The Birth of a Star
Gravity and Internal Pressure
In order to contract an isolated cloud free from external influences, it's necessary that the gravity, responsible for the contraction, prevails over the internal pressure, responsible for the scattering of the matter. Therefore, at the temperature at which are found the clouds in our galaxy, it's estimated that 1000 solar masses are necessary for one cloud to collapse. If the quantity of matter in the cloud doesn't reach that value, then it's no longer enough to overcome the pressure.
The Orion nebula: birthplace of stars (C. O'Dell, S. Wong)
The giant cloud will split in smaller fragments that will give birth to the stars. The process may still be eased by external factors, like the collision with other clouds that provoke the compression of the matter and, namely, with material proceeding from neighbouring supernovae.
The contraction of the matter will cause an increase of the density and will make the temperature rise to 1 million degrees (ºK). Nevertheless, given the transparency of the material, the energy released by the collapse is constantly irradiated (particularly by the water molecules that it contains), which keeps the cloud temperature at comparatively low levels and allows the continuation of the contraction process.
Along with the irradiation of energy, they are also observed flows of material ejected from the interior of the cloud, which counteract the action of the material that is increasingly compressed inside the cloud.
When the density reaches even higher values, the cloud becomes opaque and the energy is no longer irradiated to the space. The peripheral zones keep contracting and, therefore, are responsible for the continuation of the material and heat accumulation in the opaque zone.
Proto-star, with a bipolar jet of matter expelled from its closest neighbourhood (HST)
The Childhood of the Stars
When this material is exhausted, the collapse proceeds until the core becomes hot enough to fuse the hydrogen into helium. It's estimated that, during the collapse, the energy generated by it is transmitted to the surface in a violent and discontinuous way, so that the star may display sudden luminosity increases of as much as hundreds or thousands of times, during the scale of some dozen days. These stars are called variable T-Tauri.
In the case of the Sun, the beginning of the hydrogen fusion occurred after 10 million years of contraction, keeping contracting during the subsequent 20 million years until it reached the stability that characterizes it up to our days. The formation of smaller stars is generally slower than the formation of the bigger stars.
Surrounding the young stars are frequently observed gaseous and dusty disks that may well be the seed of planetary systems like the solar system. They are also observed holes in some of the studied disks, which may denounce the presence of planets sweeping the primordial matter from their orbits. It's estimated that the necessary period for a planetary system to form is about 10 million years.
Proto-planetary disks in the Orion nebula (M. McGaughrean, C. O'Dell)