恒星的形成与演化.ppt

2019/7/1,1,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,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？,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！ 引力为天体和整个宇宙动力学的支配者,2019/7/1,,6,2019/7/1,7,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,2019/7/1,,8,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 Jeans 1877 - 1946,2019/7/1,,9,中性氢云：n 1cm-3, T 100KMJ3104 M 暗分子云：n 106 cm-3, T 10 K MJ1 M Jeans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,2019/7/1,,12,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。,2019/7/1,,13,其它影响恒星形成的因素：,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 magnetism.,2019/7/1,,14,演化轨迹： each for a fixed mass at different times 等年龄线：each for a fixed time and different masses,2019/7/1,,15,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,2019/7/1,,16,3. 碎裂停止 (fragmentation 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）,2019/7/1,,17,4. 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,2019/7/1,,18,2019/7/1,,19,6. 零龄主序 (zero-age main-sequence stars) :恒星热核反应 (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.,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,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 IMF, x =1.35,2019/7/1,,27,恒星形成理论的观测证据,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。 Orion Nebula,2019/7/1,,31,猎户星云缩小图,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,2019/7/1,,36,2019/7/1,,37,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,2019/7/1,,38,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。,2019/7/1,,39,(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,2019/7/1,,40,恒星诞生 膨胀激波 新生恒星,2019/7/1,,41,2019/7/1,,42,(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。,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 “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 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,/xkong/introast,2019/7/1,,48,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,2019/7/1,,49,孤立的体系：只受到自引力和内部压力的作用 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.,2019/7/1,,50,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,2019/7/1,,51,2. 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 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)L(r) edM 4pr2redr dL/dr 4pr2r(r)e(r),2019/7/1,,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,2019/7/1,,54,5.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 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PraT4/3,2019/7/1,,55,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 (3) 电子简并压 非相对论性电子（v c）: Pe r 5/3 相对论性电子（vc）: Pe r 4/3 抗压缩性，简并气体的压力与温度无关 (4) 离子压强 PIr kT (XY/4 ) /mH,Pt=Pg+Pr=Pi+Pe+Pr,2019/7/1,,56,简并条件,温度一定，密度越高越容易简并 密度一定，温度越低越容易简并 温度、密度一定，粒子质量越小越容易简并,系统中粒子的德布罗意波长大于等于粒子之间的平均距离：,相对论,非相对论,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,平衡的恒星球体内的内部结构，由它的化学成份和总质量唯一确定 给定化学元素组成X Y Z，核产能率和吸收系数 边界条件： 当r0 时，M(0)0，L(0) = 0; 当rR 时，M(R)M, T(R) = 0, P(R) = 0. 恒星结构方程和物态方程 可唯一求得恒星的结构 即恒星从中心到表面不同半径r处的压强P,密度r,温度T,质量M,光度L,产能率e,和不透明度k等。 核燃烧使化学成份发生变化时，恒星的结构也随之变化,恒星结构解的唯一性定理,2019/7/1,,59,building models,Pc为恒星中心压强,M,R为恒星质量和半径,P=nkT,恒星以光度L0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,2019/7/1,,63,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-fall Thermally radiative cooling Chemically nucleosynthesis Radiatively diffusion,2019/7/1,,64,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, 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,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 26MeV total available nuclear energy Enuc=q M/4mp . Q q unity represents fraction of the star available as nuclear fuel. 主序星燃烧氢的时间, tnuc 1x1011 q (M/M)/(L/L)y,2019/7/1,,67,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=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,2019/7/1,,70,2019/7/1,,71,2019/7/1,,72,核