It all starts with a giant molecular gas cloud. These clouds can extend over several tens of light-years with an overall mass of between 10^4 and 10^6 solar masses. They have a very low average density of 1000 particles per cubic centimeter. These clouds are not stationary, they are rotating around the galaxy. Since they are very large, the portion of a cloud closest to the galactic center will move faster than the portion that is farthest from it, which will cause it to rotate on itself.
Then an event occurs which will cause the gravitational collapse of the cloud. The following factors may be the cause of this collapse.
1- A first possibility is the passage of the cloud in a zone of high density of matter. Our galaxy does not have a uniform distribution of matter but contains denser than average areas. When a giant molecular cloud passes through one of these zones, it experiences a compressive force that can upset the balance and cause gravitational collapse.
2- Another possible cause is the explosion of a supernova. This event gives rise to a formidable shock wave which violently compresses the regions it crosses and can therefore cause the gravitational collapse of a giant molecular cloud. It is this scenario that is used to explain the formation of our sun.
Once stability is broken, a giant molecular cloud will not simply contract. It begins first by fragmenting into smaller and smaller blocks. As the compression progresses, the density increases in each of these blocks. Their contraction will have the effect of increasing more and more rapidly the rotational movement of these blocks. Each block in the cloud will give birth to a star.
The fragmentation process eventually stops. As long as the blocks of the cloud were transparent, the radiation could escape freely and rid each small cloud of its excess energy. But at a certain point, the gas clouds reach a sufficient density to become opaque and prevent the radiation from eliminating the excess energy. Consequently, the temperature of the cloud which was stable until then begins to rise.
When the fragmentation stops, each small cloud of gas has become a protostar which continues to contract and heat up by converting its gravitational energy into thermal energy. The radiation can still partially escape as long as the temperature remains moderate and the light from the star is in the infrared.
But the contraction continues and the gas eventually becomes opaque. The protostar’s temperature continues to increase, reaching several thousand degrees. The star then begins to shine in the visible range. As its dimensions are enormous, the proto-star is then extremely bright. At this stage of its life, the protostar of our sun was a hundred times brighter than it is today.
In the center of the star, the density and the temperature increase more and more. Finally comes the moment when the core temperature reaches ten million degrees and when the hydrogen fusion nuclear reactions are triggered. At this moment, an enormous quantity of energy is produced and gives rise to an internal pressure which opposes the force of gravity and stabilizes the star. The contraction stops and the star’s life begins.
The duration of a star’s formation is much shorter than its longevity on the main sequence. It strongly depends on the mass of the star considered. It is thus several tens of millions of years for a star like the Sun, but less than 100 thousand years for a star of ten solar masses.
Note again for completeness that not all stars are born in giant molecular clouds. Some, among the less massive, are formed from small molecular clouds, called Bok globules, whose dimensions can go down to less than a light year.