Stars form from interstellar gas, with gravity creating an accretion disk. Hydrogen fusion is initiated by sufficient density levels, starting with deutrium. Stellar nucleosynthesis creates most atoms, with a star’s future depending on its mass. Red dwarfs form from gravitational collapse, while larger stars become red giants before collapsing into supernovae.
A star begins as a cloud of interstellar gas, composed mostly of hydrogen. Eventually, small differentials in density begin to cause the cloud to start creating gravity wells, bringing other particles closer together and condensing them. Over time, this compaction process creates a central spherical cloud, orbited by gas at the edges, creating what is called an accretion disk.
The critical step in the birth of a star is the creation of sufficient density levels 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 stellar cloud are probably atoms of deutrium, an isotope of hydrogen with one neutron. Despite their scarcity compared to conventional hydrogen, they require a lower temperature and pressure to fuse and would therefore likely start first. Fusion of atomic nuclei is difficult to achieve due to the electrostatic repulsion caused by the electron shells of both atoms.
After the deutrium in the stellar cloud ignites and begins releasing prodigious amounts of energy, it is only a matter of time before the surrounding hydrogen begins to fuse and the celestial body becomes a true star. With cores a dozen million degrees or more, baby stars are often the most energetic bodies for light-years around.
The vast majority of atoms that make up our bodies were synthesized by the fusion of atomic nuclei in a process called stellar nucleosynthesis. Most atoms other than hydrogen are formed this way.
The further future and life span of a star depends on its mass. Most stars spend most of their lives in what’s called the main sequence, fusing light nuclei together in energetic reactions. When stars start fusing all of their hydrogen together, stars start to lose energy. For stars about 0.4 times the mass of our Sun or smaller, this causes gravitational collapse. The star turns into a homogeneous red dwarf and will never fuse the elements again.
For stars from 0.4 times the mass of our Sun to about ten times the mass, helium begins to aggregate in the star’s core as the fusion process continues. Helium doesn’t melt easily, so it freezes. Its higher 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 more luminous. This produces a red giant, with a radius similar to the distance the earth orbits the sun. After the red giant consumes its fuel, it collapses violently. The shear force of matter rubbing together releases a huge amount of energy, causing a supernova explosion. Supernovae are some of the most energetic phenomena in the universe, a worthy conclusion to the majestic life of a star.
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