A stellar core is the extremely hot, dense region at the center of a star. For an ordinary
main sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...
star, the core region is the volume where the temperature and pressure conditions allow for energy production through
thermonuclear fusion
Thermonuclear fusion is the process of atomic nuclei combining or “fusing” using high temperatures to drive them close enough together for this to become possible. There are two forms of thermonuclear fusion: ''uncontrolled'', in which the re ...
of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
into
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
. This energy in turn counterbalances the mass of the star pressing inward; a process that self-maintains the conditions in
thermal
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
and
hydrostatic equilibrium
In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary ...
. The minimum temperature required for stellar
hydrogen fusion
Stellar nucleosynthesis is the creation (nucleosynthesis) of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the original creation of hydrogen, helium and lithium during the Big Bang. As a ...
exceeds 10
7 K (), while the density at the core of the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
is over . The core is surrounded by the stellar envelope, which transports energy from the core to the
stellar atmosphere
The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone.
Overview
The stellar atmosphere is divided into several regions of distinct character:
* The photosphere, whi ...
where it is radiated away into space.
[
]
Main sequence
Main sequence stars are distinguished by the primary energy-generating mechanism in their central region, which joins four hydrogen nuclei to form a single helium atom through thermonuclear fusion
Thermonuclear fusion is the process of atomic nuclei combining or “fusing” using high temperatures to drive them close enough together for this to become possible. There are two forms of thermonuclear fusion: ''uncontrolled'', in which the re ...
. The Sun is an example of this class of stars. Once stars with the mass of the Sun
The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...
form, the core region reaches thermal equilibrium after about 100 million (108)[ years and becomes radiative.][ This means the generated energy is transported out of the core via ]radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
and conduction
Conductor or conduction may refer to:
Music
* Conductor (music), a person who leads a musical ensemble, such as an orchestra.
* Conductor (album), ''Conductor'' (album), an album by indie rock band The Comas
* Conduction, a type of structured f ...
rather than through mass transport in the form of convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
. Above this spherical radiation zone
A radiation zone, or radiative region is a layer of a star's interior where energy is primarily transported toward the exterior by means of radiative diffusion and thermal conduction, rather than by convection. Energy travels through the radiatio ...
lies a small convection zone
A convection zone, convective zone or convective region of a star is a layer which is unstable due to convection. Energy is primarily or partially transported by convection in such a region. In a radiation zone, energy is transported by radiation ...
just below the outer atmosphere.
At lower stellar mass
Stellar mass is a phrase that is used by astronomers to describe the mass of a star. It is usually enumerated in terms of the Sun's mass as a proportion of a solar mass (). Hence, the bright star Sirius has around . A star's mass will vary over its ...
, the outer convection shell takes up an increasing proportion of the envelope, and for stars with a mass of around (35% of the mass of the Sun) or less (including failed stars) the entire star is convective, including the core region.[ These very low-mass stars (VLMS) occupy the late range of the M-type main-sequence stars, or ]red dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
. The VLMS form the primary stellar component of the Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...
at over 70% of the total population. The low-mass end of the VLMS range reaches about , below which ordinary (non-deuterium
Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
) hydrogen fusion
Stellar nucleosynthesis is the creation (nucleosynthesis) of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the original creation of hydrogen, helium and lithium during the Big Bang. As a ...
does not take place and the object is designated a brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
. The temperature of the core region for a VLMS decreases with decreasing mass, while the density increases. For a star with , the core temperature is about while the density is around . Even at the low end of the temperature range, the hydrogen and helium in the core region is fully ionized.[
Below about , energy production in the stellar core is predominantly through the proton–proton chain reaction, a process requiring only hydrogen. For stars above this mass, the energy generation comes increasingly from the ]CNO cycle
The CNO cycle (for carbon–nitrogen–oxygen; sometimes called Bethe–Weizsäcker cycle after Hans Albrecht Bethe and Carl Friedrich von Weizsäcker) is one of the two known sets of fusion reactions by which stars convert hydrogen to helium, ...
, a hydrogen fusion process that uses intermediary atoms of carbon, nitrogen, and oxygen. In the Sun, only 1.5% of the net energy comes from the CNO cycle. For stars at where the core temperature reaches 18 MK, half the energy production comes from the CNO cycle and half from the pp chain.[ The CNO process is more temperature-sensitive than the pp chain, with most of the energy production occurring near the very center of the star. This results in a stronger thermal gradient, which creates convective instability. Hence, the core region is convective for stars above about .][
For all masses of stars, as the core hydrogen is consumed, the temperature increases so as to maintain pressure equilibrium. This results in an increasing rate of energy production, which in turn causes the luminosity of the star to increase. The lifetime of the core hydrogen–fusing phase decreases with increasing stellar mass. For a star with the mass of the Sun, this period is around ten billion years. At the lifetime is 65 million years while at the core hydrogen–fusing period is only six million years.][ The longest-lived stars are fully convective red dwarfs, which can stay on the main sequence for hundreds of billions of years or more.][
]
Subgiant stars
Once a star has converted all the hydrogen in its core into helium, the core is no longer able to support itself and begins to collapse. It heats up and becomes hot enough for hydrogen in a shell outside the core to start fusion. The core continues to collapse and the outer layers of the star expand. At this stage, the star is a subgiant
A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution ...
. Very-low-mass stars never become subgiants because they are fully convective.[
Stars with masses between about and have small non-convective cores on the main sequence and develop thick hydrogen shells on the subgiant branch. They spend several billion years on the subgiant branch, with the mass of the helium core slowly increasing from the fusion of the hydrogen shell. Eventually, the core becomes degenerate and the star expands onto the red giant branch.][
Stars with higher masses have at least partially convective cores while on the main sequence, and they develop a relatively large helium core before exhausting hydrogen throughout the convective region, and possibly in a larger region due to ]convective overshoot Convective overshoot is a phenomenon of convection carrying material beyond an unstable region of the atmosphere into a stratified, stable region. Overshoot is caused by the momentum of the convecting material, which carries the material beyond th ...
. When core fusion ceases, the core starts to collapse and it is so large that the gravitational energy actually increases the temperature and luminosity of the star for several million years before it becomes hot enough to ignite a hydrogen shell. Once hydrogen starts fusing in the shell, the star cools and it is considered to be a subgiant. When the core of a star is no longer undergoing fusion, but its temperature is maintained by fusion of a surrounding shell, there is a maximum mass called the Schönberg–Chandrasekhar limit In stellar astrophysics, the Schönberg–Chandrasekhar limit is the maximum mass of a non-fusing, isothermal core that can support an enclosing envelope. It is expressed as the ratio of the core mass to the total mass of the core and envelope. Es ...
. When the mass exceeds that limit, the core collapses, and the outer layers of the star expand rapidly to become a red giant
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...
. In stars up to approximately , this occurs only a few million years after the star becomes a subgiant. Stars more massive than have cores above the Schönberg–Chandrasekhar limit before they leave the main sequence.[
]
Giant stars
Once the supply of hydrogen at the core of a low-mass star with at least [ is depleted, it will leave the main sequence and evolve along the ]red giant branch
The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass sta ...
of the Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosity, luminosities versus their stellar classifications or eff ...
. Those evolving stars with up to about will contract their core until hydrogen begins fusing through the pp chain along with a shell around the inert helium core, passing along the subgiant branch
A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of ...
. This process will steadily increase the mass of the helium core, causing the hydrogen-fusing shell to increase in temperature until it can generate energy through the CNO cycle. Due to the temperature sensitivity of the CNO process, this hydrogen fusing shell will be thinner than before. Non-core convecting stars above that have consumed their core hydrogen through the CNO process, contract their cores, and directly evolve into the giant stage. The increasing mass and density of the helium core will cause the star to increase in size and luminosity as it evolves up the red giant branch.[
For stars in the mass range , the helium core becomes ]degenerate
Degeneracy, degenerate, or degeneration may refer to:
Arts and entertainment
* Degenerate (album), ''Degenerate'' (album), a 2010 album by the British band Trigger the Bloodshed
* Degenerate art, a term adopted in the 1920s by the Nazi Party i ...
before it is hot enough for helium to start fusion. When the density of the degenerate helium at the core is sufficiently high − at around with a temperature of about − it undergoes a nuclear explosion known as a " helium flash". This event is not observed outside the star, as the unleashed energy is entirely used up to lift the core from electron degeneracy to normal gas state. The helium fusing core expands, with the density decreasing to about , while the stellar envelope undergoes a contraction. The star is now on the horizontal branch
The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) ...
, with the photosphere
The photosphere is a star's outer shell from which light is radiated.
The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
showing a rapid decrease in luminosity combined with an increase in the effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...
.[
In the more massive main-sequence stars with core convection, the helium produced by fusion becomes mixed throughout the convective zone. Once the core hydrogen is consumed, it is thus effectively exhausted across the entire convection region. At this point, the helium core starts to contract and hydrogen fusion begins along with a shell around the perimeter, which then steadily adds more helium to the inert core.][ At stellar masses above , the core does not become degenerate before initiating helium fusion.][ Hence, as the star ages, the core continues to contract and heat up until a ]triple alpha process
The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon.
Triple-alpha process in stars
Helium accumulates in the cores of stars as a result of the proton–pro ...
can be maintained at the center, fusing helium into carbon. However, most of the energy generated at this stage continues to come from the hydrogen fusing shell.[
For stars above , ]helium fusion
The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon.
Triple-alpha process in stars
Helium accumulates in the cores of stars as a result of the proton–pro ...
at the core begins immediately as the main sequence comes to an end. Two hydrogen fusing shells are formed around the helium core: a thin CNO cycle inner shell and an outer pp chain shell.[
]
See also
* Solar core
The core of the Sun is considered to extend from the center to about 0.2 to 0.25 of
solar radius (140,000 - 170,000 kilometres (86,000 - 110,000 miles)). It is the hottest part of the Sun and of the Solar System. It has a density of 150 g/ ...
* Stellar evolution
Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...
References
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Stellar astronomy