Nebula—a cloud of dust and gas. 70% Hydrogen, 28% Helium, 2% heavier elements. Gravity pulls the nebula together; it spins and flattens into a disk of matter with a central concentration called a protostar.
Pressure and temperature increases in the protostar, until the core reaches about 10,000,000 degrees C, and then nuclear fusion starts, releasing energy, and a star is born.
Main-Sequence Stars Main-sequence stage is the longest stage in a star’s life. Hydrogen is converted to helium by nuclear fusion, and energy is released. The star is in balance. The inward pull of gravity is balanced by the energy from fusion.
Giants and Supergiants 3 rd stage, when hydrogen in the star’s core is exhausted. Hydrogen fusion continues in the star’s outer layers. Core contracts due to gravity, and its temperature increases. Helium in the core fuses into carbon.
Combined helium and hydrogen fusion causes outer shell of star to greatly expand. Star is now a red giant (10 times bigger than sun) or a red supergiant (100 times bigger than sun)
White Dwarf Stars For medium-sized stars, after helium fusion, the giant stage is over. Outer gasses are lost, and a core is revealed, which heats and illuminates the expanding gasses, forming a planetary nebula.
The last inner matter collapses inward due to gravity, forming a hot, dense white dwarf star. It shines for billions of years, but eventually becomes cooler and fainter, until energy emmission stops, and it becomes a black dwarf.
Novas Explosions on the surface of some white dwarfs, causing brightness to increase by thousands of times for a few days. Believed to be caused by gas (from a companion star) buildup on the white dwarf’s surface.
Supernovas Occur when high-mass stars (10 to 100 times mass of the sun) finish the supergiant stage. Gravitational collapse causes fusion to restart…carbon is converted into iron…then gravity causes the iron core to collapse and fuse into all heavier elements, in a gigantic explosion.