What occurs to make a sphere of gas into a star?

Two great cosmic forces conspire to create a star from gas. Gravity is one, and time is the other. With sufficient material (the gas - mostly hydrogen), and over a great length of time, gas will be attracted by other gas due to gravity. As more gathers, more gravity attracts even more gas. It is a self-generating event if enough gas is initially present, as would be in a stellar nursery. As the "critical mass" of material accretes, the gravity - now overwhelming - pulls everything in and compresses the heck out of it. This heats the gas. At sufficient temperature, about 10 million degrees Kelvin, the proton-proton reaction that is nuclear fusion begins, and the star "turns on" for the first time. Hydrogen is being turned into helium. The star's initial fuel load will determine its type and its eventual fate. You got a link if you want it.