1、2018/7/20,1,第三章 恒星的形成与演化,恒星形成恒星结构元素合成恒星演化超 新 星密近双星,2018/7/20,,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 evid
2、ence supporting the modern theory of star formation. Explain the nature of interstellar shock waves, and discuss their possible role in the formation of stars.,2018/7/20,,3,大爆炸宇宙学宇宙的极早期，宇宙的温度和密度都极高温度不断下降，宇宙辐射为主物质为主气体逐渐凝聚成气云，然后有恒星、星系 恒星形成理论星际气体怎么会形 成光辉夺目的恒星 呢？,2018/7/20,,4,How do stars form? What fac
3、tors determine the masses, luminosities, and distribution of stars in our Galaxy?What determines which interstellar clouds collapse?引力！引力为天体和整个宇宙动力学的支配者,2018/7/20,5,恒星形成理论: 弥漫学说,散布于空间弥漫物质在引力作用下凝聚为恒星宇宙空间存在着大量的星际物质: 原子/分子/尘埃由于星际物质密度的不均匀性，形成了一些密度较大区域星际物质受到引力的作用，便聚集到这些区域，形成星云星云不断收缩，势能转换为恒星内部热能和向外的辐射能星云温
4、度不断提高，并向外辐射能量，从而形成原始恒星不同类型的恒星规模较小的星云形成一个孤立的恒星,大的星云由于密度不均匀,其中有几个质量中心,因而形成双星、聚星或星团。质量非常小的星云,不能收缩成为恒星,2018/7/20,,6,金斯（Jeans）不稳定性,由万有引力产生的一种不稳定性，因金斯在20世纪初最先研究而得名对于一个如星云的自引力体系，当星云的质量足够高时，(向内的)引力超过由热运动和湍动产生的(向外的)压力，将引起星云的收缩星云不稳定的极限质量称为金斯(Jeans)质量：,James Jeans1877 - 1946,2018/7/20,7,中性氢云：n 1cm-3, T 100KMJ3
5、104 M暗分子云：n 106 cm-3, T 10 K MJ1 MJeans质量判据给出了非相对论、无磁场星云坍缩的必要条件(并不是所有的星云都可以形成恒星),金斯（Jeans）不稳定性,2018/7/20,10,其它影响恒星形成的因素：,能量的有效辐射辐射压将反抗引力,阻碍星云塌缩星系潮汐力影响星云原初角动量Rotationthat is, spincan also compete with gravitys inward pull,垂直自转轴方向停止收缩，平行方向继续收缩，星云变得扁平且密度增大，最终星云碎裂。总角动量被分解为各个碎块的自转和轨道角动量。,2018/7/20,11,其它影
6、响恒星形成的因素：,原始星云的磁场原始星云一般具有微弱的磁场。随着星云收缩，磁场强度变大。磁场将阻止星云收缩，特别是垂直于磁场方向的收缩。,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 m
7、agnetism.,2018/7/20,12,演化轨迹： each for a fixed mass at different times等年龄线：each for a fixed time and different masses,2018/7/20,13,类太阳质量恒星的形成,星云的快速收缩过程(密度小，辐射透明，等温收缩)1. 星际云 (interstellar cloud):星际云坍缩，并分裂成小云块(密度上升，金斯质量减小)2. 星云块 (cloud fragments):星云仍十分稀薄，热量可以不受阻碍地散逸，星云内的温度没有明显上升,2018/7/20,14,3. 碎裂停止 (f
8、ragmentation ceasess):星云进一步坍缩和分裂，密度上升。核心区域变得不透明，温度迅速上升，金斯质量增大。星云停止分裂。(等温收缩绝热收缩）,2018/7/20,15,4. 形成原恒星(protostar) : 星云快速收缩过程结束，引力几乎和气体压力相等。恒星已经变得不透明，辐射只能从表面逸出，中心温度迅速升高。 5.原恒星演化(protostellar evolution): 原恒星向主序演化为主序前星，但内部温度还没有升高到H点火温度,2018/7/20,16,2018/7/20,17,6. 零龄主序 (zero-age main-sequence stars) :恒星
9、热核反应 (H燃烧)开始进行，成为零龄主序恒星。光度约为现在太阳光度的2/3。 7. 主序星 (main-sequence stars) :恒星略微收缩，完全达到流体静力学平衡，成为正常恒星.,2018/7/20,18,Prestellar evolutionary paths for stars more massive and less massive than our Sun.,2018/7/20,19,不同质量的恒星在形成过程中，在H-R图上沿不同的路径演化。小质量原恒星内部对流发展充分，温差小，收缩时表面温度几乎不变；大质量原恒星对流层浅，温度变低。质量越高的恒星，其原恒星演化到主序
10、的时间越短，在主序上的位置越高。,具有不同质量恒星的形成,2018/7/20,,20,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.,N
11、GC 3603,2018/7/20,21,褐矮星 (Brown Dwarfs),Masses 0.08 M(10MJ- 84 MJ)Central Te3 million KSurface temperature 1000 K,TWA 5 and its brown dwarf companion in Infrared (left) and in X-ray (right).,2018/7/20,22,Differences between brown dwarfs and planetsPlanets are smaller and lighterPlanets have a solid
12、coreThey are formed in a complete different way,2018/7/20,23,初始质量函数(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 I
13、MF, x =1.35,2018/7/20,24,恒星形成理论的观测证据,The evolutionary stages described are derived from numerical experiments performed on computers.,2018/7/20,25,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),
14、(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),2018/7/20,26,EVIDENCE OF CLOUD FRAGMENTS,(a) Orion (b) enlarged. The three frames at right show some of the evidence for those prot
15、ostars. (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,2018/7/20,,27,HST拍摄到了迄今为止最清晰的猎户座星云全景照片。这张照片不仅显示出大量恒星的诞生，也包含有罕见的褐矮星。Ori
16、on Nebula,2018/7/20,,28,猎户星云缩小图,最大恒星,dark red column,Failing stars,Sculpting the landscape,Pillars of gas,2018/7/20,,29,Image from Spitzer and HSTwavelengths 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 orange8-micron
17、 light is red.,2018/7/20,30,EVIDENCE OF PROTOSTARSAn 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, aroun
18、d stage 5.,2018/7/20,,31,HST在可见光波段拍摄了这个鹰状星云中的“诞生柱”，并且使得它扬名全球。Gas Pillars in the Eagle Nebula (M16): Pillars of Creation in a Star-Forming Region,2018/7/20,,32,2018/7/20,,33,Spitzer: 心宿增四星云内的年轻恒星. 发现大约有300颗正在出现和新形成的恒星，平均年龄在估计只有30万年,2018/7/20,,34,疏散星团的H-R图,星团NGC 2261的H-R图：原恒星到达主序的位置和时间随质量的变化。,2018/7/2
19、0,,35,(1) 激波压缩超新星爆发、热星辐射或银河系旋臂转动等过程产生激波。激波压缩附近的星云，使其密度增大，触发恒星的形成。 恒星形成过程可能类似于链式反应。,星云M20中的激波压缩效应,星云坍缩的触发机制,2018/7/20,,36,恒星诞生膨胀激波新生恒星,2018/7/20,,37,(2) 星云碰撞,星系自转，它的组成物质在运动中穿旋臂而过，恒星偕同星际物质在这样的过程中都是由弯曲旋臂的内侧进去，再从外侧出来。星系物质穿越密度较大的旋臂，引起产星过程。,2018/7/20,,38,2018/7/20,39,Steps to the formation of stars and pl
20、anets: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 growi
21、ng protostar at the center.At the same time, bipolar outflows from forming star/disk system begin.,2018/7/20,40,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.O
22、nce the outflow ceases and the accretion phase that lead to the buildup of the star ends, a disk of leftover 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 be
23、hind is a new star, perhaps some planets, and a disk of left-over ground-up solids,2018/7/20,41,第三章 恒星的形成与演化,恒星形成恒星结构元素合成恒星演化超 新 星密近双星,2018/7/20,42,恒星结构,恒星结构：研究恒星内部发生的各种物理过程，和由这些过程决定的恒星特征量特征参量：内部的密度、压强、温度、辐射、化学组成，以及它们的分布研究手段：理论观测合理的简化假设建立基本方程组相关的边界条件恒星的物态方程求解基本方程组与观测比较检验,2018/7/20,43,孤立的体系：只受到自引力和内部
24、压力的作用球对称体系：同心球层所组成，每一球层内物理化学性质均匀，3D1D简单的体系：忽略磁场、潮汐力和自转的影响稳定的体系：内部满足流体静力学平衡,m,l,P,T,r,M,L,0,Teff,X,Y,Z,0,0,Pc,Tc,Centre: r=0,Surface: r=R,简化假设,恒星在自身引力和内部压力作用下内部有辐射转移的流体球,满足无磁场/非相对论和球对称条件.,2018/7/20,44,基本方程组,1. 质量分布方程 考虑质量为M、半径为R的气体球，半径为r、厚度为dr 的球壳所包含的质量为： dM(r)4pr2r dr dM(r)/dr4pr2r dr/dM(r)1/4pr2r,2
25、018/7/20,45,2. 流体静力学平衡方程半径为r、厚度为dr 的球壳内面积为dA的气体元：(引力和压力平衡)重力 dFgGM(r)dM/r2 GM(r)r dAdr/r2压力 dFPPdA( P + dP ) dA dPdA0dFg+dFP GM(r)rdAdr/r2dPdAdP/drGM(r)r(r)/r2,2018/7/20,46,3. 能量平衡方程光度方程:反应光度随半径的变化率每秒由恒星表面辐射出去的总能量内部每秒产生的总能量 L(r)单位时间通过半径为r的球面的能量,辐射能流e(r)单位物质在单位时间产生的能量,总产能率半径为r、厚度为dr的球壳两侧的能量差:dLL(r+dr
26、)L(r) edM 4pr2redr dL/dr 4pr2r(r)e(r),2018/7/20,4. 能量输运方程(能流方程)恒星中心和外部温度不同，内部存在温度梯度： 能量从恒星内部传送到外部，主要由辐射和对流两种方式恒星通常由气体构成，内部热传导效应不重要，可以忽略dT/dMr = dT/dMr|rad + dT/dMr|con,dP/drGM(r)r(r)/r2,dr/dM(r)1/4pr2r,2018/7/20,48,5.物态方程 PP(T,r,X,Y,Z)表示恒星内部的压强、温度、密度和化学组成关系的方程气体内部的总压强由气体粒子运动产生的气体压强和光子产生的辐射压强 PPg Pr
27、非简并气体 (non-degenerate gas)理想气体状态方程 PgnkT (n为单位体积中粒子数)对完全电离等离子体： Pgr kT (2X3Y/4Z/2 ) /mH (其中mH为H原子质量)X/Y/Z定义为H/He/重元素各自密度与恒星总密度之比辐射压PraT4/3,2018/7/20,49,简并气体 (Degenerate Gas)(1) 电子简并条件：高密、低温 (2) 电子简并压的物理成因 ：Pauli不相容原理：对一个费米子组成的系统,不能有两个或两个以上的粒子处在同一量子态泡利不相容原理禁止不同的组成粒子占据同一量子态,因此, 就会迫使粒子进入高能态,从而产生巨大的简并压力
28、 (3) 电子简并压 非相对论性电子（v 0辐射能量时,总能量减少(dE/dt0,即温度却上升了减少的引力能一半变成了内能,一半变成了辐射能,2018/7/20,,56,Stellar timescales 恒星演化时标,Stars such as the Sun clearly do not change their properties rapidly. So how fast can they change ?Dynamically free-fallThermally radiative coolingChemically nucleosynthesisRadiatively diff
29、usion,2018/7/20,,57,动力学时标 (free-fall),从流体静力学平衡被破坏开始，到重新建立流体静力学止，中间所需要的时间如果恒星内部压力突然消失，在引力作用下恒星坍缩时间： td R/V (R3/GM)1/2 (27 min) (R/R)3/2(M/M)-1/2(由恒星内部的运动方程,忽略压力项时,即自由落体,可以求得)also:the characteristic time for a significant departure from hydrostatic equilibrium to alter the state of a star appreciably,
30、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,2018/7/20,58,开尔文时标 ：thermal time,恒星将全部势能转换为辐射能所能持续的时间tk V/L=(GM2/R)/L(3107 yr) (M/M)2 (R/R)-1 (L/L)-1计算：the time required for a body to radiate its总内能
31、UU 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 ywhere 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 cont
32、ract from infinite dispersion to its present radius at constant L,2018/7/20,,59,核反应时标 nuclear time,恒星通过核心区（约占恒星质量1/10）核反应的产能时间tn=E/LhDMc2/L0.7%0.1Mc2/L (1010 yr) (M/M) (L/L)-1计算：the fusion of four protons to create an alpha-particle releases energy Q 26MeVtotal available nuclear energy Enuc=q M/4mp
33、. Qq unity represents fraction of the star available as nuclear fuel. 主序星燃烧氢的时间, tnuc 1x1011 q (M/M)/(L/L)y,2018/7/20,,60,扩散时标 diffusion time,辐射能转移：光子与电子发生碰撞散射。 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=
34、N2To 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 sR in solar units.,2018/7/20,,61,comparative timescales,2018/7/20,,62,第三章 恒星的形成与演化,恒星形成恒星结构元素合成恒星演化超 新 星密近双星,2018/7/20,63
35、,核反应元素合成,原初元素(Primordial Elements)大部分的H, He和少量的Li, B, Be形成于宇宙大爆炸初期其他元素来源于恒星内部的核合成 (Nuclear Synthesis),2018/7/20,,64,结合能:将若干个核子结合成原子核放出的能量或将原子核的核子全部分散开来所需的能量。原子核的质量小于组成原子核的全部核子质量的总和，差额m称为质量亏损,mc2就是核结合能质量中等的核，比结合能量最大，它们最为稳定,p-p链 (Proton-Proton Chain),M2M,Tc2x107K的主序星,主要发生的核反应是CNO循环CNO循环有2个分支CNO-I: 41H
36、4He+25.01MevCNO-2: 41H4He+24.8Mev,2018/7/20,,69,2018/7/20,,70,CNO-I,2018/7/20,71,CNO cycle,The CNO cycle is a “catalytic” process. The principal reactants are neither created nor destroyed.,2018/7/20,,72,当恒星核内温度超过4x108K时,会发生3重a粒子反应,重元素开始形成3个4He核聚合为稳定12C的过程 104 105 g cm-3, and T 108KThe reaction rele
37、ases 7.27 MeV per 12C produced.,34He12C+7.27 Mev,氦燃烧： 3- Process,2018/7/20,,73,碳燃烧：恒星中心温度达到T 6x108K,氧燃烧：,2018/7/20,74,relative rates,2018/7/20,,75,比Fe峰元素更重元素的形成中子俘获反应,e.g., 56Fe + n57Fe+ n 58Fe + n 59Fe 59Co + n 60Co 60Ni,2018/7/20,,76,元素合成,福勒对宇宙中化学元素形成具有重要意义的核反应所进行的理论和实验的研究，得到Nobel prize for Physics （1983）,
