The Life of A Star
Stellar Evolution

Formation of the Heavy Elements

A nebula (gas cloud) slowly contracts under the mutual gravity of all of the atoms in the cloud.  As the nebula contracts, whatever initial tumbling/rotational motion is increased to conserve angular momentum (just like when a spinning figure skater brings in their arms).  The nebula flattens into a disk.  Gravitational contraction also causes the pressure to increase which causes the cloud to heat up.  Mass concentrates into the center and pressures and temperatures here become the greatest. The core of the nebula, the proto-star, starts to glow red hot. Soon, in less than 100,000 years, the pressure and temperature (~20 million K) in the core of the protostar are so great that the nuclei of hydrogen atoms begin to fuse together to form helium. This nuclear reaction releases huge amount of energy. The "thermonuclear fires" are lit; a star is born! The energy release yields an outward pressure and the gravitational contraction stops.  The young star goes through a period known as the T-Tauri phase during which it gives off a very active "solar" wind of protons and neutrons that clears out most of the remaining gas, dust, and fragments smaller than about 1 m from the inner nebula.

main stage
Once the thermonuclear fires are lit, a protostar becomes a "main stage" star, one that generates its energy principally by fusing hydrogen into helium.  Small stars burn their hydrogen fuel slowly and remain on the main stage for billions of years.  Our sun should survive to a ripe old age of around 9 billion years.  It is now a bit over 4.5 billion.  On the other hand, very large stars burn the fuel rapidly and may only last for on the order of 10 million years.

death and nucleosynthesis
Eventually the star fuses so much H into He that the H concentration in the core is reduced and the thermonuclear reactions becomes sluggish. The star's core cools and begins to contract again. This secondary contraction increases the pressure and temperature of the star's core.  The fate of the star depends on the star's size.

- Stars smaller than our sun will slowly cool and fade.

- In intermediate mass stars (~0.8 - 8 times the size of our sun) the temperature and pressure attained in this secondary contraction phase will allow He to fuse into carbon and heavier elements.  The outer shell of the star also expands and the surface therefore cools and reddens.  During the short, red giant phase of stellar evolution elements up to the mass of iron (Fe) may be produced in the core depending on the star's mass. The star's expanding outer shell is blown away by a strong "solar" wind and forms an expanding ring (planetary nebula) and after the fusion reactions end the core of the star collapses to a very dense state to become a white dwarf (~the size of the Earth but as massive as the sun).  The white dwarf will gradually cool and become a black dwarf.

- In high mass stars (~8 - 20 times our sun) the pressures and temperatures generated in the core from the secondary contraction phase are so great that fusion reaction produce even the most massive atomic nuclei forming the largest elements of the periodic table in a very rapid set of reactions that release prodigious amounts of energy resulting in a stupendous explosion called a supernova. Supernovae spread these elements into interstellar space where they may eventually become part of another contracting nebula. The remaining 10% of its mass remains as an extremely dense neutron star (its huge mass packed into a sphere with a radius of about 10 km).

- In very high mass stars (~20 - 100 times the size of our sun), the mass, and therefore gravity is so great that when it goes into its late contraction phase the pressure in the interior is so great that its mass collapses to a size a few kilometers across and a density greater than 1016 g/cm3  [rock has a density of 2.5-3 g/cm3].  A black hole is born.  The gravity is so great that not even light can escape.