Main sequence: Difference between revisions

Revision as of 23:19, 24 October 2023

Revision as of 23:55, 24 October 2023

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[[File:HRDiagram.png|thumb|upright=1.4|A [[Hertzsprung–Russell diagram]] plots the [[luminosity]] (or [[absolute magnitude]]) of a star against its [[color index]] (represented as B−V). The main sequence is visible as a prominent diagonal band that runs from the upper left to the lower right. This plot shows 22,000 stars from the [[Hipparcos Catalogue]] together with 1,000 low-luminosity stars (red and white dwarfs) from the [[Gliese Catalogue of Nearby Stars]].]]

''<u>'''In [[astronomy]], the '''main sequence''' is a continuous and distinctive band of [[star]]s that appears on plots of stellar [[color index|color]] versus [[absolute magnitude|brightness]]. These color-magnitude plots are known as [[Hertzsprung–Russell diagram]]s after their co-developers, [[Ejnar Hertzsprung]] and [[Henry Norris Russell]]. Stars on this band are known as '''main-sequence stars''' or [[dwarf star]]s. These are the most numerous true stars in the universe and include the [[Sun]].'''</u>''

'''''<u>After condensation and ignition of a star, it generates [[thermal energy]] in its dense [[stellar core|core region]] through [[nuclear fusion]] of [[hydrogen]] into [[helium]]. During this stage of the star's lifetime, it is located on the main sequence at a position determined primarily by its mass but also based on its chemical composition and age. The cores of main-sequence stars are in [[hydrostatic equilibrium]], where outward thermal pressure from the hot core is balanced by the inward pressure of [[gravitational collapse]] from the overlying layers. The strong dependence of the rate of energy generation on temperature and pressure helps to sustain this balance. Energy generated at the core makes its way to the surface and is radiated away at the [[photosphere]]. The energy is carried by either [[radiation]] or [[convection]], with the latter occurring in regions with steeper temperature gradients, higher opacity, or both.</u>'''''

'''''<u>The main sequence is sometimes 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 times the [[solar mass|mass of the Sun]] ({{solar mass|1.5}}), 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. Main-sequence stars with more than two solar masses 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. Main-sequence stars below {{solar mass|0.4}} undergo convection throughout their mass. When core convection does not occur, a helium-rich core develops surrounded by an outer layer of hydrogen.</u>'''''

'''''<u>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 [[stellar evolution|evolves]] away from the main sequence on the HR diagram, into a [[supergiant]], [[red giant]], or directly to a [[white dwarf]].</u>'''''

=='''''History'''''==

{{Star nav}}

In the early part of the 20th century, information about the types and distances of [[star]]s became more readily available. The [[stellar spectrum|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]] developed a method of categorization that became known as the [[stellar classification|Harvard Classification Scheme]], published in the ''Harvard Annals'' in 1901.<ref name=longair06/>

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In April 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) [[star]], named [[Icarus (star)|Icarus]] (formally, [[MACS J1149 Lensed Star 1]]), at 9 billion light-years away from [[Earth]].<ref name=" NA-20180402">{{cite journal |author=Kelly, Patrick L. |display-authors=etal |title=Extreme magnification of an individual star at redshift 1.5 by a galaxy-cluster lens |date=2 April 2018 |journal=[[Nature (journal) |Nature]] |volume=2 |issue=4 |pages=334–342 |doi=10.1038/s41550-018-0430-3 |arxiv=1706.10279 |bibcode=2018NatAs...2..334K |s2cid=125826925}}</ref><ref name=" SPC-20180402">{{cite web |last=Howell |first=Elizabeth |title=Rare Cosmic Alignment Reveals Most Distant Star Ever Seen |url=https://www.space.com/40171-cosmic-alignment-reveals-most-distant-star-yet.html |date=2 April 2018 |work=[[Space.com]] |access-date=2 April 2018}}</ref>

=='''''Formation and evolution'''''==

{{Star formation}}

{{Main|Star formation|Protostar|Pre-main-sequence star|Stellar evolution#Main sequence}}

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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.<ref name=tnc/>

=='''''Properties'''''==

The majority of stars on a typical HR diagram lie along the main-sequence curve. This line is pronounced because both the [[stellar classification|spectral type]] and the [[luminosity]] depends only on a star's mass, at least to [[order of approximation|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.<ref name=mss_atoe/>

The temperature of a star determines its [[spectral type]] via its effect on the physical properties of [[plasma (physics)|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''&nbsp;−&nbsp;''V'', which measures the star's [[apparent magnitude|magnitude]] in blue (''B'') and green-yellow (''V'') light by means of filters.<ref group=note>By measuring the difference between these values, eliminates the need to correct the magnitudes for distance. However, this can be affected by [[Extinction (astronomy)|interstellar extinction]].</ref> This difference in magnitude provides a measure of a star's temperature.

=='''''Dwarf terminology'''''==

Main-sequence stars are called dwarf stars,<ref name=smith91/><ref name=powell06/> but this terminology is partly historical and can be somewhat confusing. For the cooler stars, dwarfs such as [[red dwarf]]s, [[orange dwarf]]s, and [[yellow dwarf star|yellow dwarf]]s 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 line]]s 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.<ref name=moore06/>

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.<ref name=wd_sao/>

=='''''Parameters'''''==

[[File:Morgan-Keenan spectral classification.svg|thumb|right|upright=1.2|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]]:

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[[File:Representative lifetimes of stars as a function of their masses.svg|thumb|upright=1.35|Representative lifetimes of stars as a function of their masses]]

=='''''Energy generation'''''==

{{See also|Stellar nucleosynthesis}}

[[File:Nuclear energy generation.svg|right|upright=1.5|thumb|[[Logarithm]] of the relative energy output (ε) of [[proton–proton chain|proton–proton]] (PP), [[CNO cycle|CNO]] and [[triple-alpha process|triple-α]] fusion processes at different temperatures (T). The dashed line shows the combined energy generation of the PP and CNO processes within a star. At the Sun's core temperature, the PP process is more efficient.]]

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The observed upper limit for a main-sequence star is 120–200 {{solar mass}}.<ref name=apj620_1/> The theoretical explanation for this limit is that stars above this mass can not radiate energy fast enough to remain stable, so any additional mass will be ejected in a series of pulsations until the star reaches a stable limit.<ref name=apj162/> The lower limit for sustained proton-proton nuclear fusion is about 0.08 {{solar mass}} or 80 times the mass of [[Jupiter]].<ref name=hannu/> Below this threshold are sub-stellar objects that can not sustain hydrogen fusion, known as [[brown dwarf]]s.<ref name=apj406_1/>

=='''''Structure'''''==

{{Main|Stellar structure}}

[[File:Solar internal structure.svg|right|upright=1.0|thumb|This diagram shows a cross-section of a Sun-like star, showing the internal structure.]]

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Intermediate-mass stars such as [[Sirius]] may transport energy primarily by radiation, with a small core convection region.<ref name=lockner06/> Medium-sized, low-mass stars like the Sun have a core region that is stable against convection, with a convection zone near the surface that mixes the outer layers. This results in a steady buildup of a helium-rich core, surrounded by a hydrogen-rich outer region. By contrast, cool, very low-mass stars (below 0.4 {{solar mass}}) are convective throughout.<ref name=science295_5552/> Thus the helium produced at the core is distributed across the star, producing a relatively uniform atmosphere and a proportionately longer main-sequence lifespan.<ref name=brainerd/>

=='''''Luminosity-color variation'''''==

[[File: The Sun in white light.jpg|thumb|upright=1.0|The [[Sun]] is the most familiar example of a main-sequence star]]

As non-fusing helium ash accumulates in the core of a main-sequence star, the reduction in the abundance of hydrogen per unit mass results in a gradual lowering of the fusion rate within that mass. Since it is the outflow of fusion-supplied energy that supports the higher layers of the star, the core is compressed, producing higher temperatures and pressures. Both factors increase the rate of fusion thus moving the equilibrium towards a smaller, denser, hotter core producing more energy whose increased outflow pushes the higher layers further out. Thus there is a steady increase in the luminosity and radius of the star over time.<ref name=clayton83/> For example, the luminosity of the early Sun was only about 70% of its current value.<ref name=sp74/> As a star ages this luminosity increase changes its position on the HR diagram. This effect results in a broadening of the main sequence band because stars are observed at random stages in their lifetime. That is, the main sequence band develops a thickness on the HR diagram; it is not simply a narrow line.<ref name=padmanabhan01/>

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A nearly vertical region of the HR diagram, known as the [[instability strip]], is occupied by pulsating [[variable star]]s known as [[Cepheid variable]]s. 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, which are between one and two solar masses. Pulsating stars in this part of the instability strip intersecting the upper part of the main sequence are called [[Delta Scuti variable]]s. Main-sequence stars in this region experience only small changes in magnitude, so this variation is difficult to detect.<ref name=green04/> Other classes of unstable main-sequence stars, like [[Beta Cephei variable]]s, are unrelated to this instability strip.

=='''''Lifetime''''' ==

[[File:Isochrone ZAMS Z2pct.png|upright=1.0|right|thumb|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.]]

The total amount of energy that a star can generate through nuclear fusion of hydrogen is limited by the amount of hydrogen fuel that can be consumed at the core. For a star in equilibrium, the thermal 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 [[order of approximation|first approximation]], as the total energy produced divided by the star's luminosity.<ref name=rit_ms/>

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In high-mass main-sequence stars, the opacity is dominated by [[electron scattering]], which is nearly constant with increasing temperature. Thus the luminosity only increases as the cube of the star's mass.<ref name="prialnik00"/> For stars below 10 {{solar mass}}, the opacity becomes dependent on temperature, resulting in the luminosity varying approximately as the fourth power of the star's mass.<ref name=rolfs_rodney88>{{cite book |first=Claus E. |last=Rolfs |author2=Rodney, William S. |date=1988 |title=Cauldrons in the Cosmos: Nuclear Astrophysics |publisher=University of Chicago Press |isbn=978-0-226-72457-7}}</ref> For very low-mass stars, molecules in the atmosphere also contribute to the opacity. Below about 0.5 {{solar mass}}, 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.<ref name=science295_5552>{{cite journal |last=Kroupa |first=Pavel |title=The Initial Mass Function of Stars: Evidence for Uniformity in Variable Systems |journal=Science |date=2002 |volume=295 |issue=5552 |pages=82–91 |url=https://www.science.org/doi/10.1126/science.1067524 |access-date=2007-12-03 |doi=10.1126/science.1067524 |pmid=11778039 |arxiv=astro-ph/0201098 |bibcode=2002Sci...295...82K |s2cid=14084249}}</ref>

=='''''Evolutionary tracks'''''==

{{Main|Stellar evolution}}

[[File:Evolutionary track 1m.svg|thumb|left|Evolutionary track of a star like the sun]]

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When a [[star cluster|cluster of stars]] is formed at about the same time, the main-sequence lifespan of these stars will depend on their individual masses. The most massive stars will leave the main sequence first, followed in sequence by stars of ever lower masses. The position where stars in the cluster are leaving the main sequence is known as the [[turnoff point]]. By knowing the main-sequence lifespan of stars at this point, it becomes possible to estimate the age of the cluster.<ref name=science299_5603>{{cite journal |last=Krauss |first=Lawrence M. |author2=Chaboyer, Brian |title=Age Estimates of Globular Clusters in the Milky Way: Constraints on Cosmology |journal=Science |date=2003 |volume=299 |issue=5603 |pages=65–69 |doi=10.1126/science.1075631 |pmid=12511641 |bibcode=2003Sci...299...65K |s2cid=10814581 |url=https://semanticscholar.org/paper/4bae7262b55da8fc7d8ff28be71052c8a6bc0289}}</ref>

==See also==

=='''''See also on It’s Royal Majesty, the Wikipedia'''''==

* [[Lists of astronomical objects]]

=='''''Notes'''''==

{{Reflist|group=note}}

=='''''References'''''==

{{Reflist|30em|refs=

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}}

=='''''Further reading'''''==

===General===