Source: bioch.szote.u-szeged.hu/ astrojan/nebula1.htm

Pictured above is one such nebula

Stars are born in the interstellar clouds of gas and dust called nebulae that are primarily found in the spiral arms of galaxies. These clouds are composed mainly of hydrogen gas but also contain carbon, oxygen and various other elements, but we will see that the carbon and oxygen play a crucial role in star formation so they get special mention. A nebula by itself is not enough to form a star however, and it requires the assistance of some outside force. A close passing star or a shock wave from a supernova or some other event can have just the needed effect. It is the same idea as having a number of marbles on a trampoline and then rolling a larger ball through the middle of them or around the edges. The marbles will conglomerate around the path of the ball, and as more marbles clump together, still more will be attracted. This is essentially what happens during the formation of a star (Stellar Birth, 2004).

If the nebula is dense enough, certain regions of it will begin to gravitationally collapse after being disturbed. As it collapses the particles begin to move more rapidly, which on a molecular level is actually heat, and photons are emitted that drive off the remaining dust and gas. Once the cloud has collapsed enough to cause the core temperature to reach ten-million degrees Celsius, nuclear fusion starts in its core and this ball of gas and dust is now a star. It begins its life as a main sequence star and little does it know its entire life has already been predetermined.

Although this may sound like a simple enough process there are actually several variables that must be just right for birth to happen. For one, the mass of the collapsing particles is crucial and there are only certain allowable sizes. If the mass is too small the core will not be able to create enough compressional heat to trigger nuclear fusion and it will not become a “real” star. But instead it will become what is called a red dwarf. It still may be able to create its own heat, much like Jupiter in our own solar system, but by definition it will not be a star. On the other hand if the mass is too large, namely over a hundred times the mass of our own sun, gravity will have a death grip on it from the very beginning and the cloud will bypass all the life stages of a normal star and be crushed into oblivion.

Even if the mass is the correct size to form a star there are still a few other things that must all be in place to allow the formation. Carbon and Oxygen must both be present. Here we come back to the crucial roll these two elements play that was mentioned earlier. As the cloud of gas and dust gravitationally collapses and the particles begin to heat up i.e. pick up speed, they bang against one another violently and they begin to exert an outward pressure that would normally stop any further shrinking. Further more if there were other birthing stars nearby, as there often are, the U.V and visible light from the few beginning stars would drive off the rest of the gas and dust, putting an end to any hope of creating other stars. This does not in fact happen and it is due to the presence of carbon and oxygen. When carbon and oxygen are struck by photons or other energetic rays, they absorb the original energy and radiate it away in the form of infrared which is able to pass out of the collapsing star without further hindrance. In this way carbon and oxygen are able to keep a star cool enough in its initial collapse to keep it collapsing and keep neighboring gas and dust intact for further star formation (Brusca, 2004).