恒星的形成與演化.pptVIP

2019/7/1,1,第三章 恒星的形成與演化,恒星形成 恒星結(jié)構(gòu) 元素合成 恒星演化 超 新 星 密近雙星,2019/7/1,,2,恒星形成,Studying this chapter will enable you to: Discuss the factors that compete against gravity in the process of star formation. Explain how the process of star formation depends on stellar mass. Describe some of the observational evidence supporting the modern theory of star formation. Explain the nature of interstellar shock waves, and discuss their possible role in the formation of stars.,2019/7/1,,3,2019/7/1,,4,大爆炸宇宙學(xué) 宇宙的極早期，宇宙的溫度和密度都極高 溫度不斷下降，宇宙輻射為主物質(zhì)為主 氣體逐漸凝聚成氣云，然后有恒星、星系 恒星形成理論 星際氣體怎么會形 成光輝奪目的恒星 呢？,2019/7/1,,5,How do stars form? What factors determine the masses, luminosities, and distribution of stars in our Galaxy? What determines which interstellar clouds collapse? 引力！ 引力為天體和整個宇宙動力學(xué)的支配者,2019/7/1,,6,2019/7/1,7,恒星形成理論: 彌漫學(xué)說,散布于空間彌漫物質(zhì)在引力作用下凝聚為恒星 宇宙空間存在著大量的星際物質(zhì): 原子/分子/塵埃 由于星際物質(zhì)密度的不均勻性，形成了一些密度較大區(qū)域 星際物質(zhì)受到引力的作用，便聚集到這些區(qū)域，形成星云 星云不斷收縮，勢能轉(zhuǎn)換為恒星內(nèi)部熱能和向外的輻射能 星云溫度不斷提高，并向外輻射能量，從而形成原始恒星 不同類型的恒星 規(guī)模較小的星云形成一個孤立的恒星, 大的星云由于密度不均勻,其中有幾個質(zhì)量中心,因而形成雙星、聚星或星團(tuán)。 質(zhì)量非常小的星云,不能收縮成為恒星,2019/7/1,,8,金斯（Jeans）不穩(wěn)定性,由萬有引力產(chǎn)生的一種不穩(wěn)定性，因金斯在20世紀(jì)初最先研究而得名 對于一個如星云的自引力體系，當(dāng)星云的質(zhì)量足夠高時，(向內(nèi)的)引力超過由熱運動和湍動產(chǎn)生的(向外的)壓力，將引起星云的收縮 星云不穩(wěn)定的極限質(zhì)量稱為金斯(Jeans)質(zhì)量：,James Jeans 1877 - 1946,2019/7/1,,9,中性氫云：n 1cm-3, T 100KMJ3104 M 暗分子云：n 106 cm-3, T 10 K MJ1 M Jeans質(zhì)量判據(jù)給出了非相對論、無磁場星云坍縮的必要條件(并不是所有的星云都可以形成恒星),金斯（Jeans）不穩(wěn)定性,2019/7/1,,12,其它影響恒星形成的因素：,能量的有效輻射 輻射壓將反抗引力,阻礙星云塌縮 星系潮汐力影響 星云原初角動量 Rotationthat is, spincan also compete with gravitys inward pull,垂直自轉(zhuǎn)軸方向停止收縮，平行方向繼續(xù)收縮，星云變得扁平且密度增大，最終星云碎裂?？偨莿恿勘环纸鉃楦鱾€碎塊的自轉(zhuǎn)和軌道角動量。,2019/7/1,,13,其它影響恒星形成的因素：,原始星云的磁場 原始星云一般具有微弱的磁場。隨著星云收縮，磁場強度變大。磁場將阻止星云收縮，特別是垂直于磁場方向的收縮。,Magnetism can hinder the contraction of a gas cloud, especially in directions perpendicular to the magnetic field (solid lines). Frames (a), (b), and (c) trace the evolution of a slowly contracting interstellar cloud having some magnetism.,2019/7/1,,14,演化軌跡： each for a fixed mass at different times 等年齡線：each for a fixed time and different masses,2019/7/1,,15,類太陽質(zhì)量恒星的形成,星云的快速收縮過程(密度小，輻射透明，等溫收縮) 1. 星際云 (interstellar cloud):星際云坍縮，并分裂成小云塊(密度上升，金斯質(zhì)量減小) 2. 星云塊 (cloud fragments):星云仍十分稀薄，熱量可以不受阻礙地散逸，星云內(nèi)的溫度沒有明顯上升,2019/7/1,,16,3. 碎裂停止 (fragmentation ceasess):星云進(jìn)一步坍縮和分裂，密度上升。核心區(qū)域變得不透明，溫度迅速上升，金斯質(zhì)量增大。星云停止分裂。(等溫收縮絕熱收縮）,2019/7/1,,17,4. 形成原恒星(protostar) : 星云快速收縮過程結(jié)束，引力幾乎和氣體壓力相等。恒星已經(jīng)變得不透明，輻射只能從表面逸出，中心溫度迅速升高。 5.原恒星演化(protostellar evolution): 原恒星向主序演化為主序前星，但內(nèi)部溫度還沒有升高到H點火溫度,2019/7/1,,18,2019/7/1,,19,6. 零齡主序 (zero-age main-sequence stars) :恒星熱核反應(yīng) (H燃燒)開始進(jìn)行，成為零齡主序恒星。光度約為現(xiàn)在太陽光度的2/3。 7. 主序星 (main-sequence stars) :恒星略微收縮，完全達(dá)到流體靜力學(xué)平衡，成為正常恒星.,2019/7/1,,20,Evolution of a 1-Solar-Mass Star,2019/7/1,,21,Prestellar evolutionary paths for stars more massive and less massive than our Sun.,2019/7/1,,22,不同質(zhì)量的恒星在形成過程中，在H-R圖上沿不同的路徑演化。 小質(zhì)量原恒星內(nèi)部對流發(fā)展充分，溫差小，收縮時表面溫度幾乎不變；大質(zhì)量原恒星對流層淺，溫度變低。 質(zhì)量越高的恒星，其原恒星演化到主序的時間越短，在主序上的位置越高。,具有不同質(zhì)量恒星的形成,2019/7/1,,23,Formation of Massive Stars and Clusters,Massive stars have masses that are much larger than the Jeans mass in the cloud cores where they form. The large cloud cores might contain many small bound clumps. These cores might form groups or clusters of stars.,NGC 3603,2019/7/1,,24,褐矮星 (Brown Dwarfs),Masses 0.08 M(10MJ- 84 MJ) Central Te3 million K Surface temperature 1000 K,TWA 5 and its brown dwarf companion in Infrared (left) and in X-ray (right).,2019/7/1,,25,Differences between brown dwarfs and planets Planets are smaller and lighter Planets have a solid core They are formed in a complete different way,2019/7/1,,26,初始質(zhì)量函數(shù)(Initial Mass Function),Generally more low-mass than high-mass stars form when an interstellar cloud fragments. The stellar initial mass function (IMF) describes the probability of a star forming with a particular mass.,For Salpeter IMF, x =1.35,2019/7/1,,27,恒星形成理論的觀測證據(jù),The evolutionary stages described are derived from numerical experiments performed on computers.,2019/7/1,,28,EVIDENCE OF CLOUD CONTRACTION,The M20 region shows observational evidence for three broad phases in the birth of a star: (1) the parent cloud (stage 1), (2) a contracting fragment (between stages 1 and 2), and (3) the emission nebula (M20 itself) resulting from the formation of one or more massive stars (stages 6 and 7),2019/7/1,,29,EVIDENCE OF CLOUD FRAGMENTS,(a) Orion (b) enlarged. The three frames at right show some of the evidence for those protostars. (c) some intensely emitting molecular sites. (d) visible image of embedded nebular “knots“ thought to harbor protostars. (e) several young stars surrounded by disks of gas and dust where planets might ultimately form.,Orion,2019/7/1,,30,HST拍攝到了迄今為止最清晰的獵戶座星云全景照片。這張照片不僅顯示出大量恒星的誕生，也包含有罕見的褐矮星。 Orion Nebula,2019/7/1,,31,獵戶星云縮小圖,最大恒星,dark red column,Failing stars,Sculpting the landscape,Pillars of gas,2019/7/1,,32,Image from Spitzer and HST wavelengths of 0.43, 0.50, and 0.53 microns is blue. Light with wavelengths of 0.6, 0.65, and 0.91 microns is green. Light of 3.6 microns is orange 8-micron light is red.,2019/7/1,,33,2019/7/1,,34,EVIDENCE OF PROTOSTARS An infrared image of the nearby region containing the source Barnard 5 (indicated by the arrow, a solar-mass protostar). On the basis of its temperature and luminosity, Barnard 5 appears to be a protostar on the Hayashi track in the HR diagram, around stage 5.,2019/7/1,,35,HST在可見光波段拍攝了這個鷹狀星云中的“誕生柱”，并且使得它揚名全球。 Gas Pillars in the Eagle Nebula (M16): Pillars of Creation in a Star-Forming Region,2019/7/1,,36,2019/7/1,,37,Spitzer: 心宿增四星云內(nèi)的年輕恒星. 發(fā)現(xiàn)大約有300顆正在出現(xiàn)和新形成的恒星，平均年齡在估計只有30萬年,2019/7/1,,38,疏散星團(tuán)的H-R圖,星團(tuán)NGC 2261的H-R圖：原恒星到達(dá)主序的位置和時間隨質(zhì)量的變化。,2019/7/1,,39,(1) 激波壓縮 超新星爆發(fā)、熱星輻射或銀河系旋臂轉(zhuǎn)動等過程產(chǎn)生激波。 激波壓縮附近的星云，使其密度增大，觸發(fā)恒星的形成。 恒星形成過程可能類似于鏈?zhǔn)椒磻?yīng)。,星云M20中的激波壓縮效應(yīng),星云坍縮的觸發(fā)機制,2019/7/1,,40,恒星誕生 膨脹激波 新生恒星,2019/7/1,,41,2019/7/1,,42,(2) 星云碰撞,星系自轉(zhuǎn)，它的組成物質(zhì)在運動中穿旋臂而過，恒星偕同星際物質(zhì)在這樣的過程中都是由彎曲旋臂的內(nèi)側(cè)進(jìn)去，再從外側(cè)出來。星系物質(zhì)穿越密度較大的旋臂，引起產(chǎn)星過程。,2019/7/1,,43,2019/7/1,,44,Steps to the formation of stars and planets: Clouds of gas form within galaxies. Formation of structure within the gas clouds, due to “turbulence“ and activity of new stars. Random turbulent processes lead to regions dense enough to collapse under their own weight, in spite of a hostile environment. As blob collapses, a disk forms, with growing “protostar“ at the center. At the same time, bipolar outflows from forming star/disk system begin.,2019/7/1,,45,Material is processed, moving in from the blob to the disk. What is not lost in the outflow builds up on the protostar. When the protostar begins to undergo fusion, it becomes a real star. Once the outflow ceases and the “accretion“ phase that lead to the buildup of the star ends, a disk of “l(fā)eftover“ material is left around the star. At or near the end of the star-formation process, the remaining material in the “circumstellar disk“ forms a variety of planets. Eventually, all that is left behind is a new star, perhaps some planets, and a disk of left-over ground-up solids,SELF-TEST: TRUE OR FALSE? Given the typical temperatures found in interstellar space, a cloud containing as few as 1000 atoms has sufficient gravity for it to begin to collapse Both rotation and magnetic fields act to accelerate the gravitational collapse of an interstellar cloud. The time a solar-type star spends forming is relatively short compared to the time it spends as a main-sequence star. Stars evolve along the main sequence. Most stars form as members of groups or clusters of stars. Brown dwarfs take a long time to form but will eventually arrive as stars on the lower main sequence. Shock waves for star formation can also be produced by large stars moving rapidly through the interstellar medium. In star formation, more G, K, and M stars form than O and B stars. The gas in an emission nebula eventually dissipates into space, leaving behind a star cluster Star clusters eventually dissipate, leaving behind individual stars like the Sun.,2019/7/1,,46,2019/7/1,,47,第三章 恒星的形成與演化,恒星形成 恒星結(jié)構(gòu) 元素合成 恒星演化 超 新 星 密近雙星,/xkong/introast,2019/7/1,,48,恒星結(jié)構(gòu),恒星結(jié)構(gòu)：研究恒星內(nèi)部發(fā)生的各種物理過程，和由這些過程決定的恒星特征量 特征參量：內(nèi)部的密度、壓強、溫度、輻射、化學(xué)組成，以及它們的分布 研究手段：理論觀測 合理的簡化假設(shè) 建立基本方程組 相關(guān)的邊界條件 恒星的物態(tài)方程 求解基本方程組 與觀測比較檢驗,2019/7/1,,49,孤立的體系：只受到自引力和內(nèi)部壓力的作用 球?qū)ΨQ體系：同心球?qū)铀M成，每一球?qū)觾?nèi)物理化學(xué)性質(zhì)均勻，3D1D 簡單的體系：忽略磁場、潮汐力和自轉(zhuǎn)的影響 穩(wěn)定的體系：內(nèi)部滿足流體靜力學(xué)平衡,m,l,P,T,r,M,L,0,Teff,X,Y,Z,0,0,Pc,Tc,Centre: r=0,Surface: r=R,簡化假設(shè),恒星在自身引力和內(nèi)部壓力作用下內(nèi)部有輻射轉(zhuǎn)移的流體球,滿足無磁場/非相對論和球?qū)ΨQ條件.,2019/7/1,,50,基本方程組,1. 質(zhì)量分布方程 考慮質(zhì)量為M、半徑為R的氣體球，半徑為r、厚度為dr 的球殼所包含的質(zhì)量為： dM(r)4pr2r dr dM(r)/dr4pr2r dr/dM(r)1/4pr2r,2019/7/1,,51,2. 流體靜力學(xué)平衡方程 半徑為r、厚度為dr 的球殼內(nèi)面積為dA的氣體元：(引力和壓力平衡) 重力 dFgGM(r)dM/r2 GM(r)r dAdr/r2 壓力 dFPPdA( P + dP ) dA dPdA 0dFg+dFP GM(r)rdAdr/r2dPdA dP/drGM(r)r(r)/r2,2019/7/1,,52,3. 能量平衡方程 光度方程:反應(yīng)光度隨半徑的變化率 每秒由恒星表面輻射出去的總能量內(nèi)部每秒產(chǎn)生的總能量 L(r)單位時間通過半徑為r的球面的能量,輻射能流 e(r)單位物質(zhì)在單位時間產(chǎn)生的能量,總產(chǎn)能率 半徑為r、厚度為dr的球殼兩側(cè)的能量差: dLL(r+dr)L(r) edM 4pr2redr dL/dr 4pr2r(r)e(r),2019/7/1,,4. 能量輸運方程(能流方程) 恒星中心和外部溫度不同，內(nèi)部存在溫度梯度： 能量從恒星內(nèi)部傳送到外部，主要由輻射和對流兩種方式 恒星通常由氣體構(gòu)成，內(nèi)部熱傳導(dǎo)效應(yīng)不重要，可以忽略 dT/dMr = dT/dMr|rad + dT/dMr|con,dP/drGM(r)r(r)/r2,dr/dM(r)1/4pr2r,2019/7/1,,54,5.物態(tài)方程 PP(T,r,X,Y,Z) 表示恒星內(nèi)部的壓強、溫度、密度和化學(xué)組成關(guān)系的方程 氣體內(nèi)部的總壓強由氣體粒子運動產(chǎn)生的氣體壓強和光子產(chǎn)生的輻射壓強 PPg Pr 非簡并氣體 (non-degenerate gas) 理想氣體狀態(tài)方程 PgnkT (n為單位體積中粒子數(shù)) 對完全電離等離子體： Pgr kT (2X3Y/4Z/2 ) /mH (其中mH為H原子質(zhì)量) X/Y/Z定義為H/He/重元素各自密度與恒星總密度之比 輻射壓PraT4/3,2019/7/1,,55,簡并氣體 (Degenerate Gas) (1) 電子簡并條件：高密、低溫 (2) 電子簡并壓的物理成因 ： Pauli不相容原理：對一個費米子組成的系統(tǒng),不能有兩個或兩個以上的粒子處在同一量子態(tài) 泡利不相容原理禁止不同的組成粒子占據(jù)同一量子態(tài),因此, 就會迫使粒子進(jìn)入高能態(tài),從而產(chǎn)生巨大的簡并壓力 (3) 電子簡并壓 非相對論性電子（v c）: Pe r 5/3 相對論性電子（vc）: Pe r 4/3 抗壓縮性，簡并氣體的壓力與溫度無關(guān) (4) 離子壓強 PIr kT (XY/4 ) /mH,Pt=Pg+Pr=Pi+Pe+Pr,2019/7/1,,56,簡并條件,溫度一定，密度越高越容易簡并 密度一定，溫度越低越容易簡并 溫度、密度一定，粒子質(zhì)量越小越容易簡并,系統(tǒng)中粒子的德布羅意波長大于等于粒子之間的平均距離：,相對論,非相對論,2019/7/1,,57,EoS regimes,P=nkT,P=nkT,P=KURn4/3,P=KNRn5/3, = n/N0,PR=aT4/3,2019/7/1,,58,平衡的恒星球體內(nèi)的內(nèi)部結(jié)構(gòu)，由它的化學(xué)成份和總質(zhì)量唯一確定 給定化學(xué)元素組成X Y Z，核產(chǎn)能率和吸收系數(shù) 邊界條件： 當(dāng)r0 時，M(0)0，L(0) = 0; 當(dāng)rR 時，M(R)M, T(R) = 0, P(R) = 0. 恒星結(jié)構(gòu)方程和物態(tài)方程 可唯一求得恒星的結(jié)構(gòu) 即恒星從中心到表面不同半徑r處的壓強P,密度r,溫度T,質(zhì)量M,光度L,產(chǎn)能率e,和不透明度k等。 核燃燒使化學(xué)成份發(fā)生變化時，恒星的結(jié)構(gòu)也隨之變化,恒星結(jié)構(gòu)解的唯一性定理,2019/7/1,,59,building models,Pc為恒星中心壓強,M,R為恒星質(zhì)量和半徑,P=nkT,恒星以光度L0輻射能量時, 總能量減少(dE/dt0,即溫度卻上升了 減少的引力能一半變成了內(nèi)能,一半變成了輻射能,2019/7/1,,63,Stellar timescales 恒星演化時標(biāo),Stars such as the Sun clearly do not change their properties rapidly. So how fast can they change ? Dynamically free-fall Thermally radiative cooling Chemically nucleosynthesis Radiatively diffusion,2019/7/1,,64,動力學(xué)時標(biāo) (free-fall),從流體靜力學(xué)平衡被破壞開始，到重新建立流體靜力學(xué)止，中間所需要的時間 如果恒星內(nèi)部壓力突然消失，在引力作用下恒星坍縮時間： td R/V (R3/GM)1/2 (27 min) (R/R)3/2(M/M)-1/2 (由恒星內(nèi)部的運動方程,忽略壓力項時,即自由落體,可以求得) also: the characteristic time for a significant departure from hydrostatic equilibrium to alter the state of a star appreciably, the time taken for a body orbiting at the surface of the star to make one complete revolution, the time for a sound wave to propagate through the star,2019/7/1,,65,開爾文時標(biāo) ：thermal time,恒星將全部勢能轉(zhuǎn)換為輻射能所能持續(xù)的時間 tk V/L=(GM2/R)/L(3107 yr) (M/M)2 (R/R)-1 (L/L)-1 計算： the time required for a body to radiate its總內(nèi)能U U is related to 引力能 V by Virial theorem U = (1/2) V. But V = q GM2 / R, where q =3/5 unity, so that tK = q/2 GM2 / LR 3 107 qM2/LR y where M, L and R are in solar units. The “Kelvin time” is the relaxation time for departure of a star from thermal equilibrium. Also the time required for a star to contract from infinite dispersion to its present radius at constant L,2019/7/1,,66,核反應(yīng)時標(biāo) nuclear time,恒星通過核心區(qū)（約占恒星質(zhì)量1/10）核反應(yīng)的產(chǎn)能時間 tn=E/LhDMc2/L0.7%0.1Mc2/L (1010 yr) (M/M) (L/L)-1 計算： the fusion of four protons to create an alpha-particle releases energy Q 26MeV total available nuclear energy Enuc=q M/4mp . Q q unity represents fraction of the star available as nuclear fuel. 主序星燃燒氫的時間, tnuc 1x1011 q (M/M)/(L/L)y,2019/7/1,,67,擴(kuò)散時標(biāo) diffusion time,輻射能轉(zhuǎn)移：光子與電子發(fā)生碰撞散射。 If the photon-path is a random-walk of N steps, each of length , the total distance travelled is d=N, but the nett distance travelled is D2=N2 To escape, the photon must travel a distance R, which will take ：tdiff R2 / c 5105 R y Compare the escape time for noninteracting particles (eg neutrinos): tesc = R / c = 2.3 R s R in solar units.,2019/7/1,,68,comparative timescales,2019/7/1,,69,第三章 恒星的形成與演化,恒星形成 恒星結(jié)構(gòu) 元素合成 恒星演化 超 新 星 密近雙星,2019/7/1,,70,2019/7/1,,71,2019/7/1,,72,核