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A star begins as a cloud of interstellar gas, mostly made of hydrogen. Eventually, small density differentials begin the cloud begin to create gravity wells, pulling other particles closer and condensing them. Over time, this process of compaction creates a spherically-shaped central cloud, orbited by the gas on the fringes, creating what is called an accretion disk.
The critical step in the birth of a star is the creation of density levels sufficient to initiate hydrogen fusion. Fusion brings together atomic nuclei lighter than that of iron, releasing energy in the process. The first atoms to fuse in a condensing star-cloud are probably deutrium atoms, an isotope of hydrogen with one neutron. Despite their scarcity relative to conventional hydrogen, they require a lower temperature and pressure to fuse and therefore would probably get started first. Fusing atomic nuclei is difficult to achieve because of the electrostatic repulsion caused by the electron shells of both atoms.
After the deutrium in the star-cloud ignites and begins to release prodigious amounts of energy, it is only a matter of time until the surrounding hydrogen begins to fuse and the celestial body becomes a true star. With a core of a couple dozen million degrees or greater, infant stars are frequently the most energetic bodies for light years around.
The vast majority of atoms from which our bodies are made were synthesized by the fusion of atomic nuclei in a process called stellar nucleosynthesis. Most atoms besides hydrogen are formed in this way.
The further future and lifespan of a star depends on its mass. Most stars spend most of their lifetimes on what is called the Main Sequence, fusing together lightweight nuclei in energetic reactions. As they begin to fuse together all their hydrogen, the stars begin to lose energy. For stars about 0.4 times the mass of our Sun or below, this causes gravitational collapse. The star turns into a homogeneous red dwarf and will never fuse elements again.
For stars 0.4 times the mass of our Sun up until about ten times, helium starts to aggregate in the star's core as the fusion process continues. Helium does not fuse easily, so it just hangs around. Its greater density causes hydrogen to be pushed together very strongly in the layers above it, accelerating the fusion of the remaining hydrogen, and making the star 1,000 to 10,000 times brighter. This produces a red giant, with a radius similar to the distance at which the earth orbits the sun. After the red giant expends its fuel, it collapses violently. The shear force of the matter rubbing together releases a tremendous amount of energy, causing a supernova explosion. Supernovas are some of the most energetic phenomena in the universe, a fitting end to the majestic life of a star.
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