Stars are "born" from the gravitational collapse of cool, dense interstellar clouds. A cloud has to be of a certain mass for gravitational collapse to occur, & a trigger is needed for the collapse to start, since the clouds are held up by their own internal pressure & magnetic fields. Such a trigger might be as simple as the gravitational tug from a passing star, or it might be a shock wave caused by the blast of a supernova, or the collision of two or more galaxies. In the Milky Way, density waves move through the dust and gas in the galactic disk. As the waves pass, they temporarily increase the local density of interstellar material, triggering the collapse. The material heats up as it collapses until, in some clouds, the temperature & pressure at their centers become so great that nuclear fusion begins & a star is born.
TOWARD THE MAIN SEQUENCE:
At this stage, stars are very unstable. They lose mass by expelling strong stellar winds, which are often directed in two opposing jets chanelled by a disk of dust and gas at their equators (the accretion disk). Gradually, balance between gravity and pressure are equalized, and stars settle down on to the main sequence, where stars spend 90% of their life, fusing hydrogen into helium at its core.
GROWING OLD & BUYING THE FARM
When a star of the Sun's mass exhausts the hydrogen at its core, hydrogen-shell burning begins, and it expands into a red giant, often losing its outer layers to produce a planetary nebula. It eventually collapses, and the temperature & pressure at its core initiate helium-core burning. The star again expands as helium-shell burning occurs, before finally collapsing to become a white dwarf (about Earth-sized) that gradually fades to black.