Star

A Star is an astrographic spatial object composed of a massive incandescent sphere of plasma held together by its own self-gravity. Formed within nebulae, a star is formed as the inward gravitational pressure due to in-falling gas balances against the rising outward thermal pressure of the core of the gas cloud due to the collapse.

Description (Specifications)

General Stellar Populations

Population III stars

Population III stars, or "extremely metal-poor stars", are the oldest stars that have ever existed, having lived and died within the first 2 billion years of the existence of the universe. Population III stars are characterized by virtually zero metallicity, as it is these stars that were largely responsible for having generated the elements heavier than helium through nucleosynthesis in their cores, and having spread heavier elements into the cosmos through supernovae. Population III stars are believed to have been extremely hot and massive stars of several hundred solar masses. There are no surviving Population III stars in the universe today.

Population II stars

Population II stars, or "metal-poor stars", are old stars of generally low metallicity which were formed during the generations of star formation after the initial stars of the Population III generation. Population II stars are commonly found in the galactic halo and globular clusters, and are believed to be responsible for the creation of the remaining heavier elements of the periodic table.

Population I stars

Population I stars, or "metal-rich stars", are the most recent generation of stars, generally of higher metallicity, of which the Terran primary star Sol is an example. They are commonly found in the spiral arms of spiral galaxies.

Origin & Formation

Stars are typically formed in a specific type of collapsing interstellar cloud known as a molecular cloud. Interstellar clouds are regions of gas, dust, and plasma that have a higher average density than the general interstellar medium. They are typically composed of about 70% hydrogen, the bulk of the remainder being helium with traces of other elements. Interstellar clouds can be classified into 3 general types based on the primary form of hydrogen found within:

• Neutral Clouds (H I regions - formed of atomic hydrogen)
• Ionized Clouds (H II regions - formed of ionized hydrogen [i.e. "Plasma"])
• Molecular Clouds (Molecular Hydrogen - formed of H₂)

Under normal circumstances interstellar clouds exist in a state of hydrostatic equilibrium, in which the natural self-gravitation that would otherwise cause the cloud to collapse is balanced by the outward pressure within the cloud due to motion of the atoms and molecules that compose it. Star formation is normally caused by a triggering event which locally disturbs this equilibrium, causing runaway local collapsing reions within the cloud. Such causes can be the collision of one cloud with another, or the shock-wave within the interstellar medium caused by a nearby supernova explosion.

Once triggered, a collapsing region will begin to increase in temperature as the density of the cloud increases, while at the same time rotating with ever-increasing net angular velocity due to conservation of angular momentum acting upon the small non-zero initial net angular momentum of the collapsing region as the cloud contracts. As the cloud-region continues to contract under self-gravitation, the opacity of the gas to thermal radiation will increase (trapping the heat internally) and the growing outward thermal pressure of the cloud will cause the rate of contraction to slow, forming a hot incandescent core region known as a proto-star. As accretion of gas onto the proto-star continues, the mass of the proto-star will continue to grow, increasing its gravitational attraction and compressing the core region of the proto-star to ever higher densities. Eventually as the accretion process runs toward completion, a state of quasi-hydrostatic equilibrium will again be reached, as the outward thermal pressure within the protostar begins to counteract the inward gravitational pressure due to the proto-star's own mass. As slow compaction continues the proto-star will enter onto one of three paths dependent upon the final mass of the proto-stellar body, becoming either a Sub-Brown Dwarf, a Brown Dwarf, or a Pre-Main Sequence Star.

Sub-Brown Dwarf

A Sub-Brown Dwarf is formed from a collapsing molecular cloud whose proto-star mass produces insufficient core temperature and pressure to initiate any type of thermonuclear burning. The threshold for such pressures is typically estimated to be about 13 M_J. Having failed to reach this minimum mass threshold, a sub-brown dwarf will simply continue to slowly contract after accretion has completed, slowly radiating its heat away from its surface thermally until it eventually cools back to 2.7 K over many billions of years.

Brown Dwarf

A proto-star whose final mass falls between 13 M_J and 85 M_J (=0.08 M_Sol) will form into a Brown Dwarf. A Brown Dwarf has sufficient mass to produce temperatures and pressures sufficient to initiate deuterium-burning in its core, and if above ~ 65 M_J, to initiate lithium-burning as well. It has insufficient temperature and pressure to initiate hydrogen burning via the proton-proton chain, however. Since a small percentage of the initial cloud of gas that formed the star is composed of molecules other than light-H₂ or He, a certain small fraction of this gas will be composed of heavy hydrogen (i.e. deuterium) and lithium isotopes, among other substances. Since there is such a small percentage of HD, D₂, and Li within the gas mix, the time spent fusing these elements is relatively short on the timescale of stellar lifespans, after which all fusion reactions shut down. The Brown Dwarf will once again begin to slowly contract and radiate its heat away from its surface thermally (much as a Sub-Brown Dwarf does), its low-mass core contracting to form an electron-degenerate mass that supports the weight of the star entirely through degeneracy pressure. Brown Dwarfs typically fall within the range of stellar spectra from M6.5 V to M9.5 V (overlapping red main-sequence dwarfs), and also include classes L, T, and Y.

Pre-Main Sequence Star

A proto-star that achieves a mass greater than 85 M_J (= 0.08 M_Sol) has sufficient pressure and temperature that in addition to Lithium and Deuterium burning, it can eventually initiate light hydrogen burning via the proton-proton chain and/or the CNO cycle. The phase of compaction between the proto-star's final stage of gas-accretion and the ignition of proton-proton burning is known as the pre-main sequence phase. Stars in this phase whose mass falls between 0.08-2.0 M_Sol are generally known as T-Tauri stars, whereas those between 2-8 M_Sol form Herbig Ae/Be stars. Stars of greater than 8 M_Sol are generally massive enough to not got through a pre-main sequence phase, as they are already burning hydrogen by the time they become optically visible from within their molecular cloud.

Main Sequence

Main Sequence Stars (particularly the smaller ones in spectral classes "G" thru "M") are also sometimes referred to as "dwarf" stars or main sequence dwarfs.

A Pre-main-sequence star will eventually compact sufficiently to create pressures and temperatures conducive to thermonuclear burning via the proton-proton chain and/or the CNO Cycle, at which point the star has become a Main Sequence Star. All main sequence stars generate some fraction of their energy production via each of these processes, but as the CNO cycle generally occurs at higher ignition temperatures, the relative reaction rate for each of these reactions in any given star is expressed as a percentage called the reaction cross-section, indicating the amount of fusion taking place by each specific reaction within a given stellar core.

• Hydrogen Burning (Proton-Proton Chain). The Proton-Proton Chain is a thermonuclear fusion process which is the dominant energy generation process for main sequence stars of less than 1.3 M_Sol, and generally requires temperatures in excess of 4 megakelvins. There are several intermediary steps within the proton-proton chain, but the end result is that 4 protons (hydrogen nuclei) produce 1 helium nucleus (alpha particle) plus two positrons and two neutrinos, along with 3 gamma photons and energy. The two positrons likewise rapidly annihilate two electrons in the surrounding plasma, liberating additional energy in the form of gamma photons:

4 ¹H → ⁴He + 2e⁺ + 2ν_e + 2 γ

and

2e⁺ + 2e^- → 2 γ

• Hydrogen Burning (CNO Cycle). The CNO Cycle is a catalytic thermonuclear fusion process which is the dominant energy generation process for main sequence stars of greater than 1.3 M_Sol, and generally requires temperatures in excess of 15 megakelvins. The CNO Cycle energy output increases more rapidly with increasing temperatures than that of the proton-proton chain, however, becoming the dominant form of thermonuclear fusion at about 17 megakelvins. The net reaction for the CNO cycle is the same as that listed above for the proton-proton chain, with same number and kind of reactants and the same end products. However, the CNO cycle utilizes various isotopes of carbon, nitrogen, and oxygen catalytically in one of four possible reaction pathways in the reaction process.

In stars of sufficiently low mass (less than about 0.25-0.50 M_Sol), the heat transport mechanism within the star is primarily convective rather than radiative, meaning that heat is primarily transported by convection currents within the plasma. As a result, all of the hydrogen within the star is cycled through the reaction core, as well as the resultant helium product, and the star will continue to fuse hydrogen until the very end of its life, at which time (as helium ash continues to build up and eventually sink toward the center of the star) it will directly evolve into a helium degenerate dwarf.

Main sequence stars of higher mass typically transport heat primarily though the radiative heat transport process rather than convection, meaning that heat is primarily radiated away from the core via electromagnetic radiation. As main sequence stars of higher mass evolve, they build up an "ash" of ⁴He product that is non-reactive in the fusion process at the temperature ranges listed above. This ⁴He ash is not cycled throughout the star as it is in convective heat transport, and it thus sinks to the center of the core and absorbs heat without adding to the reaction processes within the star. As time progresses, this increasing amount of ash begins to slow the reaction rate of fusing hydrogen as it absorbs energy from the system, causing the core to begin to contract under its own weight due to the decreased reaction output, and subsequently causing a rise in core temperature. As the core temperature rises through gravitational contraction, the outer layers of the star are heated and driven slowly outward. The eventual result of this process will be the evolution of the star into a subgiant, which will itself eventually evolve into either a giant or supergiant star.

Note that some stars which are otherwise in their main sequence phase are sometimes classed as subgiants due to abnormally high luminosity as a result of unusual chemical composition or other undetermined oddities.

Stellar Evolution - Post Main Sequence

As a star ages and converts its supply of hydrogen into helium, changes in resultant energy output from the star will slowly cause the star to evolve in one of several directions depending on its mass.

Blue Dwarf

For main sequence "red dwarf" stars of less than about 0.50 M_Sol (approximately stellar types M1.5 V thru M9 V), there is insufficient mass to create the pressure and temperature necessary to fuse the helium product of proton-proton fusion into heavier elements. As the hydrogen from the star gets cycled throughout the entire star (including the reaction core), it will have the opportunity to continue hydrogen burning until virtually all of its hydrogen is used up. The helium ash from this reaction will begin to form a growing degenerate helium core within the star as hydrogen burning continues to occur at higher levels within the star. As this process continues over several hundred billion to trillions of years, the temperature of the outer layers of the star will gradually increase with increasing luminosity, the color of the star gradually evolving from the pinkish-orange coloration of a "red dwarf" toward a blue-white hue. This final "Blue Dwarf" will eventually burn out as its hydrogen supply becomes exhausted, leaving behind a radiant blue-white-hued helium degenerate dwarf star in which all nuclear reactions have ceased, slowly cooling over eons back through the spectrum down toward red/infra-red and eventually to black multiple hundreds of trillions of years in the future.

Giant Star

As a main sequence star of greater than 0.4-0.5 M_Sol ages and generates a build-up of helium ash at its core, its begins to burn its hydrogen in a shell surrounding the inert helium core, causing the outer layers of the star to begin to expand and cool as the core contracts and heats up, increasing the rate of hydrogen burning in the surrounding shell as a consequence. As the star enters this sub-giant phase, the color and temperature of the star's photosphere cools and begins to move slowly toward the "red" direction of the spectrum. Eventually, the buildup of helium ash at the core reaches sufficient mass and density to initiate helium-burning in the core via the Triple-Alpha process. The Triple-Alpha process fuses three helium nuclei (aka alpha particles) into a carbon-12 nucleus via the following mechanism:

⁴He + ⁴He → ⁸Be

and

⁸Be + ⁴He → ¹²C

In some cases, a carbon-12 atom will fuse with an additional helium nucleus to form an oxygen-16 nucleus:

¹²C + ⁴He → ¹⁶O + γ

A few particularly high mass stars just under the 9 M_Sol threshold will further fuse oxygen-16 into neon-20:

¹⁶O + ⁴He → ²⁰Ne + γ

The additional energy generated by this process causes the outer layers of the star to expand and cool even further, resulting in the star transforming into a red giant star.

Supergiant Star

Stars whose initial masses are greater than 9-10 M_Sol will not only initiate core helium fusion after they have exhausted their core hydrogen, but will continue to fuse heavier elements after its core helium has been exhausted, successively burning the ash of previous fusion products beginning with carbon, and continuing with oxygen, neon, and silicon, until finally an iron-ash core is produced. Each phase of core burning is accompanied by a build up of reaction-product ash which slows the reaction, leading to further core collapse and resultant higher core temperatures, which initiates the next phase of core-burning. As the reactions progress through each of these phases, a "shell" surrounding prior thermonuclear burning phases develops in which earlier thermonuclear burning reactions are still taking place, much like the layers of an onion, with hydrogen burning still occurring in the outermost layer, followed by helium burning in the next layer inward, etc., down to the current level of burning occurring in the core at the very center.

When thermonuclear burning reaches the point at which iron-ash is being produced within the core, due to several factors including the mass of the particular supergiant star in question, sustained iron-burning either will not occur prior to the iron core reaching the Chandrasekhar limit at which the core is no longer able to support itself even by electron degeneracy pressure, or if the core does reach temperatures and pressures sufficient for iron burning prior to reaching this limit, will not proceed due to the tendency of iron nuclei to undergo photodisintegration due to the energies involved. The result in either case will be core collapse as the supergiant star experiences the loss of outward pressure from core fusion, the core collapsing under its own gravity and resulting either in the formation of a neutron degenerate dwarf (or "neutron star") or a black hole, with or without an accompanying supernova explosion.

Stellar Evolution - Stellar End

Degenerate Dwarf

• "White" Dwarf

1. Helium Degenerate Dwarf
2. Carbon-Oxygen Degenerate Dwarf
3. Neon Degenerate Dwarf

• Neutron Star

• Neutron Degenerate Dwarf

• Exotic Star

1. Quark Degenerate Dwarf
2. Strange Degenerate Dwarf

• Black Dwarf. As a degenerate dwarf slowly cools over eons, its color will slowly devolve toward the "red" end of the spectrum from its initial blue-white hue, slowly devolving through the spectrum down toward red/infra-red and eventually to black multiple hundreds of trillions of years in the future. A black dwarf should not be confused with a black hole.

Black Hole

1. Schwarzschild Black Hole
2. Kerr Black Hole

History & Background (Dossier)

No information currently available.

Early Classification Terminology

• Giant Stars & Dwarf Stars
• Early Spectral Classes

Current Stellar Spectral Types & Luminosity Class Terminology

No information currently available.

References & Contributors (Sources)

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This page uses content from Wikipedia. The original article was at Star. The list of authors can be seen in the page history. The text of Wikipedia is available under the Commons Attribution-ShareAlike 3.0 Unported License.

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