Star formation takes place within high molecular density clouds in the interstellar medium. The interstellar medium consists of particles of gas and dust of which 90% is hydrogen, 9 % helium and the remaining is traces of other elements. A molecular cloud will remain in hydrostatic equilibrium if the internal gas pressure is in equal balance with the force of gravity holding it; however, when this balance is disrupted by external forces, the molecular cloud collapses under its own gravity starting the formation of stars.
In the dense regions of the interstellar medium most hydrogen is found in its molecular form. These giant clouds may have diameters of approximately one hundred years and around six million solar masses, and an estimated temperature of 10-100 K in its interior. The core is the region where the stars form. A giant molecular cloud is maintained in balance by gravity which tends to push matter inward and the small quantity of heat exerting pressure outwards produced by its atoms. This balance will be maintained unless external forces act upon the molecular cloud to upset its equilibrium.
A dense molecular cloud will remain in balance or in hydrostatic equilibrium, meaning that the force of gravity pushing inwards is in balance with the heat pressure pushing outwards, but if the dense cloud acquires sufficient mass, it will collapse under its own gravity. Other stellar events may trigger the collapse of a dense molecular cloud, including the shock wave of a supernova explosion, reaching the interior of the cloud; the collision between dense molecular clouds of gas; or the collision between galaxies, producing massive starbursts of star formation.
Low mass star formation
During the collapse, a molecular cloud splits into smaller cloud fragments. Each fragment accrues its own stellar mass. In the process of accretion, the collapsing molecular fragment radiates energy; however, as the density increases, the cloud becomes opaque, preventing the radiation of energy, thus, the temperature increases. At this stage, the cloud has now condensed onto a rotating sphere of gas. The accretion of material continues in the outer portions of the rotating sphere, and jets of outpouring material known as Herbig-Haro objects appear on bot poles of the cloud. Despite the pressure produced by its internal heat, the gravitational force is still greater. When the star reaches a temperature of 10 million K, it starts to fuse hydrogen into helium and is supported against further collapse by hydrostatic equilibrium.
Massive star formation
Stars of distinct masses are believed to form through different processes. Although the mechanism by which stars greater than 8 solar masses form is not completely understood, there are some theories which seek to explain their formation. The competitive accretion theory suggests that stars are small when they´re born with an initial mass of 1.5 solar masses, and soon they begin accreting gas to which they were initially not bound. Another theory states that massive stars may originate by the combination of a number of other low-mass stars.
Molecular clouds of star formation exists I the entire interstellar medium, and a single dense molecular cloud may produce a collection of high and low mass stars. In the Milky Way there are more than 5,000 molecular clouds with up to 100,000 solar masses. The Orion Nebula is the nearest region to the solar system where massive stars are being formed. The most recently discovered main sequence star is known as IRS-8, and was discovered in 2006. This star is believed to be 3.5 million years of age.
