The length of a star’s life and what takes place in that life span depends mainly on its total mass. The more massive the star, the longer and more dramatically it lives, and dies. The smallest stars go out with a quiet unseen whimper, while the biggest ones never really die but become black holes.
Regardless of mass, all stars share the same general birth, formed within diffuse nebulae, molecular clouds sometimes referred to as stellar nurseries; the nearest example of which is Orion’s Nebula in the constellation Orion. These nebulae possess a size and density conducive to the formation of molecules, primarily hydrogen with a tiny amount of helium. Within this nebular haze of gas and dust, gravity exerts an unequal force, forming the molecules into clumps. These clumps possess their own gravitational force. This gravity attracts more atoms to its mass. As the clumps grow, gravity increases, causing molecular collapse within the clump, which in turn, increases the mass of the core and subsequently, its gravity, and more atoms are gathered. This gathering of molecular mass is called accretion and results in the formation of the protostar.
As the protostar eliminates binding gravitational energy, heat is lost through the shell via radiant energy from gas expansion. This loss of energy causes the core of the protostar to continue to collapse, resulting in increased core temperature, pressure, and density.
In order for a protostar to become a star, it must reach equilibrium between these opposing forces of expansion and contraction before nuclear fusion can begin. Eventually, the core is so dense that it becomes opaque. When gas molecules fall toward this opacity, shock waves create additional heat. When the core reaches 2000 Kelvin, nuclear fusion of hydrogen into helium begins, and lasts until the hydrogen is depleted.
If it cannot do this, then it becomes a brown dwarf. The brownish red shade of failed protostars is remnant radiant energy from the failed fusion.
Thus begins the battle of equilibrium that will last for the duration of the star’s energetic life. The major portion of any star’s life, spent in fusion, is the main sequence phase. When there is no more hydrogen, helium is fused into carbon. The more massive stars continue the process of fusion, creating increasingly heavier elements according to the periodic table, while maintaining equilibrium.
Toward the end of the fuel supply, the core contracts further while the atmosphere inflates and expands, transforming the star into a Giant or Super Giant; usually Red but sometimes Blue, dependent upon mass and fusion activity, respectively. Sun-like stars, with a mass less than 1.5 times that of our sun, become Giants and the larger stars become Super Giants. Blue Giants are not blue in color but rather are an advanced stage of stellar evolution in which there is still fusion of helium taking place in the core. Red Giants have inactive helium cores surrounded by shells still in the process of fusion.
Following the Red Giant stage, the sun-like smaller stars lose most of their mass when their outer layers drift outward in clouds called Planetary Nebula. This leaves the core behind to cool and shrink as a White Dwarf. When all heat and energy is finally lost, the star fades into the cold darkness around it, ending its life as a Black Dwarf.
Huge stars have a mass between 1.5 to 3 times that of our sun and slow down as Super Giants until gravity wins the battle of equilibrium with a violent demonstration. There is an instant collapse in the core followed by a short, but massive, explosion of the shell and outer layers. The result is the brilliant display of a Supernova, now destined to become a Neutron Star. Just as with the smaller stars, the loss of the outer shell leaves the leftover core to collapse, but in a far more dramatic fashion. Such a star, having a greater mass than our sun, potentially even three times as much, is suddenly compressed into an area that is only about 10 miles in diameter. This results in both a very rapid spin as well as a gravitational field so strong that is difficult to quantify and/or imagine. Five milliliters, one teaspoon, of neutron star material would weigh fifty trillion kilograms!
The biggest stars are called Giant Stars and have a mass three times greater than the sun. After the Supernova phase, their size further intensifies the collapse event, causing a force of gravity in the core so strong that it the star falls in on itself to become a black hole.
