Chapter10.ppt

1、Chapter 10 The formation of stars,10.1 Interstellar medium Fig. 10-1 The Horsehead nebula (星云). Dark nebula forms the shape of a horsehead (right). Emission nebula is also seen behind the dark nebula.,Nebulae: Higher density clouds of gas and dust. Emission nebula (H II regions): An excited cloud of

2、 relatively low-density gas. Hydrogen gas is ionized by ultraviolet radiation from a nearby hot star. It emits light (mostly red) when the ionized gas recapture electrons. Reflection nebula: Dust in a gas cloud reflecting light from other stars (Fig. 10-1). Dark nebula: Dark, thick clouds blocking l

3、ight from other stars. Bok globules (博克球状体): Collapsing small dark clouds, possible star formation sites. Many of them contain infrared sources at centers.,Tiny amounts of molecules (some complex, e.g., alcohol) are also found in nebulae; some are good emitters of radio waves. can be detected by rad

4、io telescopes Giant molecular clouds typically span over a few hundred pc, containing up to 0.5 million solar masses. High density (106 particles cm-3 ) and low temperature (10 K). They contain infrared sources and bright H II regions excited by young, hot stars. Cooler regions are sites of active s

5、tellar formation.,10.2Birth of stars Gravitational force mutual attraction in the clouds contraction of gas clouds to form stars Thermal motions particles in clouds move around in random directions expansion of clouds Turbulence and rotational motions also resist contraction Most clouds would not co

6、ntract by own gravity.,Shock waves trigger fragments formation (Fig. 10-2), compressing some clouds to density high enough to form new stars. Shock waves may come from supernovae: exploding stars in the latest stage of stellar evolution. radiation pressure due to ignition of hot, young stars. collis

7、ions of molecular clouds. bulk motions of gas clouds in the spiral arms of our Milky Way Galaxy collisions of galaxies,Fig. 10-2 An interstellar gas cloud has such a low density that it is unlikely to form stars without an outside stimulus. Computer models show that shock waves can compress and frag

8、ment a cloud, driving some regions to high enough density and trigger star formation.,Contraction of the cloud the smaller the cloud becomes, the larger the gravitational force (law of gravitation: shorter distances larger forces) particles fall faster and faster towards the center collisions betwee

9、n particles are more often temperature and pressure rise Instabilities in a large contracting cloud a large cloud fragments into many pieces, each forming a new star a group of young stars form at about the same time,Protostars (原恒星): A contracting gas cloud becomes hot enough to emit much infrared

10、radiation, but not hot enough to ignite nuclear reactions (Fig. 10-3). Large size (bright) but low temperature. enters the upper-right area of the H-R diagram. Many are surrounded by a cocoon (茧)- a cloud of dust and gas absorbing light from the protostar, re-radiating the energy as infrared radiati

11、on.,Fig. 10-3 A column of cool molecular hydrogen gas and dust in M16 - Eagle nebula. Newly formed stars are embedded inside finger-like structures extending from the top of the nebula. Each fingertip is somewhat larger than our Solar System.,When a protostar is hot enough, its radiation vaporizes a

12、nd pushes away the cocoon (茧). the star becomes visible e.g., T Tauri stars: young stars surrounded by expanding clouds of gas; luminosity varies irregularly and quite rapidly. Many young stars are associated with disks of material called protoplanetary disks (原行星盘) which may finally evolve into pla

13、netary systems (Fig. 10-4).,Fig. 10-4 A Hubble Space Telescope view of a small portion of the Orion nebula (猎户星云) revealed four protoplanetary disks around young stars. They are gas and dust disks long suspected by astronomers to be in an early stage of planetary systems formation.,Central temperatu

14、re is high enough nuclear fusion is ignited at the core produces energy; temperature of the star further increases rapidly expanding thermal pressure balances the collapsing force of gravity (Fig. 10-5) contraction of gas stops a stable, shining star enters the main sequence in the H-R diagram (Fig.

15、 10-6),Fig. 10-5 In a steady burning star on the main sequence, the outward pressure of hot gas counter-balances the inward pressure of gravity. This is true at every point inside the star, guaranteeing its stability.,Fig. 10-6 The paths of the protostars entering the main sequence in the H-R diagra

16、m. The more massive a protostar is, the larger is its gravity and the faster it contracts. A star of one solar mass requires 30 million years to reach the main sequence.,10.4Stellar energy A series of nuclear reactions fusing four protons (the nucleus of a hydrogen atom is a single proton) into a he

17、lium nucleus. 4 protons 1 helium nucleus + positrons + neutrinos + energy Four protons is more massive than a helium nucleus some mass is converted to energy according to E = mc2 Rate of reactions is very sensitive to temperature higher temperature much faster reaction rate,Energy is released in the

18、 form of high energy electromagnetic waves. Positron: A particle very similar to an electron, except that it carries positive charge. Neutrino: A very light, neutral particle, interacting with matter very weakly. Energy generated at the core is slowly transported to the surface by radiation and conv

19、ection. Radiation: Energy carried by electromagnetic radiation Convection: Hot gas rises and cooler gas sinks, carrying energy outward.,Solar neutrino problem Neutrinos hardly interact with matter (Fig. 10-7). they penetrate the interior layers of the sun and reach the earth directly detection of wh

20、ich is an indirect evidence of nuclear reactions in the solar core but we detect only 1/3 of the neutrinos from the sun as theoretically expected something wrong with our theory?,Fig. 10-7 Neutrinos hardly interact with matter. An enormous number of neutrinos coming from the sun pass through our bod

21、ies every second.,Pressure-temperature thermostat (温度调节) controls the rate of energy generation gravity and thermal pressure counterbalances stability of a star If nuclear reactions slow down temperature decreases, lower thermal pressure gravity contracts the star compression heats the core, increas

22、es the density increases nuclear energy generation,If nuclear reactions are too fast temperature increases; higher thermal pressure stellar core expands expansion cools the core, decreases the density slows down the nuclear reactions rate A star is very stable in the main sequence (a star of 1 solar

23、 mass lasts for 10 billion years),2002年度的诺贝尔物理奖是奖给雷蒙德戴维斯（美国宾州大学物理天文系）、小柴昌俊（日本东京大学国际基本粒子物理中心）和里卡尔多贾科尼（美国华盛顿特区联合大学公司）在天体物理上的先驱性贡献。这里包括两项成就，前两位是因为宇宙中微子的探测，后一位是因为发现宇宙X射线源。 雷蒙德戴维斯和小柴昌俊开辟了中微子天文学 中微子是基本粒子中最难探测的一种粒子。要探测一个粒子，就要使这个粒子至少在探测器中发生一次碰撞，产生一个信号。但是，中微子与物质的作用很弱，即使一 个中微子穿过上百亿个地球，也很难发生一次碰撞。因此，中微子是最难发现、最难

24、探测的一种粒子。要探测中微子，必须用非常庞大的探测器，对着非常强的中微子源，持续非常长的时间。,中微子天文学,历史上，在1931年，中微子是泡利为了解释贝塔衰变中的电子连续能谱而预言的一种电中性的粒子，它的质量极其微小，几乎等于零。第一个提出验证中微子存在 的实验方案的是我国的王淦昌。早在1942年，他就建议用7Be来做实验。7Be通过俘获一个电子而变成7Li并放出一个中微子。只要测定7Li的反冲动量，就可以推知中微子的性质。50年代初，阿伦、戴维斯等才完成这个实验，显示中微子的确存在。这是间接的测量方法。直接方法要到1955年才由柯温和莱茵斯完成。他们用了200公升的水和370加仑的液体闪烁体做探测器，埋在很深的地下，对着核反应堆放射出来的极强的中微子（实际是反中微子）束，经过很长的时间，才成功地测量到为数不多的中微子。可惜柯温死得太早，1995年迟到的诺