Stars

Fission has nothing to do with it. Fusion does, however. Fission is the process by which large nuclei (like Uranium) split apart releasing energy. Fusion is the joining of nuclei to make larger nuclei, with the release of energy. A ball of contracting interstellar gas becomes a star like the Sun when fusion reactions start in its center. Then, we say "a star is born."

No specific gases are actually needed. It's more like this: the interstellar gas is predominately hydrogen gas. It is hydrogen that is fused at the center of the gas sphere when the gas sphere becomes a star. However, if the cloud were made of carbon gas, for instance, the sphere could eventually fuse carbon nuclei together (to make larger nuclei), as it first "turned on." Just about any gas will do. The different sorts of gas spheres would produced different sorts of stars (brighter or dimmer, larger or smaller,...).

Stars appear to be born together in large "clusters" (groups) in clouds of large enough density (so gravity can cause the stars to "condense" in the gas). The clouds in which the conditions are right are clouds made mostly of molecular hydrogen, the so-called "molecular clouds." The clusters of young stars are found in or near these molecular clouds.

If you mean, "at what temperature does fusion start at the center?", then the answer is about 10 million degrees Kelvin (10 million degrees above absolute zero). In terms of Fahrenheit temperature that's about 20 million degrees Fahrenheit, in round numbers. In answer to why it must get so hot, consider the following. In order for hydrogen nuclei (one hydrogen nucleus is a proton) to fuse, they must come into contact with each other. However, a proton is a positively electrically charged particle. Like electrically charged particles repel each other with a strong electric force. Therefore, two separate protons will repel each other strongly. The only way they can come into contact is if they run at each other at very high speed (in order to overcome the repulsive force). It's as if you were wearing a large balloon around wrapped all around your waist, and you wished to touch a wall, but could not reach it since the balloon was preventing you from getting close enough to the wall. One way you could "overpower" the balloon would be to step back, and run at the wall --- the balloon would slow you down quite a bit, but you would get much closer to the wall, even perhaps close enough to touch it. So, in the star, the protons need to run around at high speeds to be able to touch during collisions. When the particles in a gas have high speeds, we say the "temperature of the gas is high." Actually, temperature is just a measure of the speeds of motion of the particles in the gas.

To make a star like the Sun, the process takes some tens of millions of years. As to why, that's actually a very complex question. When the gas sphere that will become a starfirst condenses out of the interstellar gas cloud, the sphere is quite large in radius (compared to the Sun). It must shrink in size, squashing itself under its own gravity until, because of the pressure of the squashing, the central temperature reaches 10 million degrees Kelvin and fusion starts (it becomes a Sun). The shrinking is only possible since the sphere is losing energy from its surface (light is escaping), and so the material is settling down under gravity. Once the central temperature rises to 10 million Kelvin, fusion starts, energy is released by the fusion, replacing the lost energy from the surface, and the sphere stops contracting --- it is now a star like the Sun. The process of losing energy, and shrinking, while the central temperature rises as the pressure builds, all takes 10 million years or more. Why so long? Well, it' a combination of two things. First, such a large amount of gas (as is contained in the star) must lose a large amount of energy (by light lost from the surface) before it can settle down to the radius of the Sun. Second, it is losing that energy at a specific rate dictated by the way energy is transported up the surface from the interior. Both of these facts determine how long the entire shrinking process will take place. As I said, it is complex, but one can perhaps think of it as taking a long time because there is so much work (energy loss) that a gas sphere must do to shrink to the size of the Sun.