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lot shows 22 000 stars from the hipparcos catalog together with 1 000 low luminosity stars red and white dwarfs from the gliese catalogue of nearby stars in astronomy the main sequence is a classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band stars spend the majority of their lives on the main sequence during which core hydrogen burning is dominant these main sequence stars or sometimes interchangeably dwarf stars are the most numerous true stars in the universe and include the sun color magnitude plots are known as hertzsprung russell diagrams after ejnar hertzsprung and henry norris russell when a gaseous nebula undergoes sufficient gravitational collapse the high pressure and temperature concentrated at the core will trigger the nuclear fusion of hydrogen into helium see stars the thermal energy from this process radiates out from the hot dense core generating a strong pressure gradient it is this pressure gradient that counters the star s collapse under gravity maintaining the star in a state of hydrostatic equilibrium the star s position on the main sequence is determined primarily by the mass but also by age and chemical composition as a result radiation is not the only method of energy transfer in stars convection plays a role in the movement of energy particularly in the cores of stars greater than 1 3 to 1 5 times the sun s mass again depending on age and chemical composition when discussing chemical composition astronomers generally refer to the metallicity of the star this is the abundance of heavier than helium elements present in the star for example the fraction of the sun by mass currently composed of hydrogen denoted x is 74 9 for helium denoted y it is 23 8 meaning the star s metallicity or mass fraction of all other elements is 1 3 denoted z this is a typical range for similar mass main sequence stars in fact a higher metallicity leads to a higher opacity whereby the energy production can remain concentrated in the core without being radiated or transferred away to the star s outer layers this hotter environment speeds up nuclear fusion and decreases the amount of time the star will spend on the main sequence the main sequence is divided into upper and lower parts based on the dominant process that a star uses to generate energy the sun along with main sequence stars below about 1 5 m primarily fuse hydrogen atoms together in a series of stages to form helium a sequence called the proton proton chain above this mass in the upper main sequence the nuclear fusion process mainly uses atoms of carbon nitrogen and oxygen as intermediaries in the cno cycle that produces helium from hydrogen atoms the proton proton chain is still occurring but it produces less energy than the cno cycle main sequence stars where the cno cycle is the dominant energy production process undergo convection in their core regions which acts to stir up the newly created helium and maintain the proportion of fuel needed for fusion to occur below this mass stars have cores that are entirely radiative with convective zones near the surface with decreasing stellar mass the proportion of the star forming a convective envelope steadily increases the main sequence stars below 0 4 m undergo convection throughout their mass when core convection does not occur a helium rich core develops surrounded by an outer layer of hydrogen the more massive a star is 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 on the hr diagram into a supergiant red giant or directly to a white dwarf history edit hertzsprung russell diagram spectral type o b a f g k m l t brown dwarfs white dwarfs red dwarfs subdwarfs main sequence dwarfs subgiants giants red giants blue giants bright giants supergiants red supergiant hypergiants absolute magni tude m v in the early part of the 20th century information about the types and distances of stars became more readily available the spectra of stars were shown to have distinctive features which allowed them to be categorized annie jump cannon and edward charles pickering at harvard college observatory developed a method of categorization that became known as the harvard classification scheme 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 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 for these stars he published the first plots of color versus luminosity 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 he was studying the relationship between the spectral classification of stars and their actual brightness as corrected for distance their absolute magnitude 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 giant stars are much brighter than dwarfs and so do not follow the same relationship russell proposed that 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 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 with the same composition the star s mass determines its luminosity and radius conversely when a star s chemical composition and its position on the main sequence are known the star s mass and radius can be deduced this became known as the vogt russell theorem named after heinrich vogt and henry norris russell it was subsequently discovered that this relationship breaks down somewhat for stars of the non uniform composition 5 a refined scheme for stellar classification was published in 1943 by william wilson morgan and philip childs keenan 6 the mk classification assigned each star a spectral type based on the harvard classification and a luminosity class the harvard classification had been developed by assigning a different letter to each star based on the strength of the hydrogen spectral line before the relationship between spectra and temperature was known when ordered by temperature and when duplicate classes were removed 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 a popular mnemonic for memorizing this sequence of stellar classes is oh be a fine girl guy kiss me the luminosity class ranged from i to v in order of decreasing luminosity stars of luminosity class v belonged to the main sequence 7 in april 2018 astronomers reported the detection of the most distant ordinary i e main sequence star named icarus formally macs j1149 lensed star 1 at 9 billion light years away from earth 8 9 formation and evolution edit star formation object classes interstellar medium molecular cloud bok globule dark nebula young stellar object protostar pre main sequence star t tauri star herbig ae be star herbig haro object theoretical concepts accretion initial mass function jeans instability kelvin helmholtz mechanism nebular hypothesis planetary migration v t e main articles star formation protostar pre main sequence star and stellar evolution main sequence stellar mass objects zero age main sequence and evolutionary tracks the violent youth of stars like the sun 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 approximately 70 hydrogen 28 helium and trace amounts of other elements by mass 10 the initial mass of the star depends on the local conditions within the cloud the mass distribution of newly formed stars is described empirically by the initial mass function 11 during the initial collapse this pre main sequence star generates thermal energy through the increase in pressure arising due to its gravitational contraction during this phase before hydrogen ignition the star will spend a length of time contracting known as the kelvin helmholtz or thermal timescale this timescale describes the length of time a star can last by radiating its internal kinetic energy once sufficiently dense stars begin converting hydrogen into helium and producing energy through an exothermic nuclear fusion process 7 the nuclear timescale is useful to describe the length of time a star can last during this next phase when nuclear fusion of hydrogen becomes the dominant energy production process and the excess energy gained from gravitational contraction has been lost 12 the star lies along a curve on the hertzsprung russell diagram or hr diagram called the standard main sequence astronomers will sometimes refer to this stage as zero age main sequence or zams 13 14 the zams curve can be calculated using computer models of stellar properties at the point when stars begin hydrogen fusion from this point the brightness and surface temperature of stars typically increase with age 15 a 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 classification edit hot and brilliant o type main sequence stars in star forming regions these are all regions of star formation that contain many hot young stars including several bright stars of spectral type o 16 further information ob star main sequence stars are divided into the following types o type main sequence star b type main sequence star a type main sequence star f type main sequence star g type main sequence star k type main sequence star m type main sequence star m type and to a lesser extent k type 17 main sequence stars are usually referred to as red dwarfs properties edit size comparison of main sequence stars including proxima centauri the sun and sirius and the blue giant spica the majority of stars on a typical hr diagram lie along the main sequence curve this line is pronounced because both the spectral type and the luminosity depends 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 lives doing 18 the temperature of a star determines its spectral type via its effect on the physical properties of plasma in its photosphere a star s energy emission as a function of wavelength is influenced by both its temperature and composition a key indicator of this energy distribution is given by the color index b v which measures the star s magnitude in blue b and green yellow v light by means of filters note 1 this difference in magnitude provides a measure of a star s temperature dwarf terminology edit main sequence stars are called dwarf stars 19 20 but this terminology is partly historical and can be somewhat confusing for the cooler stars dwarfs such as red dwarfs orange dwarfs and yellow dwarfs are indeed much smaller and dimmer than other stars of those colors however for hotter blue and white stars the difference in size and brightness between so called dwarf stars that are on the main sequence and so called giant stars that are not becomes smaller for the hottest stars the difference is not directly observable and for these stars the terms dwarf and giant refer to differences in spectral lines which indicate whether a star is on or off the main sequence nevertheless very hot main sequence stars are still sometimes called dwarfs even though they have roughly the same size and brightness as the giant stars of that temperature 21 the common use of dwarf to mean the main sequence is confusing in another way because there are dwarf stars that are not main sequence stars for example a white dwarf is the dead core left over after a star has shed its outer layers and is much smaller than a main sequence star roughly the size of earth these represent the final evolutionary stage of many main sequence stars 22 parameters edit comparison of main sequence stars of each spectral class by treating the star as an idealized energy radiator known as a black body the luminosity l and radius r can be related to the effective temperature t eff by the stefan boltzmann law l 4 π σ r 2 t eff 4 displaystyle l 4 pi sigma r 2 t_ text eff 4 where σ is the stefan boltzmann constant as the position of a star on the hr diagram shows its approximate luminosity this relation can be used to estimate its radius 23 the mass radius and luminosity of a star are closely interlinked and their respective values can be approximated by three relations first is the stefan boltzmann law which relates the luminosity l the radius r and the surface temperature t eff second is the mass luminosity relation which relates the luminosity l and the mass m finally the relationship between m and r is close to linear the ratio of m to r increases by a factor of only three over 2 5 orders of magnitude of m this relation is roughly proportional to the star s inner temperature t i and its extremely slow increase reflects the fact that the rate of energy generation in the core strongly depends on this temperature whereas it has to fit the mass luminosity relation thus a too high or too low temperature will result in stellar instability a better approximation is to take ε l m the energy generation rate per unit mass as ε is proportional to t i 15 where t i is the core temperature this is suitable for stars at least as massive as the sun exhibiting the cno cycle and gives the better fit r m 0 78 24 sample parameters edit 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 the actual values ...
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