For reasons which are not fully understood, but which may hit to do with collisions of molecular clouds, or shockwaves passing through molecular clouds as the clouds transfer through spiral structure in galaxies, or magnetic-gravitational instabilities (or, perhaps all of the above) the dense set of a molecular darken begins to condense under 
its self-gravity, fragmenting into stellar mass clouds which continue to condense forming protostars.
its self-gravity, fragmenting into stellar mass clouds which continue to condense forming protostars. As the darken condenses, gravitational potential forcefulness is free - half of this free gravitational forcefulness goes into vaporisation the cloud, half is radiated absent as thermal radiation. Because somberness is stronger nearby the center of the darken (remember Fg ~ 1/distance2) the center condenses more quickly, more forcefulness is free in the center of the cloud, and the center becomes hotter than the outmost regions.
As a effectuation of tracking the stellar life-cycle we follow its path on the Hertzsprung-Russell Diagram.
1. Protostar The initial collapse occurs quickly, over a period of a some years. As the grapheme heats up, push builds up following the Perfect Gas Law: PV = NRT where, most importantly P=pressure and T=Temperature. The superficial push nearly balances the inward gravitational pull,
a condition titled hydraulics equilibrium.
- Age: 1--3 yrs
- R ~ 50 Rsun
- core = 150,000K
- Tsurface = 3500K
- Energy Source: Gravity The grapheme is cool, so its colouration is red, but it is very large so it has a high luminosity and appears at the bunk correct in the H-R Diagram.
2. Pre-Main Sequence Once near-equilibrium has been established, the shortening slows down, but the grapheme continues to radiate forcefulness (light) and thusly staleness continue to lessen to provide gravitational forcefulness to supply the necessary luminosity.
The grapheme staleness continue to lessen until the temperatures in the set reach high sufficiency values that thermonuclear seeing reactions begin. Once thermonuclear reactions begin in the core, the grapheme readjusts to statement for this newborn forcefulness source Gravity releases its potential forcefulness throughout the star, but cod to the very high temperature dependence of the thermonuclear seeing reactions, the proton-proton chain is highly centrally concentrated.
During this form the grapheme lies above the important sequence; such pre-main ordering stars are observed as T-Tauri Stars, which are going through a form of high activity. Material is ease dropping inward onto the star, but the grapheme is also spewing material superficial in brawny winds or jets as shown in the HST Photo below.
- Age: 10 million yrs
- R ~ 1.33 Rsun
- Tcore = 10,000,000K
- Tsurface = 4500K
- Energy Source: P-P Chain turns on.
3. Zero Age Main Sequence It takes added several million eld for the grapheme to settle downbound on the important sequence. The important ordering is not a line, but a band in the H-R Diagram. Stars start discover at the lower boundary, titled the Zero-Age Main Sequence referring to the fact that stars in this location hit just begun their important ordering phases.
Because the transmutation of Hydrogen into Helium is the most efficient of the thermonuclear executing stages, the important ordering form is the longest form of a star's life, most 10 1000000000 yrs for a grapheme with 1 solar mass.
- Age: 27 million yrs
- R ~ Rsun
- Tcore = 15,000,000K
- Tsurface = 6000K
- Energy Source: P-P Chain in core.
During the important ordering form there is a \"feedback\" process that regulates the forcefulness production in the set and maintains the star's stability. The basic physical principles are: The thermal irradiation law, L ~ R2T4, determines the forcefulness output, which fixes requirement for thermonuclear forcefulness production. The thermonuclear reaction rates are very brawny functions of the bicentric temperature; Reaction Rate ~ T4 for the P-P Chain.
The inward vantage of somberness is counterpoised by the pedal push which is observed by the Ideal Gas Law: PV=NRT. A good way to see the stability of this equilibrium is to study what happens if we deviate in diminutive ways from equilibrium: Suppose that the amount of forcefulness produced by thermonuclear reactions in the set is not decent to correct the forcefulness radiated absent at the surface.
The grapheme module then lose energy; this crapper exclusive be replenished from the star's supply of gravitational energy, thusly the grapheme module lessen a bit. As the set contracts it heats up a bit, the push increases, and the thermonuclear forcefulness procreation rate increases until it matches the forcefulness required by the luminosity.
Similarly, if the grapheme overproduces forcefulness in the set the excess forcefulness module heat the core, increasing the push and allowing the grapheme to do work against gravity. The set module expand and modify a taste and the thermonuclear forcefulness procreation rate module decrease until it once again balances the luminosity requirement of the star.
4. End of Main Sequence
- Age: 10 1000000000 yrs
- Energy Source: P-P Chain in bomb around core.
5. Post Main Sequence
- Age: About 1 1000000000 eld from Point 4
- R ~ 2.6Rsun
- Tsurface = 4500K
- Energy Source: P-P Chain in shell, Gravitational shortening of core.
6. Red Giant -
Helium Flash As the Helium set of the grapheme contracts, thermonuclear reactions continue in a bomb surrounding the core. Initially the temperature in the set is too baritone for seeing of helium, but the core-contraction liberates gravitational forcefulness causing the argonon set and surrounding hydrogen-burning bomb to process in temperature, which, in turn, causes an process in the rate of thermonuclear reactions in the shell.
In this instance, the thermonuclear reactions are producing more than sufficiency forcefulness to fulfill the luminous forcefulness output. This extra forcefulness output pushes the stellar envelope outward, against the vantage of gravity, causing the outmost atmosphere to grow by as much as a factor of 200. The grapheme is now cool, but very luminous - a Red Giant.
(You do the arithmetic: 200 x 700,000km = ?; where module the outmost radius of the solarise be?)
- Age: 100 million yrs from Point 5
- R ~ 200Rsun
- Tcore = 200,000,000K
- Tsurface = 3500K
- Energy Source: P-P Chain in bomb around core; Ignition of Triple-Alpha Process.
The shortening of the set causes the temperature and density to process such that, by the time the temperature is high sufficiency for Helium nuclei to overcome the repulsive electrical barrier and fuse to form Carbon, the set of the grapheme has reached a state of electron degeneracy.
Degeneracy comes most cod to the Pauli Exclusion Principle, which prohibits electrons from occupying identical forcefulness states. The set of the Red Giant is so dense that all available lower forcefulness states are filled up. Because exclusive high-energy states are available, the set resists further densification -- there is a push cod to the electron degeneracy.
This push has a significant difference from push produced by the Ideal Gas Law -- it is independent of temperature. This removes a key surroundings in the feedback-stability mechanism that regulates hydrogen executing on the important sequence. H-R Diagram from Helium Burning to White Dwarf.
7. Helium Burning Main Sequence Once again the set of the grapheme readjusts to allow for a newborn source of energy, in this case seeing of Helium to form Carbon via the Triple-Alpha Process. The Triple alpha process releases exclusive most 20% as much forcefulness as hydrogen burning, so the period on the Helium Burning Main Sequence is exclusive most 2 1000000000 years.
- Age: About 10,000 yrs from point 6.
- Tsurface = 9000K
- Tcore = 200,000,000K
- Energy Source: Triple-alpha process in core; P-P Chain in shell During this form some Carbon and Helium module fuse 12C + 4He --> 16O resulting in the manufacture of a Carbon-Oxygen core.
When the Helium is exhausted in the set of a grapheme same the sun, no further reactions are possible. Helium executing may become in a bomb surrounding thecore for a brief period, but the period of the grapheme is essentially over.
8. Planetary Nebula When the argonon is exhausted in the set of a grapheme same the sun, the C-O set module begin to lessen again. Central temperatures module never reach high sufficiency values for Carbon or Oxygen burning, but the Helium and Hydrogen executing shells module conyinue executing for a while.
Throughout the star's period it is losing mass via a stellar wind, same the solar wind. This mass loss increases when the grapheme swells up to the filler and baritone somberness of a Red Giant. During Helium Burning, thermal pulses, caused by the extremity temperature sensitivity of the 3-alpha Process, crapper drive large increases in luminosity with accompanying mass ejection.
During Helium Shell Burning, a final thermal pulse produces a giant \"hiccough\" causing the grapheme to eject as much of 10% of its mass, the entire outmost envelope, revealing the blistering intrinsic regions with temperatures in excess 100,000K, shown in this aliveness of the Helix, below.
The resulting Planetary Nebuala is the interaction of the newly ejected bomb of pedal with the more slowly moving ejecta from preceding events and the ultraviolet light from the blistering stellar remnant, which heats the pedal and causes it to fluoresce.
9. White Dwarf As the nebula disperses, the bomb thermonuclear reactions die discover leaving the stellar remnant, supported by electron degeneracy, to fade absent as it cools down. The albescent dwarf is small, most the filler of the earth, with a density of order 1 million g/cm3, most equivalent to prevention a volkswagen downbound to a cubic centimeter or a \"ton per teaspoonful.\"
- R ~ Rearth (a some thousand km)
- surface = 30000K - 5000K
- Energy Source: \"Cooling Off\".
A albescent dwarf grapheme module take billions of eld to radiate absent its store of thermal forcefulness because of its diminutive surface area. The albescent dwarf module slowly advise downbound and to the correct in the H-R Diagram as it cools until it fades from view as a \"black dwarf\". To the correct is the albescent dwarf companion to the nearby grapheme Sirius.
