A Hertzsprung-Russell diagram plots the actual brightness (or absolute magnitude) of a star against its color index (represented as B-V). In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard In Astronomy, the color index is a simple numerical expression that determines the color of an object which in the case of a Star gives its Temperature The main sequence is visible as a prominent diagonal band that runs from the upper left to the lower right.

The main sequence is the name for a continuous sequence of stars that appear on a plot of color versus brightness for groups of stars. In Astronomy, the color index is a simple numerical expression that determines the color of an object which in the case of a Star gives its Temperature These color-magnitude plots are known as Hertzsprung-Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell. In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD Ejnar Hertzsprung ( October 8, 1873, Copenhagen &ndash October 21, 1967, Roskilde) was a Danish chemist Henry Norris Russell ( October 25, 1877 &ndash February 18, 1957) was an American Astronomer who along with Ejnar Stars on this band are known as main-sequence stars or dwarf stars.

After a star has formed, it generates energy at the hot, dense core region through the nuclear fusion of hydrogen atoms into helium. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical During this stage of the star's lifetime, it is located along the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and other factors. In general, the more massive the star the shorter its lifespan on the main sequence. After the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence.

The main sequence is sometimes divided into upper and lower parts, based on the processes that stars use to generate energy. Stars below about 1. 5 times the mass of the Sun (or 1. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. 5 solar masses) fuse hydrogen atoms together in a series of stages to form helium; a sequence called the proton-proton chain. The proton-proton chain reaction is one of several fusion reactions by which Stars convert Hydrogen to Helium, the primary alternative being the Above this mass, in the upper main sequence, the nuclear fusion process can instead use atoms of carbon, nitrogen and oxygen as intermediaries in the production of helium from hydrogen atoms. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the

Because there is a temperature gradient between the core of a star and its surface, energy is steadily transported upward through the intervening layers until it is radiated away at the photosphere. The two mechanisms used to carry this energy through the star are radiation and convection, with the type used depending on the local conditions. Radiation, as in Physics, is Energy in the form of waves or moving Subatomic particles emitted by an atom or other body as it changes from a higher energy Convection in the most general terms refers to the movement of molecules within Fluids (i Convection tends to occur in regions with steeper temperature gradients, higher opacity or both. When convection occurs in the core region it acts to stir up the helium ashes, thus maintaining the proportion of fuel needed for fusion to occur.

History

In the early part of the twentieth century, information about the types and distances of stars became more readily available. The twentieth century of the Common Era began on A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth The spectra of stars were shown to have distinctive features, which allowed them to be categorized. Annie Jump Cannon and Edward C. Pickering at Harvard College Observatory had developed a method of categorization that became known as the Harvard classification scheme. Annie Jump Cannon ( December 11, 1863 &ndash April 13, 1941) was an American Astronomer whose cataloging work was instrumental Edward Charles Pickering ( July 19 1846 – February 3 1919) was an American Astronomer and Physicist, brother The Harvard College Observatory (or HCO is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Department of Astronomy This scheme was published in the Harvard Annals in 1901. ^[1]

In Potsdam in 1906, the Danish astronomer Ejnar Hertzsprung noticed that the reddest stars—classified as K and M in the Harvard scheme—could be divided into two distinct groups. Also see Potsdam New York (in the USA For the Potsdam Conference see Potsdam Conference. Ejnar Hertzsprung ( October 8, 1873, Copenhagen &ndash October 21, 1967, Roskilde) was a Danish chemist These stars are either much brighter than the Sun, or much fainter. To distinguish these groups, he called them "giant" and "dwarf" stars. The following year he began studying star clusters; large groupings of stars that are co-located at approximately the same distance. Star clusters are groups of Stars which are gravitationally bound He published the first plots of color versus luminosity for these stars. These plots showed a prominent and continuous sequence of stars, which he named the main sequence. ^[2]

At Princeton University, Henry Norris Russell was following a similar course of research. Princeton University is a private Coeducational research university located in Princeton, New Jersey. Henry Norris Russell ( October 25, 1877 &ndash February 18, 1957) was an American Astronomer who along with Ejnar He was studying the relationship between the spectral classification of stars and their actual brightness as corrected for distance—their absolute magnitude. In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard For this purpose he used a set of stars that had reliable parallaxes and many of which had been categorized at Harvard. When he plotted the spectral types of these stars against their absolute magnitude, he found that dwarf stars followed a distinct relationship. This allowed the real brightness of a dwarf star to be predicted with reasonable accuracy. ^[3]

Of the red stars observed by Hertzsprung, the dwarf stars also followed the spectra-luminosity relationship discovered by Russell. However, the giant stars are much brighter than dwarfs and so do not follow the same relationship. Russell proposed that the "giant stars must have low density or great surface-brightness, and the reverse is true of dwarf stars". The same curve also showed that there were very few faint white stars. ^[3]

In 1933, Bengt Strömgren introduced the term Hertzsprung-Russell diagram to denote a luminosity-spectral class diagram. Bengt Georg Daniel Strömgren ( January 21 1908 &ndash July 4 1987) was a Danish astronomer and astrophysicist ^[4] This name reflected the parallel development of this technique by both Hertzsprung and Russell earlier in the century. ^[2]

As evolutionary models of stars were developed during the 1930s, it was shown that, for stars of a uniform chemical composition, a relationship exists between a star's mass and its luminosity and radius. That is, for a given mass and composition is known, there is a unique solution determining the star's radius and luminosity. This became known as the Vogt-Russell theorem; named after Heinrich Vogt and Henry Norris Russell. The Vogt-Russell theorem, named after Heinrich Vogt and Henry Norris Russell, states that The mass and the composition structure throughout a star uniquely determine its By this theorem, once a star's chemical composition and its position on the main sequence is known, so too is the star's mass and radius. (However, it was subsequently discovered that the theorem breaks down somewhat for stars of non-uniform composition. )^[5]

A refined scheme for stellar classification was published in 1943 by W. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics W. Morgan and P. C. Keenan. ^[6] The MK classification assigned each star a spectral type—based on the Harvard classification—and a luminosity class. For historical reasons, the spectral types of stars followed, in order of decreasing temperature with colors ranging from blue to red, the sequence O, B, A, F, G, K and M. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics (A popular mnemonic for memorizing this sequence of stellar classes is "Oh Be A Fine Girl/Guy, Kiss Me". A mnemonic device (nəˈmɒnɪk is a Memory aid Commonly met mnemonics are often verbal something such as a very short poem or a special word used to help a person remember ) The luminosity class ranged from I to V, in order of decreasing luminosity. Stars of luminosity class V belonged to the main sequence. ^[7]

Characteristics

Main sequence stars have been extensively studied through stellar models, allowing their formation and evolutionary history to be relatively well understood. The position of the star on the main sequence provides information about its physical properties.

The temperature of a star can be approximately determined by treating it as an idealized energy radiator known as a black body. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature In Physics, a black body is an object that absorbs all light that falls on it In this case, the luminosity L and radius R are related to the temperature T by the Stefan-Boltzmann Law:

L = 4πσR²T⁴

where σ is the Stefan–Boltzmann constant. The Stefan–Boltzmann law, also known as Stefan's law, states that the total Energy radiated per unit surface Area of a Black body in unit The Stefan–Boltzmann constant (also Stefan's constant) a Physical constant denoted by the Greek letter σ, is the Constant of proportionality The temperature and composition of a star's photosphere determines the energy emission at different wavelengths. The color index, or B − V, measures the difference in this energy emission by means of filters that capture the star's magnitude in blue (B) and green-yellow (V) light. In Astronomy, the color index is a simple numerical expression that determines the color of an object which in the case of a Star gives its Temperature The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value (By measuring the difference between these values, this eliminates the need to correct the magnitudes for distance. ) Thus the position of a star on the HR diagram can be used to estimate its radius and temperature. ^[8] By modifying the physical properties of the plasma in the photosphere, the temperature of a star also determines its spectral type. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics

Formation

When a protostar is formed from the collapse of a giant molecular cloud of gas and dust in the local interstellar medium, the initial composition is homogeneous throughout, consisting of about 70% hydrogen, 28% helium and trace amounts of other elements, by mass. A protostar is a large object that forms by contraction out of the gas of a Giant molecular cloud in the Interstellar medium. See also Solar nebula A molecular cloud, sometimes called a stellar nursery if Star formation is occurring within is a type of Interstellar ^[9] During the initial collapse, this pre-main sequence star generates energy through gravitational contraction. A pre-main sequence star (PMS star or PMS object is a Star in the stage when it has not yet reached the Main sequence. Upon reaching a suitable density, energy generation is begun at the core using an exothermic nuclear fusion process that converts hydrogen into helium. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus ^[7]

Once nuclear fusion of hydrogen becomes the dominant energy production process and the excess energy gained from gravitational contraction has been lost,^[10] the star lies along a curve on the Hertzsprung-Russell diagram (or HR diagram) called the standard main sequence. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics Brown dwarfs are sub- stellar objects with a mass below that necessary to maintain Hydrogen -burning Nuclear fusion reactions in their cores as do stars A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool Star, of the Main sequence, either late K A subdwarf star, sometimes denoted by "sd" is Luminosity class VI under the Yerkes spectral classification system Subgiant star is a class of Stars that are slightly brighter than normal Main sequence (dwarf stars of the same spectral class but not as bright as A giant star is a Star with substantially larger Radius and Luminosity than a Main sequence star of the same surface temperature. The luminosity class II in the Yerkes spectral classification is given to bright giants. Supergiants are among the most massive Stars In the Hertzsprung-Russell diagram they occupy the top region of the diagram A hypergiant ( luminosity class 0) is a Star with a tremendous Mass and Luminosity, showing signs of a very high rate of mass loss In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard In Mathematics, the concept of a curve tries to capture the intuitive idea of a geometrical one-dimensional and continuous object The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD Astronomers will sometimes refer to this stage as "zero age main sequence", or ZAMS. ^[11] This is a line calculated by computer models of where a star will be when it begins hydrogen fusion; its brightness and surface temperature typically increase from this point with age. ^[12]

The star remains near its initial position on the main sequence until a significant amount of hydrogen in the core has been consumed, then begins to evolve into a more luminous star. (On the HR diagram, the evolving star moves up and to the right of the main sequence. ) Thus the main sequence represents the primary hydrogen-burning stage of a star's lifetime. ^[7]

The majority of stars on a typical HR diagram lie along the main sequence curve. This line is so pronounced because both the spectral type and the luminosity depend only on a star's mass, at least to zeroth order approximation, as long as it is fusing hydrogen at its core—and that is what almost all stars spend most of their "active" life doing. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics Luminosity has different meanings in several different fields of science Orders of approximation have been used not only in Science, Engineering, and other quantitative disciplines to make Approximations with various degrees ^[13] These main-sequence (and therefore "normal") stars are called dwarf stars. This is not because they are unusually small, but instead they have smaller radii and are less luminous than the other main type of stars, the giant stars. A giant star is a Star with substantially larger Radius and Luminosity than a Main sequence star of the same surface temperature. ^[14] White dwarfs are a different kind of star which are smaller than main sequence stars—roughly the size of the Earth. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 These represent the final evolutionary stage of many main sequence stars. ^[15]

Energy generation

All main sequence stars have a core region where energy is generated by nuclear fusion. The temperature and density of this core are at the levels necessary to sustain the energy production needed to support the remainder of the star. A reduction of energy production would cause the overlaying mass to compress, increasing the temperature and pressure needed for fusion. Likewise an increase in energy production would cause the star to expand, lowering the pressure at the core. Thus the star forms a self-regulating system in hydrostatic equilibrium that is stable over the course of its main sequence lifetime. Hydrostatic equilibrium occurs when compression due to Gravity is balanced by a Pressure gradient which creates a Pressure gradient force in the opposite ^[16]

Astronomers divide the main sequence into upper and lower parts, based on the type of fusion process at the core. Stars in the upper main sequence have sufficient mass to use the CNO cycle to fuse hydrogen into helium. The CNO cycle (for Carbon - Nitrogen - Oxygen) or sometimes Bethe-Weizsäcker-cycle, is one of two sets of fusion reactions This process uses atoms of carbon, nitrogen and oxygen as intermediaries in the fusion process. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the In the lower main sequence, energy is generated as the result of the proton-proton chain, which directly fuses hydrogen together in a series of stages to produce helium. The proton-proton chain reaction is one of several fusion reactions by which Stars convert Hydrogen to Helium, the primary alternative being the ^[17]

At a stellar core temperature of 18 million kelvins, both fusion processes are equally efficient. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic This is the core temperature of a star with 1. 5 solar masses. Hence the upper main sequence consists of stars above this mass. The apparent upper limit for a main sequence star is 120-200 solar masses. ^[18] The lower limit for sustained nuclear fusion is about 0. 08 solar masses. ^[17]

Structure

This diagram shows a cross-section of a Sun-like star, showing the internal structure.

Because there is a temperature difference between the core and the surface, or photosphere, energy is transported outward. The two modes for transporting this energy are radiation and convection. Radiation, as in Physics, is Energy in the form of waves or moving Subatomic particles emitted by an atom or other body as it changes from a higher energy Convection in the most general terms refers to the movement of molecules within Fluids (i A radiation zone, where energy is transported by radiation, is stable against convection and there is very little mixing of the plasma. The radiation zone is the middle zone in the Sun 's interior Energy travels out of the core into the radiation zone Radiation, as in Physics, is Energy in the form of waves or moving Subatomic particles emitted by an atom or other body as it changes from a higher energy By contrast, in a convection zone the energy is transported by bulk movement of plasma, with hotter material rising and cooler material descending. The convection zone of a Star is the range of radii in which energy is transported primarily by Convection. Convection is a more efficient mode for carrying energy than radiation, but it will only occur under conditions that create a steep temperature gradient. ^[19][16]

In massive stars, the rate of energy generation by the CNO cycle is very sensitive to temperature, so the fusion is highly concentrated at the core. Consequently, there is a high temperature gradient at the core, which results in a convection zone for more efficient energy transport. ^[17] The mixing of material around the core removes the helium ashes from the hydrogen burning region, allowing more of the hydrogen in the star to be burned. The outer regions of a massive star transport energy by radiation, with little or no convection. ^[16]

Intermediate mass, class A stars such as Sirius may transport energy entirely by radiation. Sirius is the brightest star in the night sky with a visual Apparent magnitude of &minus1 ^[20] Medium-sized, low mass stars like the Sun have a core region that is stable against convection and a convection zone near the surface. This produces mixing of the outer layers, but a less efficient burning of the hydrogen in the star. The eventual result is the buildup of a helium-rich core, surrounded by a hydrogen-rich region. By contrast, cool, low-mass stars are convective throughout. The helium produced at the core is distributed across the star, producing a relatively uniform atmosphere. ^[16]

Luminosity-color variation

As non-fusing helium ash accumulates in the core, the reduction in the abundance of hydrogen per unit mass results in a gradual lowering of the fusion rate within that mass. To compensate, the core temperature and pressure slowly increase, which actually causes a net increase in the overall fusion rate (to support the greater density of the inner star). This produces a steady increase in the luminosity and radius of the star over time. ^[12] Thus, for example, the luminosity of the early Sun was only about 70% of its current value. ^[21] The luminosity increase of a star changes its position on the HR diagram; resulting in a broadening of the main sequence band because stars are observed at random stages in their lifetime. ^[22]

The stars in the main sequence do not lie upon a narrow curve on the HR diagram. This is primarily because of the observational uncertainties that mainly affect the distance of the star in question, but also because of factoring in unresolved binary stars. A binary star is a Star system consisting of two Stars orbiting around their Center of mass. However, even perfect observations would lead to a fuzzy main sequence, because mass is not a star's only parameter. In addition to variations in chemical composition—both because of the initial abundances and the star's evolutionary status,^[23] the presence of a close companion,^[24] rapid rotation,^[25] or a magnetic field can also move a star slightly on the main sequence, to name just a few factors. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime A binary star is a Star system consisting of two Stars orbiting around their Center of mass. Stellar rotation is the angular motion of a Star about its axis A stellar magnetic field is a Magnetic field generated by the motion of conductive plasma inside a Main sequence ( Hydrogen -burning For example, there are stars with a very low abundance of elements with higher atomic numbers than helium—known as metal-poor stars—that lie just below the main sequence. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen Also known as subdwarfs, these stars are also fusing hydrogen in their core and so they mark the lower edge of the main sequence's fuzziness due to chemical composition. A subdwarf star, sometimes denoted by "sd" is Luminosity class VI under the Yerkes spectral classification system ^[26]

A nearly vertical region of the HR diagram is known as the instability strip and is occupied by pulsating variable stars. The Instability strip is a nearly vertical region in the HR diagram which is occupied by pulsating Variable stars (including RR Lyrae variable, Cepheid For the astronomical object see Variable star. Variable Star is a 2006 novel written by Spider Robinson These stars vary in magnitude at regular intervals, giving them a pulsating appearance. The strip intersects the upper part of the main sequence in the region of class A and F stars; between one and two solar masses. However, main sequence stars in this region experience only small variations in magnitude and so are hard to detect. ^[27]

Lifetime

The lifespan that a star spends on the main sequence is governed by two factors. The total amount of energy that can be generated through nuclear fusion of hydrogen is limited by the amount of available hydrogen fuel that can be consumed at the core. For a star in equilibrium, the energy generated at the core must be at least equal to the energy radiated at the surface. Since the luminosity gives the amount of energy radiated per unit time, the total life span can be estimated, to first approximation, as the total energy produced divided by the star's luminosity. ^[28]

Our Sun has been a main sequence star for about 4. The Sun (Sol is the Star at the center of the Solar System. 5 billion years and will continue to be one for another 5. 5 billion years, for a total main sequence lifetime of 10¹⁰ years. After the hydrogen supply in the core is exhausted, it will expand to become a red giant and fuse helium atoms to form carbon. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 As the energy output of the helium fusion process per unit mass is only about a tenth the energy output of the hydrogen process, this stage will only last for about 10% of a star's total active lifetime. Thus, on average, about 90% of the observed stars will be on the main sequence. ^[29]

On average, main sequence stars are known to follow an empirical mass-luminosity relationship. ^[30] The luminosity (L) of the star is approximately related to the total mass (M) as the following power law:

The amount of fuel available for nuclear fusion is proportional to the mass of the star. A power law is any Polynomial relationship that exhibits the property of Scale invariance. Thus, the lifetime of a star on the main sequence can be estimated by comparing it to the Sun:^[31]

where M and L are the mass and luminosity of the star, respectively, is a solar mass, is the solar luminosity and τ_ms is the star's estimated main sequence lifetime. The solar luminosity, L_\odot is a unit of Luminosity ( power emitted in the form of Photons conventionally used by Astronomers to

This plot gives an example of the mass-luminosity relationship for zero-age main sequence stars. The mass and luminosity are relative to the present-day Sun.

This is a counter-intuitive result, as more massive stars have more fuel to burn and might be expected to last longer. Instead, the lightest stars, of less than a tenth of a solar mass, may last over a trillion years. ^[32] For the heaviest stars, however, this mass-luminosity relationship poorly matches the estimated lifetime, which last at least a few million years. A more accurate representation gives a different function for various ranges of mass.

The mass-luminosity relationship depends on how efficiently energy can be transported from the core to the surface. A higher opacity has an insulating effect that retains more energy at the core, so the star does not need to produce as much energy to remain in hydrostatic equilibrium. Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation especially visible Light. Hydrostatic equilibrium occurs when compression due to Gravity is balanced by a Pressure gradient which creates a Pressure gradient force in the opposite By contrast, a lower opacity means energy escapes more rapidly and the star must burn more fuel to remain in equilibrium. ^[33] Note, however, that a sufficiently high opacity can result in energy transport via convection, which changes the conditions needed to remain in equilibrium. Convection in the most general terms refers to the movement of molecules within Fluids (i ^[34]

In high mass main sequence stars, the opacity is dominated by electron scattering, which is nearly constant with increasing temperature. Electron scattering is the process whereby an Electron is deflected from its original Trajectory. Thus the luminosity only increases as the cube of the star's mass. ^[35] For stars below 10 times the solar mass, the opacity becomes dependent on temperature, resulting in the luminosity varying approximately as the fourth power of the star's mass. ^[36] For very low mass stars, molecules in the atmosphere also contribute to the opacity. Below about 0. 5 solar masses, the luminosity of the star varies as the mass to the power of 2. 3, producing a flattening of the slope on a graph of mass versus luminosity. Even these refinements are only an approximation, however, and the mass-luminosity relation can vary depending on a star's composition. ^[37]

Evolutionary tracks

Once a main sequence star consumes the hydrogen at its core, the loss of energy generation causes gravitational collapse to resume. The hydrogen surrounding the core reaches sufficient temperature and pressure to undergo fusion, forming a hydrogen-burning shell surrounding a helium core. In consequence of this change, the outer envelope of the star expands and decreases in temperature, turning it into a red giant. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 At this point the star is evolving off the main sequence and entering the giant branch. (The path the star now follows across the HR diagram is called an evolutionary track. ) The helium core of the star continues to collapse until it is entirely supported by electron degeneracy pressure—a quantum mechanical effect that restricts how closely matter can be compacted. Electron degeneracy pressure is a consequence of the Pauli exclusion principle, which states that two Fermions cannot occupy the same Quantum state at the Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons For stars of more than about 0. 5 solar masses,^[38] the core can reach a temperature where it becomes hot enough to burn helium into carbon via the triple alpha process. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. The triple alpha process is a set of Nuclear fusion reactions by which three Helium nuclei ( Alpha particles are transformed into Carbon. ^[39][40]

This shows the Hertzsprung-Russell diagrams for two open clusters. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD NGC 188 is older, and shows a lower turn off from the main sequence than that seen in M67. NGC 188 is an Open cluster in the constellation Cepheus. It was discovered by John Herschel in 1825 Messier 67 (also known as M67 or NGC 2682) is an Open cluster, or galactic cluster in the constellation Cancer.

When a cluster of stars is formed at about the same time, the life span of these stars will depend on their individual masses. Star clusters are groups of Stars which are gravitationally bound The most massive stars will leave the main sequence first, followed steadily in sequence by stars of ever lower masses. Thus the stars will evolve in order of their position on the main sequence, proceeding from the most massive at the left toward the right of the HR diagram. The current position where stars in this cluster are leaving the main sequence is known as the turn-off point. By knowing the main sequence lifespan of stars at this point, it becomes possible to estimate the age of the cluster. ^[41]

Stellar parameters

The table below shows typical values for stars along the main sequence. The values of luminosity (L), radius (R) and mass (M) are relative to the Sun—a dwarf star with a spectral classification of G2 V. Luminosity has different meanings in several different fields of science Remote Authentication Dial In User Service ( RADIUS) is a networking protocol that provides centralized access authorization and accounting management for people or computers Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object The actual values for a star may vary by as much as 20-30% from the values listed below. ^[42] The coloration of the stellar class column gives an approximate representation of the star's photographic color, which is a function of the effective surface temperature. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature

Table of main sequence stellar parameters^[43]

Stellar Class	Radius	Mass	Luminosity	Temperature	Examples
	R/R☉	M/M☉	L/L☉	K
O5	18	40	500,000	38,000	Sanduleak −66° 41, Zeta Puppis
B0	7. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics Remote Authentication Dial In User Service ( RADIUS) is a networking protocol that provides centralized access authorization and accounting management for people or computers In Astronomy, the solar radius is a unit of Length used to express the size of Stars It is equal to the current radius of the Sun. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. The solar luminosity, L_\odot is a unit of Luminosity ( power emitted in the form of Photons conventionally used by Astronomers to The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic Zeta Puppis (ζ Pup / ζ Puppis is a Star in the Constellation of Puppis. 4	18	20,000	30,000	Phi1 Orionis
B5	3. The Bayer designation Phi Orionis (φ Ori/ φ Orionis is shared by two Star systems φ¹ Orionis and φ² Orionis, in the Constellation 8	6. 5	800	16,400	Pi Andromedae A
A0	2. Pi Andromedae (π And / π Andromedae is a Star system in the Constellation Andromeda. 5	3. 2	80	10,800	Alpha Coronae Borealis A
A5	1. Alpha Coronae Borealis (α CrB / α Coronae Borealis is a Binary star in the Constellation Corona Borealis. 7	2. 1	20	8,620	Beta Pictoris
F0	1. Beta Pictoris (β Pic / β Pictoris is the second brightest Star in the Constellation Pictor. 4	1. 7	6	7,240	Gamma Virginis
F5	1. 2	1. 29	2. 5	6,540	Eta Arietis
G0	1. Eta Arietis (η Ari / η Arietis is a Star in the Constellation Aries. 05	1. 10	1. 26	6,000	Beta Comae Berenices
G2	1. Beta Coma Berenices (β Comae Berenices / β Com is a Main sequence dwarf Star in the Constellation of Coma Berenices. 00[44]	1. 00[44]	1. 00[44]	5,920	Sun, Alpha Centauri A
G5	0. The Sun (Sol is the Star at the center of the Solar System. Alpha Centauri (α Centauri / α Cen also known as Rigil Kentaurus, Rigil Kent, or Toliman, is the brightest Star in the southern Constellation 93	0. 93	0. 79	5,610	Alpha Mensae
K0	0. Alpha Mensae is the brightest star in the Constellation Mensa. 85	0. 78	0. 40	5,150	70 Ophiuchi A
K5	0. 70 Ophiuchi is a binary Star system 166 Light years away from Earth. 74	0. 69	0. 16	—	61 Cygni A
M0	0. Not be confused with 16 Cygni, a more distant system containing two G-type stars harboring the Gas giant planet 16 Cygni Bb. 63	0. 47	0. 063	3,920	Gliese 185
M5	0. 32	0. 21	0. 0079	3,120	EZ Aquarii A
M8	0. EZ Aquarii is a Triple star system approximately 34 pc or 113 Light-years from the Sun, in the Constellation Aquarius 13	0. 10	0. 0008	—	Van Biesbroeck's star

See also

• Hertzsprung-Russell diagram
• Hydrogen burning process

References

External links

• Table of Features of the "Life Zones" of Main Sequence Stars
• A java based applet for stellar evolution.