Star formation and stellar ignition is believed to be the final stage in a long process by which a giant molecular cloud collapses into a large central mass and achieves such heat and density that hydrogen atoms in the core begin to fuse together, producing helium. These hydrogen fusion reactions then sustain the star throughout its lifetime, until the core hydrogen is depleted and the star undergoes one of several forms of collapse, depending on its size.
– Interstellar and Giant Molecular Clouds –
Star formation relies on the coalescing of a large cloud of previously free-floating gas and dust in the interstellar medium, so it is believed that stars form most commonly and most quickly in places where this matter is particularly rich: so-called stellar nurseries within planetary nebulae. Nebulae are the massive and beautiful formations shed from the outer layers of long-dead stars; in a sort of slowly declining cycle of life, the remaining material from dead stars is recycled in the birth of new ones. Eventually, not enough free-floating gas and dust remains in a region of space for new stars to form, at which point stellar formation in that region ceases. The nearest large stellar nursery is located within the Orion Nebula, over one thousand light-years away.
In these regions, by far the most prevalent form of matter is free-floating molecular hydrogen, taking the form of what are referred to as giant molecular clouds which stretch over many light-years but may be only modestly more dense than the interstellar medium itself. If a cloud grows large enough, then the gravity of its mass exceeds the energy of the gas pressure and the cloud collapses inward. Within the cloud, several separate and increasingly massive fragments begin to grow where gravity has drawn large amounts of matter together. As the gas and dust moves inward, it begins to heat up, although the entire process generally occurs at only a few dozen degrees above absolute zero.
– From Protostar to Star –
Eventually, this collapsing process is halted by the growing density and heat at the centre. The core itself heats up to thousands of degrees Celsius, and as it does so, the hydrogen and helium molecules separate, forming free-floating hydrogen atoms. As accretion of new material onto the outer surface continues, this core material begins to heat even further, until it grows hot enough and dense enough that the first hydrogen fusion reactions occur, creating a helium atom from a pair of hydrogen atoms. This early formation is known as a protostar, and it continues to draw more energy from the process of gravitational collapse than from hydrogen fusion.
However, as the amount of material being drawn in decreases and the amount of hydrogen being fused in the core increases, eventually the opposite process occurs: hydrogen fusion becomes the main source of energy. The resulting formation becomes what is known as a main-sequence star: a star which falls along a predictable curve defined by the relationship between stellar mass and its colour and brightness. A star will leave the main sequence only once its hydrogen core is exhausted, or as a result of interactions with other stars or interstellar phenomena.
Essentially, the most important variable in determining how quickly and in what particular way this process occurs is the amount of material drawn from the giant molecular cloud. The model described above seems to break down with large stars (those more than several times the mass of our own Sun). In addition, there is something of a grey area at the other end of the scale: so-called brown dwarfs, which are several times the mass of Jupiter but still not large enough to begin stellar ignition and rely on internal hydrogen fusion. Research continues into precisely how high-mass stars are able to make the transition from relying on gravitational collapse to relying on hydrogen fusion, as well as how low-mass objects become either brown dwarfs or red dwarfs.