<>    A star begins its life in the form of a Giant Molecular Cloud. The density of empty space is around 0.1 to 1 particles per cm³; the cloud, on the other hand, contains hundreds per cm³. The cloud is stable, but its  molecules are too widely spaced for gravity to draw them closer, until a supernova explodes nearby, sending out a shockwave of successive compression forming knots of matter or cores of greater density. When density exceeds 100,000 atoms / cm³, gravity takes over  and the region begins to collapse into a protostar (each dense core will produce anywhere from 1 protostar to tens of thousands). The atoms gain speed in their fall toward the center, providing the protostar with heat and a weak infrared glow. In some protostars, contraction remains the only source of energy; these are brown dwarfs, and they die away slowly, over hundreds of billions of years (did I say slowly?). If a protostar is massive, though, if matter at the center has enough on top of it, then the threshold is around 15 MK (15 million degrees Celsius) and the electrons are stripped from their parent atoms, creating a plasma. Contraction continues, and eventually the speed of the atomic nuclei is great enough to overcome the electrical repulsion keeping them apart and nuclear fusion occurs: hydrogen nuclei fuse to form helium in the proton-proton chain or by the CNO cycle. In doing so the protostar gives off a tremendous amount of energy, which pours out from the core, setting up an outward pressure in the gas around it that balances the inward pull of gravity, stopping the protostar's contraction. When the energy reaches the outer layers, it continues into space in the form of electromagnetic radiation and among other things, visible light.

< style="color: rgb(255, 204, 0);">When a star is formed, it takes on a color from blue to red that depends on its temperature, composition, and mass.The future of the star depends on which road it takes; either allowing gravity to overcome its fusion reaction that pushes out or the opposite. When a star forms and takes its position it will rest there for a period of millions (for the biggest and hottest stars) to billions (for mid-sized stars like the Sun) to tens or hundreds of billions (for red dwarfs) of years, expending most of the hydrogen in its core. Eventually the supply of hydrogen runs out and the star begins its demise. Once the star's hydrogen runs out there will be no more nuclear processes and therefore no outward presssure. The outer layers of the star will begin to collapse and during the process the temperature and pressure increase forcing helium fusion. This heat temporarily counteracts gravity and the outer layers expand as much as 100 times its original size, being known as a red giant. What happens next depends on the star's mass. If the star has a relatively low mass, then all scientists have to go upon is theory and computer generations that predict either red or brown dwarfs because as a small star, it would burn its fuel less quickly and live much longer. The universe as we know it is too young  and this has not happened yet.

                                                                                                  Doomed Star Eta Carinae
                                                Credit: J. Morse (Arizona State U.), K. Davidson (U. Minnesota) et al., WFPC2, HST, NASA

  If the star is about .4-1.4 times the mass of the Sun then its fate is a white dwarf, where it undergoes a red giant phase and then shrinks to about 20% of its original mass and cools for millions of years until it becomes a black dwarf, but none of these exitst yet due to the age of the universe. The fate of large stars is known to be one of the most remarkable, but not completely understood phenomenae in the universe, the super novae.  The star first takes the form of a red supergiant and causes a series of nuclear reactions causing the combination of its heavier elements, and therefore removing its reaction capability and then losing its ability to counteract gravity. What happens next is not clearly understood, but a super novae takes place in matters of a fraction of a second. Its  neutrinos send a  shockwave out blasting most of the stars materials into space, but  many of the elements get caught up in  neutrinos and combine to create  heavier elements. Without supernovae there would be no heavier elements than Iron-56.