Meta tags:
Headings (most frequently used words):
parameters, main, sequence, contents, history, formation, and, evolution, classification, properties, dwarf, terminology, energy, generation, structure, luminosity, color, variation, lifetime, evolutionary, tracks, see, also, notes, references, further, reading, sample, general, technical,
Text of the page (most frequently used words):
the (505), and (167), stars (155), main (124), #sequence (123), star (121), mass (70), this (63), energy (56), #luminosity (51), for (51), stellar (47), core (45), are (42), hydrogen (38), that (38), from (37), bibcode (34), temperature (32), with (29), sun (28), helium (27), university (27), doi (27), dwarf (25), fusion (25), evolution (24), diagram (24), astrophysics (23), 978 (23), isbn (23), its (23), edit (22), can (22), 000 (22), their (21), astronomy (20), retrieved (20), dwarfs (20), red (20), type (20), solar (19), press (19), russell (18), process (18), known (18), which (18), structure (17), giant (17), cambridge (17), these (16), when (16), hertzsprung (15), convection (15), spectral (15), will (15), than (15), more (14), about (13), white (13), massive (13), low (13), s2cid (13), nuclear (13), evolutionary (13), relationship (13), not (13), only (13), most (12), color (12), burning (12), cno (12), proton (12), blue (12), classification (12), 2007 (12), age (12), upper (12), composition (12), years (12), magnitude (11), arxiv (11), pressure (11), cycle (10), 1086 (10), journal (10), time (10), lower (10), where (10), have (10), thus (10), below (10), region (10), into (10), was (9), science (9), outer (9), surface (9), new (9), very (9), astrophysical (9), astro (9), generation (9), called (9), lifetime (9), radius (9), wikipedia (8), may (8), properties (8), radiation (8), 521 (8), archived (8), original (8), example (8), initial (8), also (8), has (8), much (8), class (8), brightness (8), object (7), metallicity (7), black (7), track (7), formation (7), york (7), springer (7), 2008 (7), see (7), history (7), relation (7), masses (7), shows (7), between (7), position (7), layers (7), increase (7), two (7), high (7), opacity (7), higher (7), equilibrium (7), amount (7), other (7), parameters (7), production (7), harvard (7), text (6), types (6), brown (6), zone (6), nucleosynthesis (6), yellow (6), instability (6), john (6), 2000 (6), introduction (6), 1146 (6), annurev (6), annual (6), review (6), william (6), 2004 (6), principles (6), chicago (6), models (6), 2006 (6), april (6), physics (6), giants (6), astronomical (6), atoms (6), limit (6), generated (6), hot (6), difference (6), values (6), same (6), become (6), cores (6), collapse (6), stage (6), but (6), remain (6), fuel (6), displaystyle (6), been (6), roughly (6), above (6), chemical (6), there (6), rate (6), during (6), plots (6), table (5), last (5), different (5), system (5), space (5), luminous (5), cluster (5), absolute (5), hole (5), strip (5), branch (5), 2003 (5), theoretical (5), within (5), physical (5), objects (5), current (5), billion (5), further (5), 2005 (5), tracks (5), times (5), zero (5), function (5), however (5), point (5), becomes (5), elements (5), they (5), lives (5), move (5), observed (5), consumed (5), gravitational (5), like (5), power (5), depends (5), changes (5), over (5), bigodot (5), left (5), right (5), total (5), output (5), spend (5), band (5), because (5), all (5), rich (5), stable (5), each (5), dominant (5), toggle (4), contents (4), search (4), page (4), articles (4), index (4), planetary (4), distant (4), bright (4), earth (4), envelope (4), binary (4), systems (4), variable (4), kelvin (4), alpha (4), chain (4), shell (4), carbon (4), nebula (4), herbig (4), pre (4), protostar (4), young (4), cloud (4), steven (4), off (4), 1983 (4), 337 (4), 555 (4), princeton (4), clusters (4), 0004 (4), january (4), universe (4), near (4), present (4), became (4), produces (4), while (4), henry (4), cool (4), zams (4), interstellar (4), medium (4), 2018 (4), spectra (4), century (4), typical (4), distance (4), lifespan (4), first (4), followed (4), approximately (4), relatively (4), sufficiently (4), away (4), few (4), less (4), directly (4), range (4), article (4), increases (4), versus (4), approximation (4), transported (4), contrast (4), must (4), result (4), transport (4), radiated (4), proportional (4), least (4), per (4), unit (4), results (4), length (4), following (4), through (4), thermal (4), divided (4), part (4), classes (4), fusing (4), material (4), occur (4), primarily (4), against (4), gradient (4), efficient (4), regions (4), hotter (4), astronomers (4), based (4), 800 (4), along (4), eff (4), after (4), size (4), curve (4), were (4), hide (4), sidebar (4), view (3), contact (3), available (3), under (3), terms (3), non (3), use (3), planet (3), neutron (3), gravity (3), related (3), supernovae (3), way (3), nearest (3), historical (3), brightest (3), strömgren (3), loss (3), helmholtz (3), cosmic (3), dust (3), accretion (3), photosphere (3), atmosphere (3), supernova (3), dark (3), supergiant (3), early (3), molecular (3), modern (3), book (3), theory (3), james (3), verlag (3), 1996 (3), 1967 (3), michael (3), fowler (3), caughlan (3), georgeanne (3), zimmerman (3), barbara (3), thermonuclear (3), reaction (3), rates (3), hansen (3), carl (3), kawaler (3), interiors (3), clayton (3), 226 (3), education (3), bahcall (3), 2001 (3), general (3), reading (3), pmid (3), 1126 (3), isochrones (3), intermediate (3), long (3), november (3), 2012 (3), end (3), 387 (3), 1993 (3), cosmos (3), smith (3), institute (3), subdwarfs (3), 113 (3), 128 (3), 540 (3), research (3), sample (3), pdf (3), course (3), encyclopedia (3), 200 (3), local (3), formed (3), enough (3), fuse (3), eventually (3), follow (3), across (3), supergiants (3), diagrams (3), start (3), appears (3), sufficient (3), undergo (3), forming (3), expand (3), since (3), predicted (3), longer (3), begin (3), producing (3), even (3), vary (3), conditions (3), hydrostatic (3), million (3), estimated (3), frac (3), form (3), law (3), generate (3), gives (3), relative (3), pulsating (3), them (3), appearance (3), one (3), variation (3), beta (3), variables (3), would (3), abundance (3), stages (3), convective (3), throughout (3), concentrated (3), plasma (3), cooler (3), create (3), series (3), processes (3), together (3), uses (3), line (3), stefan (3), boltzmann (3), smaller (3), many (3), terminology (3), refer (3), sometimes (3), both (3), order (3), information (3), timescale (3), named (3), scheme (3), published (3), had (3), developed (3), decreasing (3), norris (3), continuous (3), denoted (3), tools (3), languages (2), privacy (2), policy (2), additional (2), using (2), wikimedia (2), inc (2), commons (2), categories (2), december (2), short (2), description (2), wikidata (2), concepts (2), portal (2), galaxy (2), year (2), sub (2), substellar (2), nebulae (2), remnants (2), milky (2), multiple (2), volume (2), proper (2), lists (2), photometric (2), standard (2), extinction (2), radio (2), emission (2), super (2), globular (2), open (2), symbiotic (2), common (2), rotation (2), magnetic (2), field (2), effective (2), mechanism (2), eddington (2), circumstellar (2), protoplanetary (2), disk (2), nova (2), triple (2), ray (2), extreme (2), gap (2), subgiant (2), late (2), agb (2), post (2), horizontal (2), haro (2), tauri (2), bok (2), globule (2), list (2), wiley (2), sons (2), prialnik (2), dina (2), padmanabhan (2), 2002 (2), city (2), interior (2), kippenhahn (2), rudolf (2), 1990 (2), 464 (2), iben (2), icko (2), 090167 (2), 1967ara (2), iii (2), second (2), donald (2), san (2), richard (2), jones (2), eds (2), pinsonneault (2), basu (2), sarbani (2), epoch (2), dependences (2), neutrinos (2), helioseismological (2), 1012 (2), 13798091 (2), 321493 (2), 2001apj (2), 990b (2), 990 (2), 1989 (2), arnett (2), david (2), technical (2), 100 (2), lawrence (2), cosmology (2), 299 (2), march (2), bressan (2), bertelli (2), chiosi (2), 1051 (2), 141 (2), staff (2), australia (2), telescope (2), outreach (2), adams (2), fred (2), laughlin (2), gregory (2), 1997 (2), term (2), 2021 (2), spectroscopic (2), 6361 (2), 2010 (2), february (2), 1995 (2), 482 (2), 1994 (2), 94138 (2), 1993apj (2), 418 (2), future (2), studies (2), how (2), 386 (2), monthly (2), notices (2), royal (2), society (2), green (2), mark (2), adam (2), july (2), metal (2), poor (2), 701 (2), any (2), 1981 (2), variations (2), 2014 (2), 102 (2), 406 (2), set (2), 162 (2), 620 (2), nearby (2), 129 (2), abundances (2), model (2), listed (2), swinburne (2), sao (2), astronomer (2), patrick (2), 2009 (2), atlas (2), 1999 (2), 217 (2), 5538 (2), 293 (2), 2188 (2), 295 (2), rare (2), ever (2), individual (2), morgan (2), keenan (2), 1943 (2), bengt (2), 1933 (2), observatory (2), references (2), need (2), notes (2), leaving (2), estimate (2), heavier (2), top (2), show (2), prominently (2), usually (2), either (2), reaches (2), lifetimes (2), causes (2), evolves (2), nearly (2), constant (2), resulting (2), contribute (2), depending (2), efficiently (2), effect (2), does (2), produce (2), means (2), rapidly (2), burn (2), via (2), needed (2), radiate (2), proportionately (2), greater (2), state (2), maintain (2), outward (2), tenth (2), tau (2), shorter (2), compared (2), produced (2), plot (2), small (2), factors (2), include (2), caused (2), name (2), just (2), lie (2), close (2), reduction (2), gradual (2), lowering (2), around (2), out (2), consequently (2), reflected (2), such (2), surrounded (2), uniform (2), sirius (2), mixing (2), little (2), movement (2), internal (2), 120 (2), until (2), sustain (2), generates (2), primary (2), almost (2), parts (2), temperatures (2), intermediaries (2), oxygen (2), nitrogen (2), density (2), cause (2), 240 (2), note (2), orionis (2), actual (2), better (2), fit (2), three (2), relates (2), fact (2), too (2), used (2), comparison (2), confusing (2), somewhat (2), colors (2), still (2), determines (2), distribution (2), light (2), majority (2), what (2), including (2), contraction (2), newly (2), phase (2), before (2), dense (2), shown (2), discovered (2), vogt (2), brighter (2), true (2), showed (2), similar (2), studying (2), categorized (2), distinct (2), allowed (2), groups (2), prominent (2), ejnar (2), distinctive (2), method (2), jump (2), proportion (2), fraction (2), upload (2), file (2), links (2), read (2), english (2), norsk (2), bahasa (2), subsection (2), log (2), account (2), donate (2), menu (2), add, topic, mobile, cookie, statement, statistics, developers, code, conduct, legal, safety, contacts, disclaimers, apply, site, you, agree, registered, trademark, profit, organization, foundation, creative, attribution, sharealike, license, edited, 2026, utc, hidden, featured, needing, clarification, 2023, https, org, php, title, main_sequence, oldid, 1353867558, portals, category, tidal, disruption, event, engulfment, collision, hosting, merger, intergalactic, guest, galactic, desert, timeline, candidates, novae, exoplanets, resolved, images, smallest, largest, lowest, highest, extremes, chinese, arabic, names, parallax, motion, radial, velocity, photographic, apparent, asterism, constellation, circumpolar, pole, sunlight, eclipse, centric, observations, eclipsing, kraft, break, sphere, starlight, darkening, kinematics, dynamics, designation, helioseismology, asteroseismology, proplyd, bipolar, outflow, bubble, wind, alfvén, corona, chromosphere, starspot, oscillations, microturbulence, micronova, recurrent, remnant, flash, lithium, deuterium, blitzar, iron, thorne, żytkow, gravastar, quasi, quark, planck, preon, strange, electroweak, boson, exotic, hypothetical, sgr, burster, supermassive, magnetar, pulsar, quiet, compact, straggler, technetium, lead, lambda, boötis, barium, weak, hgmn, cemp, chemically, peculiar, hypergiant, subdwarf, hypernova, superluminous, population, dredge, pg1159, wolf, rayet, loop, asymptotic, clump, henyey, hayashi, shore, hoboken, tapestry, 2000itss, novotny, eva, 1973, oxford, atmospheres, liebert, probst, ronald, 1987, 437, 090187, 002353, 1987ara, 473l, weigert, alfred, berlin, iglesias, carlos, rogers, forrest, updated, opal, opacities, 943, 177381, 1996apj, 943i, 571, 003035, 571i, harris, 165, 090183, 001121, 1983ara, 165h, trimble, virginia, edition, 1975, 090175, 000441, 1975ara, 69f, 525, 002521, 525f, cox, giuli, 1968, gordon, breach, 1968pss, 10952, dover, 1967aits, study, chandrasekhar, chabrier, gilles, baraffe, isabelle, 377, 59325115, 2000ara, 337c, 0006383, carroll, bradley, ostlie, dale, francisco, pearson, addison, wesley, 8053, 0402, bowers, deeming, terry, 1984, boston, bartlett, barnes, schramm, 1982, essays, 0010346, 37975, neutrino, basic, books, suns, krauss, chaboyer, brian, estimates, constraints, 5603, 10814581, 12511641, 1075631, 2003sci, 65k, sitko, cincinnati, girardi, 383, 14566232, aas, 2000126, 2000a, 371g, 9910164, 371, supplement, october, 2013, dying, fate, 372, 12173790, 1103, revmodphys, 1997rvmp, 337a, 9701131, reviews, martins, palacios, a67, 202039337, 2021a, 645a, 67m, 13430, 645, 1481, 107, 01657, imamura, oregon, bodenheimer, peter, 432, 304125, 1997apj, 420l, 420, birkhäuser, sackmann, juliana, boothroyd, arnold, kraemer, kathleen, 468, 173407, 457s, 457, our, rolfs, claus, rodney, 1988, 72457, cauldrons, detailed, reconstruction, derivation, 1924, lecchini, stefano, bern, philosophy, 9522882, discovery, 691, 01147, investigation, matter, big, bang, schröder, connon, robert, 163, 10073988, 1111, 1365, 2966, 13022, 2008mnras, 155s, 0801, 4031, 155, revisited, 65937, richmond, rochester, technology, burnell, jocelyn, 54622, burgasser, kirkpatrick, davy, lépine, sébastien, hamburg, germany, dordrecht, reidel, publishing, 237, 2005esasp, 560, 237b, proceedings, 13th, workshop, spitzer, ultracool, sweet, roy, 1953, 715, 1093, mnras, 1953mnras, 701s, rotating, tayler, roger, 45885, wright, know, maunder, minimum, 1278, 118975831, 423221, 2004aj, 1273w, 0406338, 1273, thanu, 56241, gough, 120541081, 1007, bf00151270, 1981soph, 21g, lochner, jim, gibb, meredith, newman, phil, september, nasa, overshooting, 1981a, 25b, aller, 1991, 31040, burrows, hubbard, saumon, didier, lunine, jonathan, 172427, 158b, 158, expanded, ziebarth, kenneth, 1970, 962, 150726, 1970apj, 947z, 947, oey, clarke, statistical, confirmation, l46, 7280299, 428396, 2005apj, 620l, 43o, 0501135, l43, salaris, maurizio, cassisi, santi, 470, 09220, populations, 0212331, karttunen, hannu, 00179, fundamental, brainerd, jerome, spectator, consortium, hundred, luck, earle, heiter, ulrike, 1083, 427250, 2005aj, 1063l, 1063, parsecs, northern, centre, données, astronomiques, strasbourg, simbad, database, zombeck, martin, 2nd, 34787, handbook, compare, siess, lionel, institut, astronomie, astrophysique, université, libre, bruxelles, computation, andrews, 2020, nebraska, origin, 85233, 878, amateur, moore, powell, harding, center, sciences, california, diego, gene, tutorial, pettersen, hawley, survey, flare, issn, 1989a, 187p, 187, eso, release, don, live, alone, 10953, library, business, media, schilling, govert, 2189, 33059330, 11567116, kroupa, pavel, evidence, uniformity, 5552, 14084249, 11778039, 1067524, 2002sci, 82k, 0201098, gloeckler, george, geiss, johannes, diagnosed, pickup, ions, 1016, asr, 054, 2004adspr, 53g, advances, howell, elizabeth, com, alignment, reveals, seen, kelly, magnification, redshift, lens, 342, 125826925, 1038, s41550, 018, 0430, 2018natas, 334k, 1706, 10279, 334, nature, unsöld, albrecht, 1969, 268, 90886, kellman, illinois, outline, schatzman, evry, praderie, francoise, 54196, interpretation, 248, 1933za, 222s, 222, zeitschrift, für, astrophysik, 1913, 329, 1913obs, 324r, 324, laurie, 1696, 33102501, oclc, 7503, 0310, american, bristol, twentieth, pippard, pais, abraham, longair, malcolm, 47436, g2v, measuring, eliminates, correct, magnitudes, affected, depend, leave, knowing, possible, turnoff, instead, quickly, leading, behind, degenerate, opaque, shells, lived, surrounding, causing, brief, comes, halt, old, occurred, older, turn, dots, outside, sequences, mostly, foreground, background, m67, ngc, 188, resume, path, follows, terminal, tams, depleted, dominated, increasing, cube, dependent, varying, fourth, molecules, varies, flattening, slope, graph, refinements, electron, scattering, exact, insulating, retains, escapes, although, might, intuitively, expected, increased, required, equation, rise, match, titanic, inward, trillion, respectively, approx, comparing, towards, hence, applies, propto, supply, exhausted, expands, average, empirical, 130, limited, equal, life, span, day, vertical, occupied, regular, intervals, giving, intersects, intersecting, experience, difficult, detect, unstable, unrelated, cephei, delta, scuti, cepheid, broaden, uncertainty, presence, unresolved, alter, perfect, observation, fuzzy, parameter, affects, interaction, slightly, change, atomic, numbers, edge, fuzziness, variance, rapid, companion, status, accumulates, supplied, maintains, supports, gradually, gets, compressed, brings, depth, pushes, value, ages, broadening, contains, various, familiar, sized, mixes, steady, buildup, distributed, sensitive, highly, removes, ash, allowing, modes, transporting, bulk, rising, descending, mode, carrying, steep, cross, section, showing, explanation, fast, ejected, pulsations, sustained, threshold, jupiter, equally, half, net, consists, speaking, belong, transition, spans, single, entire, employ, divide, fuses, chart, levels, necessary, support, remainder, overlaying, compress, likewise, forms, self, regulating, dashed, combined, logarithm, representative, 2mass, j0523, 1403, 00017, van, biesbroeck, 650, aquarii, 0027, lacaille, 8760, 072, cygni, 410, ophiuchi, mensae, 610, 780, comae, berenices, 920, eta, arietis, gamma, virginis, pictoris, coronae, borealis, andromedae, 400, phi, theta, 180, 253, examples, temp, why, take, suitable, exhibiting, closely, interlinked, respective, approximated, relations, finally, linear, ratio, factor, inner, extremely, slow, reflects, strongly, whereas, orders, approximate, sigma, treating, idealized, radiator, body, mean, another, dead, shed, represent, final, partly, indeed, dimmer, those, hottest, observable, differences, indicate, whether, nevertheless, though, lines, orange, wavelength, influenced, key, indicator, given, measures, filters, provides, measure, pronounced, active, doing, zeroth, spica, proxima, centauri, lesser, extent, referred, brilliant, contain, several, remains, significant, then, begins, evolve, evolving, moves, represents, excess, gained, lost, lies, calculated, computer, typically, gas, homogeneous, consisting, trace, amounts, described, empirically, arising, due, ignition, contracting, describes, radiating, kinetic, once, converting, exothermic, useful, describe, next, violent, youth, migration, nebular, hypothesis, jeans, reported, detection, ordinary, formally, macs, j1149, lensed, icarus, refined, assigned, assigning, letter, strength, ordered, duplicate, removed, ranging, popular, memorizing, fine, girl, guy, kiss, ranged, belonged, mnemonic, philip, childs, wilson, 1930s, conversely, deduced, subsequently, breaks, down, heinrich, theorem, introduced, denote, parallel, development, technique, earlier, proposed, great, reverse, faint, corrected, purpose, reliable, plotted, found, real, reasonable, accuracy, parallaxes, 1906, danish, noticed, reddest, classified, could, fainter, distinguish, began, large, groupings, located, potsdam, 20th, distances, readily, features, categorization, 1901, annals, college, edward, charles, pickering, annie, cannon, magni, tude, hypergiants, subgiants, mainly, occurring, acts, stir, created, entirely, radiative, zones, steadily, develops, layer, discussing, generally, currently, composed, meaning, leads, whereby, without, being, transferred, environment, speeds, decreases, undergoes, trigger, generating, strong, counters, maintaining, determined, transfer, plays, role, particularly, again, radiates, gaseous, appear, interchangeably, numerous, represented, visible, diagonal, gliese, catalogue, hipparcos, catalog, racehorse, horse, free, item, projects, printable, version, download, print, export, get, shortened, url, cite, permanent, link, here, actions, talk, tiếng, việt, українська, татарча, tatarça, türkçe, ไทย, తెలుగు, svenska, српски, srpski, slovenščina, slovenčina, simple, srpskohrvatski, српскохрватски, русский, română, português, polski, occitan, bokmål, nynorsk, nederlands, नेपाली, plattdüütsch, မြန်မာဘာသာ, melayu, മലയാളം, македонски, latviešu, lietuvių, lëtzebuergesch, latina, 한국어, қазақша, taqbaylit, ქართული, 日本語, italiano, igbo, indonesia, magyar, hrvatski, हिन्दी, עברית, galego, gaeilge, français, suomi, فارسی
Text of the page (random words):
ral 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 for a star may vary by as much as 20 30 from the values listed below 25 why table of main sequence stellar parameters 26 stellar class radius r r mass m m luminosity l l temp k examples 27 o2 12 100 800 000 50 000 bi 253 o6 0 9 8 0 35 180 000 38 000 theta 1 orionis c b0 0 7 4 0 18 0 20 000 30 000 phi 1 orionis b5 0 3 8 00 6 5 000 800 16 400 pi andromedae a a0 0 2 5 00 3 2 000 0 80 10 800 alpha coronae borealis a a5 0 1 7 00 2 1 000 0 20 0 8 620 beta pictoris f0 0 1 3 00 1 7 000 00 6 0 7 240 gamma virginis f5 0 1 2 00 1 3 000 00 2 5 0 6 540 eta arietis g0 0 1 05 00 1 10 000 00 1 26 0 5 920 beta comae berenices g2 0 1 00 00 1 00 000 00 1 00 0 5 780 sun note 2 g5 0 0 93 00 0 93 000 00 0 79 0 5 610 alpha mensae k0 0 0 85 00 0 78 000 00 0 40 0 5 240 70 ophiuchi a k5 0 0 74 00 0 69 000 00 0 16 0 4 410 61 cygni a 28 m0 0 0 51 00 0 60 000 00 0 072 0 3 800 lacaille 8760 m5 0 0 18 00 0 15 000 00 0 0027 0 3 120 ez aquarii a m8 0 0 11 00 0 08 000 00 0 0004 0 2 650 van biesbroeck s star 29 l1 0 0 09 00 0 07 000 00 0 00017 0 2 200 2mass j0523 1403 representative lifetimes of stars as a function of their masses energy generation edit see also stellar nucleosynthesis logarithm of the relative energy output ε of proton proton pp cno and 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 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 that will support the remainder of the star a reduction of energy production would cause the overlaying mass to compress the core resulting in an increase in the fusion rate because of higher temperature and pressure 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 30 main sequence stars employ two types of hydrogen fusion processes and the rate of energy generation from each type depends on the temperature in the core region astronomers divide the main sequence into upper and lower parts based on which of the two is the dominant fusion process in the lower main sequence energy is primarily generated as the result of the proton proton chain which directly fuses hydrogen together in a series of stages to produce helium 31 stars in the upper main sequence have sufficiently high core temperatures to efficiently use the cno cycle see chart this process uses atoms of carbon nitrogen and oxygen as intermediaries in the process of fusing hydrogen into helium at a stellar core temperature of 18 million kelvin the pp process and cno cycle are equally efficient and each type generates half of the star s net luminosity as this is the core temperature of a star with about 1 5 m the upper main sequence consists of stars above this mass thus roughly speaking stars of spectral class f or cooler belong to the lower main sequence while a type stars or hotter are upper main sequence stars 15 the transition in primary energy production from one form to the other spans a range difference of less than a single solar mass in the sun a one solar mass star only 1 5 of the energy is generated by the cno cycle 32 by contrast stars with 1 8 m or above generate almost their entire energy output through the cno cycle 33 the observed upper limit for a main sequence star is 120 200 m 34 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 35 the lower limit for sustained proton proton nuclear fusion is about 0 08 m or 80 times the mass of jupiter 31 below this threshold are sub stellar objects that can not sustain hydrogen fusion known as brown dwarfs 36 structure edit main article stellar 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 a radiation zone where energy is transported by radiation is stable against convection and there is very little mixing of the plasma by contrast in a convection zone the energy is transported by bulk movement of plasma with hotter material rising and cooler material descending convection is a more efficient mode for carrying energy than radiation but it will only occur under conditions that create a steep temperature gradient 30 37 in massive stars above 10 m 38 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 in the core region which results in a convection zone for more efficient energy transport 31 this mixing of material around the core removes the helium ash from the hydrogen burning region allowing more of the hydrogen in the star to be consumed during the main sequence lifetime the outer regions of a massive star transport energy by radiation with little or no convection 30 intermediate mass stars such as sirius may transport energy primarily by radiation with a small core convection region 39 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 m are convective throughout 11 thus the helium produced at the core is distributed across the star producing a relatively uniform atmosphere and a proportionately longer main sequence lifespan 30 luminosity color variation edit the sun is the most familiar example of a main sequence star as non fusing helium 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 fusion supplied power that maintains the pressure of the core and supports the higher layers of the star the core gradually gets compressed this brings hydrogen rich material into a shell around the helium rich core at a depth where the pressure is sufficient for fusion to occur the high power output from this shell pushes the higher layers of the star further out this causes a gradual increase in the radius and consequently luminosity of the star over time 15 for example the luminosity of the early sun was only about 70 of its current value 40 as a star ages it thus changes its position on the hr diagram this evolution is reflected in a broadening of the main sequence band which contains stars at various evolutionary stages 41 other factors that broaden the main sequence ...
|
