How do stars form?
The universe was created about 14 billion years ago with the "Big Bang," a cosmic explosion that resulted in an expanding cloud of the two lightest elements, the gases hydrogen and helium. At that time there were no other elements. Where there were higher concentrations of gases, the mutual gravitational attractions of the gas molecules led to the growth of the first generation of stars. As more and more material fell into a new star, the pressure at its center finally became high enough to start the process of nuclear fusion, in which the nuclei of hydrogen and helium merge to form heavier elements. This was accompanied by the release of energy, which made the star begin to shine.
Eventually, all the hydrogen and helium, and those products that could be used to generate energy in the core of the star, were exhausted, and the nuclear furnace was extinguished. The outer layers of the star could no longer resist the central force of gravity, which was pulling the star's outer matter inward toward its core. What happened next depended on the mass of the star. In some cases, the star became a supernova, exploding violently, and rapidly creating even heavier elements, spewing much of the stellar material into space. In other cases, the process was slower: instead of an explosion, elements from the star's interior zones rose to the surface and were then lost to space when the outer layers blew off. The end results were similar: the space between the stars was enriched with heavy elements, many of which condensed to form small solid grains. The processes of the birth and death of stars occurred over and over again, with each successive generation of stars starting off with a greater quantity of heavy elements than the previous generation.
Eagle Nebula, Lagoon Nebula, Orion Nebula (Hubble Space Telescope)
These images are examples of regions in our Milky Way galaxy that are enriched areas capable of undergoing star formation. The Eagle, Lagoon and Orion Nebulas all show the presence of heavy elements, an abundance of dust, and physical structures that appear to be fragmenting into newly formed protostars.
The birth of our Sun
The Sun and our solar system formed at nearly the same time, out of the solar nebula — an enriched interstellar gas and dust cloud that existed where our solar system now resides. The order of events for the birth of our Sun was similar to what we see in newly forming stars today. Compression from nearby disturbances causes the interstellar medium to collapse. Small clumps of gas and dust break away from the larger cloud. These denser knots have gravity and begin to gather other gas and dust around them. Temperatures begin to rise due to the further increase in gravity from material in the collapsing gas cloud. The center of this fragmented gas cloud evolves quickly (over the next 100,000 years) to form a protostar. Once pressure and temperatures reach a peak, nuclear fusion begins.
All stars in our galaxy, and in all galaxies, use this process of nuclear fusion to create energy, light and heat. The initial mass of the star is an important factor involved with the life of a star, and helps to answer many other questions: how long will the star live, at what temperature will it emit radiation, and at what part of the electromagnetic spectrum will the radiation be emitted. Stars also make up much of the visible portion of the universe, and help to illuminate gas, dust and even planets that may orbit a star.