR source.This process of becoming a protostar to a young star could be understood in following cotegories-

1. When gas clumps breaks off from molecular clouds and collapses. This process causes production of angular momentum. This Angular momentum turns irregular gas clump into rotating disc.

2. The central denser region of this disc is protostar. Now this very dense part attracts matters, and infalling matters to the prorostar increses int's size rapidely about 100 times to the initial size.

3. Now after such a bigger size in the core of this growing star temperature, pressure rises very high causes to start thermo-nuclear fusion. And this stage infall stops and strong steller wind produces.

The protostar is now considered a young star since its mass is fixed, and its future evolution is now set.