Main sequence

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In astronomy, the main sequence is a continuous and distinct star band that appears in the plot of star versus brightness. This colorful plot is known as the Hertzsprung-Russell diagram after its development partners, Ejnar Hertzsprung and Henry Norris Russell. Stars in this band are known as main sequence stars or dwarf stars. This is the most true star in the universe, and includes the Earth's Sun.

After the mass condensation and ignition of the star, it generates heat energy in the solid core region through the nuclear fusion of a hydrogen atom into helium. During the life stage of this star, it lies along the main sequence in positions determined primarily by its mass, but also based on its chemical composition and other factors. All major consecutive stars are in hydrostatic equilibrium, where the thermal stress out of the heat core is balanced by the inward pressure of the gravitational collapse of the top layer. The strong dependence of the rate of generation of energy in the core at temperature and pressure helps to maintain this balance. The energy generated in the core makes its way to the surface and radiates deep in the photosphere. Energy is carried by radiation or convection, with the latter occurring in areas with steep temperature gradients, higher opacities or both.

The main sequence is sometimes divided into upper and lower parts, based on the dominant process that stars use to generate energy. Stars below about 1.5 times the mass of the Sun (1.5Ã, M _? ) primarily merge the hydrogen atoms together in a series of stages to form helium, a sequence called proton- proton. Above this mass, in the top sequence, the nuclear fusion process primarily uses carbon atoms, nitrogen and oxygen as an intermediate in the CNO cycle that produces helium from a hydrogen atom. The main sequence stars with more than two solar masses experience convection in their core region, which acts to generate freshly created helium and retain the proportion of fuel needed for fusion to occur. Below this mass, the star has a fully radiative core with a convective zone near the surface. With a decrease in star mass, the proportion of stars forming convective envelopes is increasing. The main sequence stars below 0.4Ã, M _? is undergoing convection across their mass. When nuclear convection does not occur, a helium-rich nucleus develops surrounded by an outer layer of hydrogen.

In general, the more massive star is, the shorter its age in the main sequence. Once the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence on the HR diagram. Star behavior now depends on its mass, with stars below 0.23 M _? into a white dwarf directly, while stars with up to ten solar masses pass through red giant stages. The more massive stars can explode as supernovas, or collapse directly into the black hole.

In April 2018, astronomers reported the most distant "star" detection (ie, the main sequence), named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light-years from Earth.

Video Main sequence

Histori

At the beginning of the 20th century, information about the type and distance of stars became more readily available. The spectra of the stars proved to have distinctive features, which allowed them to be categorized. Annie Jump Cannon and Edward C. Pickering at the Harvard College Observatory developed a categorization method which came to be known as Harvard Classification Scheme, published in Harvard Annals in 1901.

In Potsdam in 1906, Danish astronomer Ejnar Hertzsprung noticed that the most red stars - classified as K and M in the Harvard scheme - can be divided into two distinct groups. These stars are much brighter than the Sun, or more dim. To distinguish these groups, he calls them "giant" and "dwarf" stars. The following year he began studying star clusters; a large grouping of stars located at about the same distance. He published the first plot of color versus luminosity for these stars. These plots show a prominent and continuous star sequence, which he names the Main Order.

At Princeton University, Henry Norris Russell followed a similar study. He studied the relationship between the star spectrum classification and their actual brightness as their absolute correction of magnitude. For this purpose he uses a set of stars that have reliable parallax and many have been categorized in Harvard. When he charted the spectral types of these stars against his absolute magnitude, he found that the dwarf star followed a different relationship. This allows the real brightness of a dwarf star to be predicted with reasonable accuracy.

From the red star observed by Hertzsprung, the dwarf star also follows the luminosity-spectra-relationship found by Russell. However, the giant star is much brighter than the dwarf and so does not follow the same relationship. Russell proposes that "a gigantic star should have a low density or large surface brightness, and vice versa is true of a dwarf star". The same curve also shows that there are very few dim white stars.

In 1933, Bengt StrÃÆ'¶mgren introduced the Hertzsprung-Russell diagram to show a diagrams of luminosity-spectral classes. This name reflects the parallel development of this technique by Hertzsprung and Russell at the beginning of this century.

As a model of star evolution developed during the 1930s, it shows that, for stars of a uniform chemical composition, there is a relationship between the mass of the star and its luminosity and radius. Namely, for given masses and compositions, there is a unique solution for determining the radius and luminosity of stars. This is known as the Vogt-Russell theorem; named after Heinrich Vogt and Henry Norris Russell. With this theorem, when the chemical composition of the star and its position in the main sequence is known, so does the mass and radius of the star. (However, it was later discovered that his theory was broken somewhat for the stars of non-uniform compositions.)

The subtle scheme for star classification was published in 1943 by William Wilson Morgan and Philip Childs Keenan. The MK classification assigned each star is spectral type - based on the Harvard classification - and the luminosity class. The Harvard classification has been developed by assigning different letters to each star based on the strength of the hydrogen spectrum line, before the relationship between the spectra and the temperature is known. When ordered by the temperature and when the duplicate class is removed, the spectrum of the star type is followed, in order of decrease in temperature with colors ranging from blue to red, the order O, B, A, F, G, K and M. (A popular mnemonic to memorize the class sequence This star is "Oh Be A Fine Girl/Guy, Kiss Me".) Class luminosity ranges from I to V, in order to reduce luminosity. Stars of class V luminosity included in the main sequence.

In April 2018, astronomers reported the most distant "star" detection (ie, the main sequence), named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light-years from Earth.

Maps Main sequence

Formation and evolution

When a protostar is formed from the collapse of a cloud of gas molecules and giant dust in the local interstellar medium, the initial composition is homogeneous, comprising about 70% hydrogen, 28% helium and a small number of other elements, by mass. The star's initial mass depends on local conditions within the cloud. (The mass distribution of newly formed stars is explained empirically by the function of the initial mass.) During the initial collapse, this pre-main sequence star produces energy through gravitational contractions. After reaching the appropriate density, energy generation begins at the core using an exothermic nuclear fusion process that converts hydrogen into helium.

When nuclear fusion becomes the dominant energy production process and the excess energy obtained from the gravitational contraction has been lost, the star lies along the curve in the Hertzsprung-Russell diagram (or HR diagram) called the standard main sequence. Astronomers will sometimes refer to this stage as the "main sequence of zero age", or ZAMS. The ZAMS curve can be calculated using the computer model of the star property at a point when the star initiates a hydrogen fusion. From this point, the brightness and temperature of the star surface usually increases with age.

A star remains near its initial position in the main sequence until a large amount of hydrogen in the core has been consumed, then begins to evolve into a lighter star. (In the HR diagram, the evolved star moves up and to the right of the main sequence.) Thus the main sequence represents the primary hydrogen combustion stage of a star's life span.

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Properties

The majority of stars in typical HR diagrams are located along the main sequence curves. This sentence is spoken because both types of spectra and luminosity depend only on the mass of stars, at least for the zeroth-order approach, as long as the hydrogen blends in essence - and that's what almost all stars spend most of the "Active" life doing.

The temperature of a star determines its spectral type through its effect on the physical properties of the plasma within its photosphere. The emission of star energy as a function of wavelength is influenced by its temperature and composition. The main indicator of this energy distribution is given by the color index, B Ã, - V , which measures the magnitude of the star in blue ( B ) and the green light yellow ( V ) through the filter. This magnitude difference gives a measure of star temperature.

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Dwarf terminology

The main sequence stars are called dwarf stars, but the terminology is partly historical and can be somewhat confusing. For cooler stars, dwarfs like red dwarfs, orange, and yellow dwarfs are much smaller and fainter than other stars of those colors. However, for warmer blue and white stars, the difference in size and brightness between so-called "dwarf" stars is in the main sequence and so-called "giant" stars that do not become smaller; for the hottest stars was not immediately observed. For the stars, the terms "dwarf" and "giant" refer to the spectral line differences that indicate if the star is in the main sequence or outside it. Nevertheless, the ultra-hot main sequence stars are still sometimes called dwarves, although they have almost the same size and brightness as the "giant" stars of that temperature.

The common use of "dwarfs" to define the main sequence is confusing in other ways, since there are dwarf stars that are not the main sequential stars. For example, the white dwarf is the dead core of the star that remains after it has released its outer layer, which is much smaller than the main sequence star, approximately the size of Earth. This represents the final evolutionary stage of many major sequence stars.

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Parameters

Dengan memperlakukan bintang sebagai radiator energi ideal yang dikenal sebagai tubuh hitam, luminositas L dan radius R dapat dikaitkan dengan suhu efektif T _eff oleh hukum Stefan-Boltzmann:

${\ displaystyle L = 4 \ pi \ sigma R ^ {2} T_ {eff} ^ {4}}  ÂÂ$

where ? is the Stefan-Boltzmann constant. Since the star position on the HR diagram shows the approximate luminosity, this relationship can be used to estimate its radius.

The mass, radius and luminosity of stars are intimately connected, and each value can be approached by three relations. First is Stefan-Boltzmann's law, which connects luminosity L , radius R and surface temperature T _eff . The second is the mass-luminosity relationship, which connects the luminosity L and the mass M . Finally, the relationship between M and R is linearly approximated. The M ratio for R increases by a factor just three-and-a-half over the magnitude of M . This relationship is approximately proportional to the inner temperature of the T _I star, and its very slow increase reflects the fact that the rate of generation of energy in the nucleus is highly dependent on this temperature, whereas it should be appropriate with a mass-luminosity relationship. Thus, temperatures that are too high or too low will result in star instability.

A better approximation is to take ? = L/M , the rate of energy generation per unit mass, such as? proportional to T _I ¹⁵ , where T _I is the core temperature. This is suitable for stars at least by the Sun, showing the CNO cycle, and giving better R ? M ^0.78 .

Example parameters

The table below shows the typical values ​​for stars along the main sequence. The value of luminosity ( L ), the radius ( R ) and the mass ( M ) relative to the Sun - dwarf star with spectral classification G2 V. The true value for stars can vary by 20-30% of the values ​​listed below.

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Energy generation

All the major sequential stars have a core region where energy is generated by nuclear fusion. This temperature and core density are at the level necessary to sustain energy production that will support the rest of the stars. Reduced energy production will cause overlaying of the masses to compress the nucleus, resulting in an increase in the fusion rate due to higher temperatures and pressures. Likewise an increase in energy production will cause the star to expand, lowering the pressure on the core. Thus stars form self-regulating systems in stable hydrostatic equilibrium during the period of the main sequence.

The main sequence stars use two types of hydrogen fusion processes, and the rate of energy formation of each type depends on the temperature in the core region. Astronomers divide the main sequence into upper and lower parts, based on which of them are the dominant fusion processes. In the lower main sequence, energy is mainly produced as a result of proton-proton chains, which directly combine hydrogen together in a series of stages to produce helium. Stars in the top main series have a high enough core temperature to efficiently use the CNO cycle. (See chart.) This process uses carbon, nitrogen and oxygen atoms as an intermediate in the process of melting hydrogen into helium.

At a core temperature of 18 million Kelvin stars, the PP process and CNO cycle are equally efficient, and each type produces half the luminosity of a star net. Since this is the core temperature of a star with about 1.5 M _? , the top main order consists of stars above this mass. Thus, speaking harshly, the spectral class stars F or coolers belong to the lower main order, while the stars of type A or hotter are the main top-order stars. The transition in primary energy production from one form to another varies in the distance of less than a single solar mass. In the Sun, one solar-mass star, only 1.5% of the energy is generated by the CNO cycle. Conversely, stars with 1.8 M _? or above generates almost all of their energy output through the CNO cycle.

The observed upper limit for the main sequence star is 120-200 M _? . The theoretical explanation for this limit is that the stars above this mass can not emit energy fast enough to remain stable, so that any additional mass will be expelled in a series of vibrations until the star reaches a stable threshold. The lower limit for sustained nuclear fusion of protons is about 0.08 M _? or 80 times the mass of Jupiter. Below this threshold is a sub-star object that can not sustain hydrogen fusion, known as a brown dwarf.

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Structure

Because there is a temperature difference between core and surface, or photosphere, energy is transported outward. Two modes to transport this energy are radiation and convection. The radiation zone, where energy is transported by radiation, is stable against convection and there is little plasma mixing. In contrast, in energy convection zones are transported by plasma mass transfer, with warmer materials rising and cooler materials down. Convection is a more efficient mode to carry energy than radiation, but will only occur in conditions that create a steep temperature gradient.

In big stars (above 10 M _? ) the energy generation rate by the CNO cycle is very sensitive to temperature, so the fusion is highly concentrated in the core. As a result, there is a high temperature gradient in the core region, which results in a convection zone for more energy-efficient transportation. Mixing the material around this core removes the helium ash from the combustion zone of hydrogen, allowing more hydrogen in the star to be consumed during the main lifetime of the sequence. The outside area of ​​massive massive transport energy by radiation, with little or no convection.

Middle mass stars such as Sirius can transport energy primarily by radiation, with a small nuclear convection region. Medium, low-mass stars such as the Sun have a stable core region against convection, with a zone of near-surface convection that mixes the outer layers. This results in a buildup of helium-rich cores, surrounded by a hydrogen-rich outer region. Conversely, cool, very low-mass stars (below 0.4 M _? ) are convective throughout. Thus the helium produced at the core is distributed across the star, resulting in a relatively uniform atmosphere and a proportionally longer main length sequence.

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Color-luminosity variation

Because the non-fused helium ash accumulates in the core main star cores, the reduction of the abundance of hydrogen per unit mass results in a gradual decrease in the rate of fusion in the mass. Due to the fusion-supplied energy flow that supports the higher star layer, the core is compressed, resulting in higher temperature and pressure. Both factors increase the fusion rate so that moving the balance toward a smaller, denser, warmer nucleus produces more energy that increases the outflow pushing the higher layer further away. Thus there is a steady increase in luminosity and star radius over time. For example, the initial solar luminosity is only about 70% of its current value. As the age star of this increase in luminosity changes its position on the HR diagram. This effect produces the expansion of the main sequence bands because the star is observed at random stages in its lifetime. That is, the main sequence sequence develops the thickness in the HR diagram; this is not just a narrow line.

Other factors that broaden the main sequence bands in the HR diagram include the uncertainty in the distance to the star and the presence of unresolved binary stars that can alter the observed star parameters. However, even perfect observation will show a vague main sequence since mass is not the only parameter affecting the star's color and luminosity. Variations in chemical composition caused by initial abundance, stellar evolution status, interaction with close friends, rapid rotation, or magnetic fields can all slightly alter the position of the main sequence SDM diagram, to name just a few factors. For example, there are poor metal-stars (with very low abundance of elements with higher atomic numbers than helium) located just below the main sequence and known as subdwarves. These stars combine hydrogen in their nuclei and they mark the bottom edge of the main sequence irregularities caused by differences in chemical composition.

An almost vertical region of the HR diagram, known as the instability strip, is occupied by a pulsed variable star known as the Cepheid variable. These stars vary greatly in bulk periodically, giving them a throbbing appearance. The strip crosses the top of the main sequence in the class of A and F stars, which are between one and two solar masses. The star's pulsation in this section of the instability strip that cuts off the top of the main sequence is called the Delta Scuti variable. The main sequence stars in this region have only minor changes in magnitudes and these variations are difficult to detect. Other classes of unstable main stars, such as the Beta Cephei variables, are not related to this path of instability.

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Lifetime

The total amount of energy that can be generated by stars through nuclear fusion of hydrogen is limited by the amount of hydrogen fuel that can be consumed in the core. For stars in equilibrium, the energy produced at the nucleus must be at least equal to the energy emitted on the surface. Since luminosity gives the amount of energy emitted per unit of time, the total life span can be estimated, to the first estimate, since the total energy produced is divided by the luminosity of the star.

For stars with at least 0.5 M _? , when the supply of hydrogen in its core runs out and extends into a red giant, it can start to blend the helium atom into carbon. The energy output of the helium fusion process per unit mass is only about a tenth of the energy output of the hydrogen process, and the star's luminosity increases. This results in a much shorter length of time in this stage compared to the main lifetime sequence. (For example, the Sun is estimated to spend <130 million years burning helium, compared to about 12 billion years of burning hydrogen.) So, about 90% of the stars are observed above 0.5 M _? will be in the main sequence. On average, the main sequence stars are known to follow the empirical mass-luminosity relationship. The luminosity ( L ) of a star is roughly proportional to the total mass ( M ) as the law of the following forces:

${\ displaystyle {\ begin {smallmatrix} L \ \ propto \ M ^ {3.5} \ end {smallmatrix}}}$

This connection applies to the main series of stars in the range of 0.1-50 M _? .

di mana M dan L adalah massa dan luminositas bintang, masing-masing, ${\ displaystyle {\ begin {smallmatrix} M _ {\ bigodot} \ end {smallmatrix}}}  ÂÂ$ adalah massa matahari, ${\ displaystyle {\ begin {smallmatrix} L _ {\ bigodot} \ end {smallmatrix}}}  ÂÂ$ adalah luminositas matahari dan ${\ displaystyle \ tau _ {\ rm {MS}}}  ÂÂ$ adalah perkiraan masa pakai urutan utama bintang.

Although more massive stars have more fuel to burn and may be intuitively expected to last longer, they also emit a greater proportional amount with an increased mass. This is required by the state star's equations; for a massive star to maintain balance, the exit pressure from the radiated energy produced in the nucleus not only has to be increased but will to titanic gravitational pressure into the envelope. Thus, the most massive stars can remain in the main sequence for only a few million years, while stars with less than one-tenth of the Sun's mass can survive for more than a trillion years.

The appropriate mass-luminosity relationship depends on how efficiently the energy can be transported from the nucleus to the surface. Higher opacities have an insulating effect that retains more energy at the core, so the star does not need to generate much energy to stay in the hydrostatic equilibrium. Conversely, lower opacities mean faster exit energy and stars should burn more fuel to stay in equilibrium. Note, however, that a high enough opacity can produce energy transport through convection, which changes the conditions necessary to stay in equilibrium.

In major high-frequency stars, opacities are dominated by electron scattering, which is almost constant with increasing temperatures. Thus the luminosity only increases as the cube of the star's mass. For stars below 10 M _? , opacity becomes temperature dependent, generating luminosity which varies approximately as the fourth power of the star's mass. For very low mass stars, molecules in the atmosphere also contribute to opacity. Below about 0.5 M _? , the luminosity of a star varies as mass to a strength of 2.3, resulting in a slope inclination on the mass graph versus luminosity. Even these improvements are only approximate, and the mass-luminosity relationship may vary depending on the composition of the star.

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Evolution track

When the main sequence star consumes hydrogen at its core, the loss of energy generation causes its gravitational collapse to continue. Stars with less than 0.23 M _? , is thought to instantly become a white dwarf when the energy generated by the nuclear fusion of hydrogen at their nucleus is stalled. On the stars between this threshold and 10 M _? , the hydrogen that surrounds the helium nuclei reaches a temperature and sufficient pressure to undergo fusion, forming a hydrogen-burning shell. As a result of this change, the outer envelope expands and lowers the temperature, turning it into a red giant. At this point the star progresses from the main sequence and enters a giant branch. The path that is now followed by a star along the HR diagram, to the upper right of the main sequence, is called the path of evolution.

The red giant helium core continues to collapse until it is fully supported by the pressure of electron degeneration - a quantum mechanical effect that limits how closely the material can be compacted. For stars more than about 0.5 M _? , the core eventually reaches the temperature at which it becomes hot enough to burn helium into carbon through the alpha alpha process. Stars with more than 5-7.5 M _? can also combine elements with higher atomic numbers. For stars with ten or more solar masses, this process can cause an increasingly solid core that eventually collapses, catapulting the star top layer in a Type II supernova explosion, Type I supernovae or Type Ic supernovae.

When a group of stars is formed at the same time, the life span of these stars will depend on the mass of each. The most massive stars will leave the main sequence first, followed steadily sequentially by the stars of the lower mass. So the stars will evolve in the order of their positions in the main sequence, moving from the most massive left to the right of the HR diagram. The current position in which the stars in this group leave the main sequence known as the turn-off point. By knowing the main sequence of the star's life at this point, it becomes possible to estimate the age of the cluster.

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See also

• The hydrogen burning process
• Red dwarf
• Supergiant
• Sun

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Note

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References

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Further reading

General

• Kippenhahn, Rudolf, <100 Billion Suns , Basic Book, New York, 1983.

Technical

• Arnett, David, Supernova and Nucleosynthesis , Princeton University Press, Princeton, 1996.
• Bahcall, John N., Neutrino Astrophysics , Cambridge University Press, Cambridge, 1989.
• Bahcall, John N., Pinsonneault, MH, and Basu, Sarbani, "Solar Model: Current Epoch and Time Dependency, Neutrino, and Helioseismological Properties," The Astrophysical Journal, 555, 990 , 2001.
• Barnes, C. A., Clayton, D. D., and Schramm, D. N. (eds.), Essays in Nuclear Astrophysics , Cambridge University Press, Cambridge, 1982.
• Bowers, Richard L., and Deeming, Terry, Astrophysics I: Stars , Jones and Bartlett, Publishers, Boston, 1984.
• Bradley W. Carroll & amp; Dale A. Ostlie (2007). Introduction to Modern Astrophysics . Educational People Addison-Wesley San Francisco. ISBN 0-8053-0402-9.
• Chabrier, Gilles, and Baraffe, Isabelle, "The Theory of Low-Mass Star and Substar Object," Annual Reviews of Astronomy and Astrophysics , 38, 337, 2000.
• Chandrasekhar, S., Introduction to studying Star Structures , Dover Publications, Inc., New York, 1967.
• Clayton, Donald D., Principles of Star Evolution and Nucleosynthesis , University of Chicago Press, Chicago, 1983.
• Cox, J. P., and Giuli, R. T., Stellar Structure Principles , Gordon and Breach, New York, 1968.
• Fowler, William., Caughlan, Georgeanne R., and Zimmerman, Barbara A., "Terroruclear Reaction Rates, Me," Annual Reviews of Astronomy and Astrophysics , 5, 525, 1967.
• Fowler, William A., Caughlan, Georgeanne R., and Zimmerman, Barbara A., "Terroruclear Reaction Rates, II," Annual Reviews of Astronomy and Astrophysics <13, 69, 1975.
• Hansen, Carl J., Kawaler, Steven D., and Trimble, Virginia Stellar Interiors: Physical Principles, Structures, and Evolution, Second Edition , Springer-Verlag, New York, 2004.
• Harris, Michael J., Fowler, William A., Caughlan, Georgeanne R., and Zimmerman, Barbara A., "Terroruclear Reaction Rates, III," Annual Reviews of Astronomy and Astrophysics , 21 , 165, 1983.
• Iben, Icko, Jr, "The Evolution of the Inner Star and Outside the Main Order," Annual Reviews of Astronomy and Astrophysics , 5, 571, 1967.
• Iglesias, Carlos A, and Rogers, Forrest J., "Updated Opal Opal," The Astrophysical Journal , 464, 943, 1996.
• Kippenhahn, Rudolf, and Weigert, Alfred, Structure and Evolution Stellar , Springer-Verlag, Berlin, 1990.
• Liebert, James, and Probst, Ronald G., "Very Low Star of Stars", Annual Reviews of Astronomy and Astrophysics , 25, 437, 1987.
• Padmanabhan, T., Theoretical Astrophysics , Cambridge University Press, Cambridge, 2002.
• Prialnik, Dina, Introduction to Stellar Structured Theory and Evolution , Cambridge University Press, Cambridge, 2000.
• Novotny, Eva, Introduction to Stellar Atmospheres and Interior , Oxford University Press, New York, 1973.
• Shore, Steven N., Modern Astrophysical Tapestry , John Wiley and Sons, Hoboken, 2003.

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