恒星的形成、结构及元素合成.ppt

2018/10/16,1,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,2018/10/16,xkong@,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.,2018/10/16,xkong@,3,2018/10/16,xkong@,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？,2018/10/16,xkong@,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,2018/10/16,6,2018/10/16,7,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,2018/10/16,xkong@,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 - 1946,2018/10/16,9,中性氢云：n ~1cm-3, T ~100K→MJ~3×104 M⊙ 暗分子云：n ~106 cm-3, T ~10 K → MJ~1 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,2018/10/16,12,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 Rotation—that is, spin—can also compete with gravity's 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2018/10/16,13,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.,2018/10/16,14,演化轨迹： each for a fixed mass at different times等年龄线：each for a fixed time and different masses,,,2018/10/16,15,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,2018/10/16,16,3. 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）,2018/10/16,17,4. 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,2018/10/16,18,2018/10/16,19,6. 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2/37. 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.,2018/10/16,20,Prestellar evolutionary paths for stars more massive and less massive than our Sun.,2018/10/16,21,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,2018/10/16,xkong@,22,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 3603,2018/10/16,23,褐矮星 (Brown Dwarfs),Masses < 0.08 M⊙(10MJ- 84 MJ) Central Te<3 million K Surface temperature ~ 1000 K,,,TWA 5 and its brown dwarf companion in Infrared (left) and in X-ray (right).,2018/10/16,24,Differences between brown dwarfs and planets Planets are smaller and lighter Planets have a solid core They are formed in a complete different way,2018/10/16,25,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 =1.35,2018/10/16,26,恒星形成理论的观测证据,The evolutionary stages described are derived from numerical experiments performed on computers.,2018/10/16,27,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),2018/10/16,28,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,2018/10/16,xkong@,29,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,2018/10/16,xkong@,30,猎户星云缩小图,最大恒星,dark red column,Failing stars,Sculpting the landscape,Pillars of gas,2018/10/16,xkong@,31,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.,。