Astrophysics & Cosmology Now we leap to the largest stars and galaxies. These two extreme realms, elementary particles and the cosmos, are among the most intriguing(引起兴趣的) and exciting subjects in science. Use of the techniques and ideas of physics to study the heavens is often referred to as astrophysics. At the base of our present theoretical understanding of the universe is Einsteins general theory of relativity and its theory of gravitation stars 星体 galaxy glksi 星系,银河 cosmos kzms 宇宙astrophysics strfIzIks 天体物理学general theory of relativity 广义相对论专 业 英 语Physics Englishfor in the large-scale structure of the universe, gravity is the dominant force. General relativity serves also as the foundation for modern cosmology, which is the study of the universe as a whole. Cosmology deals especially with the search for a theoretical framework to understand the observed universe, its origin, and its future.cosmology kzmld i 宇宙学宇宙学专 业 英 语Physics Englishplanet 行星行星 telescopic telisk pik 用望远镜看的用望远镜看的1 Stars and Galaxies According to the ancients, the stars, except for the few that seemed to move (the planets), were fixed on a sphere. The universe was neatly self-contained, and we on Earth were at or near its center. But in the centuries following Galileos first telescopic observations of the heavens in 1610, our view of the universe has changed dramatically. 专 业 英 语Physics English We no longer place ourselves at the center, and we view the universe as vastly larger. The distances involved are so great that we specify them in terms of the distance traveled (in a vacuum) by light in one year: 1 light-year (ly) = 9.471015 m. Besides light-year, in astrophysics, the following distance units are often used:Astronomical unit (AU) This is the mean radius of the Earths orbit around the Sun. 1 AU = 1.50 1011 m.light-year 光年 astronomical strnmIk()l unit(AU) 天文单位专 业 英 语Physics EnglishParsec (pc) This is the distance at which the mean radius of the Earths orbit has an angular displacement of one arc second (1/3600 degree).1 pc = 3.26 ly = 2.06 105 AU = 3.09 1016 m.EarthSun1 AU1 pc1/3600 degreeparsec (pc)秒差距秒差距专 业 英 语Physics English The Earth is one of many planets in orbit around the Sun. The Sun, planets, and other objects in orbit are together known as the Solar System. Most of the planets move in near-circular orbits. Many have smaller moons orbiting them. Comets are small, icy objects in highly elliptical orbits around the Sun. Planets, moons, and comets are only visible because they reflect the Suns light.Solar System Solar System 太阳系太阳系moon moon 卫星卫星comet comet 彗星彗星专 业 英 语Physics English On a clear moonless night, thousands of stars of varying degrees of brightness can be seen, as well as the elongated(伸长的) cloudy stripe(条纹) known as the Milky Way. It was Galileo who first observed, about 1610, that the Milky Way is comprised of countless individual stars. A section of the Milky WayMilky Way 银河银河 A century and a half later, Thomas Wright suggested that the Milky Way was a flat disc of stars extending to great distances in a plane, which we call the Galaxy (Greek for “milky way”). Our Galaxy has a diameter of almost 100,000 light-years and a thickness of very roughly 2000 light-years. It has a bulging(凸出) central nucleus and spiral arms. The total mass of all the stars in our Galaxy is about 31041 kg. Our Sun, which seems to be just another star, is located more than half way from the center to the edge, about 28,000 lys from the center. Our Galaxy contains about 1011 stars. The sun orbits the galactic glktik center approximately once every 200 million years, so its speed is about 250 km/s relative to the center of the Galaxy. 专 业 英 语Physics English100,000 lysOur Sunend view in the plane of the disctop view, looking down on the disc2000 lys牛郎牛郎 Altair lt天鹰座天鹰座 织女织女 Vega vi:g天琴座天琴座专 业 英 语Physics English We can see stars both within and outside the Milky Way by telescope, many faint cloudy patches（ 斑 点 ） in the sky which were all referred to once as “nebula” (Latin for “clouds”). A few of these, such as those in the constellations Andromeda and Orion, can actually be discerned with the naked eye on a clear night. This globular star cluster is located in the constellation Hercules h:kjuli:z (武仙座武仙座).nebula 星云Constellation knstlein 星座Andromeda ndrmid 仙女座Orion rain 猎户座专 业 英 语Physics EnglishOthers are glowing clouds of gas or dust, and it is for these that we now mainly reserve the word nebula. Most fascinating are those that belong to a third category: they often have fairly regular elliptical shapes and seem to be a great distance beyond our Galaxy. They appear elliptical because we see them at an angle, and are faint because they are so distant. At first it was not universally accepted that these objects were extragalactic that is, outside our Galaxy. The very large telescopes constructed in this century revealed that individual stars could be resolved within these extragalactic objects and that many contained spiral arms. extragalactic ,ekstrgl ktik 银河系外的银河系外的, 天河外的天河外的专 业 英 语Spiral galaxy in the constellation Hydra长蛇座长蛇座The Whirlpool galaxyThe gaseous nebula专 业 英 语Physics English Edwin Hubble (1889-1953), who did much of this observational work in the 1920s using the telescope, was also able to demonstrate that these objects were indeed extragalactic because of their great distances. These nebula are galaxies similar to ours. Today, the largest telescopes can see about 1011 galaxies. Galaxies tend to be grouped in galaxy clusters, with anywhere from a few to many thousands of galaxies in each cluster. Furthermore, clusters themselves seem to be organized into even larger aggregates(集 合 体 ): clusters of clusters of galaxies, or superclusters. The galaxies nearest us are about 2 million light-years away.supercluster 超星系团超星系团专 业 英 语Physics English Besides the usual stars, clusters of stars, galaxies, and clusters and superclusters of galaxies, the universe contains a number of other interesting objects. Among these are stars known as red giants, white dwarfs, neutron stars, black holes and so on.red giant 红巨星红巨星white dwarf 白矮星白矮星neutron star 中子星中子星black hole 黑洞黑洞专 业 英 语Physics English2 Birth of a starStars form in huge clouds of gas (mainly hydrogen) and dust called nebulae (星星云云). The sun formed in a nebula about 5 109 years ago. The process took about 5 107 years.Gravity pulled more and more nebular matter into a concentrated clump called a protostar (原原始始星星体体). The loss of Gravity caused a rise in core temperature which triggered the fusion (聚变) of hydrogen and the release of energy. Thermal activity stopped further gravitational collapse (引力坍缩). The sun had become a main sequence（主星序）star.专 业 英 语Physics English3 Life and death of a star The sun gets most of its energy from the proton-proton chain (质质 子子 质质 子子 链链 ), a multi-stage fusion process which converts hydrogen-1 into helium-4. Hotter, more massive stars use the CNO cycle. This also changes hydrogen-1 into helium-4, but involves carbon, nitrogen (氮), and oxygen nuclei. The Sun is about half way through its life on the main sequence (about 1010 year). Hotter, more massive stars consume hydrogen more quickly and have shorter main sequence lives.专 业 英 语Physics EnglishWhen all its hydrogen has been converted into helium, the Sun will take the path shown on the H-R diagram below.1231. The core collapses. The sun becomes a red giant (红红巨巨星星) as its outer layers expand and cool (and engulf (吞没) the Earth). With the core temperature rising to over 108 K, energy is released by the fusion of helium into carbon.专 业 英 语Physics English专 业 英 语Physics English2. After further changes, the outer layers expand and drift off into space. The core and inner layers123become a white dwarf (白白矮矮星星) whose core is so dense that the normal atomic structure breaks down. The electrons form a degenerate electron gas (派派 生生 电电 子子 气气 体体 ) whose pressure stops further collapse.3. Fusion cease. The white dwarf cools and fades for ever.专 业 英 语Physics EnglishNote: Stars less massive than the Sun end up as white dwarfs, without going through the giant stages. Massive stars become giants or supergiants, then end up as neutron stars or black holes.supergiant star 超巨星超巨星neutron star 中子星中子星black hole 黑洞黑洞专 业 英 语Physics English4 More stellar objectsSupernova (超超新新星星) When a massive star enters its giant phase, its core becomes so hot that carbon is fused into heavier elements. If the star exceeds about 8 solar (Suns) masses, iron is produced. As this is at the top of the binding energy (结合能) curve, fusion no longer supplies energy. The core collapses, causing a shock wave (冲击波) which blows away the stars outer layers in a huge explosion called a supernova ,sju:pnuv. For a few days, a supernova is millions of times brighter than a star. Elements ejected from a supernova eventually “seed” the nebulae (星云) in which new stars (恒星) and planets (行星) will form.专 业 英 语Physics EnglishNeutron stars (中中子子星星) If the core of a supernova (超新星) exceeds about 1.4 solar masses, the degenerate electron gas (派生电子气体) cannot resist gravitational collapse (引力坍缩). Electrons and protons are pushed together to form neutrons. The result is a neutron star - essentially a giant nucleus about 10 - 30 km across.Pulsars pls: (脉冲星) These emit radio, light, or X-ray pulses at up to 500 times per second. They are believed to be rapidly spinning neutron stars. Pulsars are detected because the star sends out two narrow radiation beams which rotate with it, rather like the beams of light from a lighthouse (灯塔).专 业 英 语Physics EnglishPulsarsneutron starmagnetic axisradiationspin axisThis shows how radio pulsars radiate their radio waves 专 业 英 语Physics EnglishBlack holes If the core of a supernova exceeds about 2.5 solar masses, even the neutrons formed cannot resist gravitational collapse. The core shrinks (收缩) to become a black hole from which no particles or radiation can escape.the galaxy NGC4261 the black hole专 业 英 语Physics EnglishBinary stars (双子星) These are two stars which rotate about a common center of mass. If they are close, gravity may pull material from one to the other. If one is a neutron star or black hole, material falling into it will give off X-rays.Quasars (类星体) These have red shifts which suggest that they are the most distant objects in the Universe. If they really are distant, they radiate as much energy as some galaxies, but have only the volume of a solar system. Each may be the active center of a galaxy where nebular matter surrounds a supermassive (特大质量的) black hole.vvoidgalaxies5 The structure of the UniverseThe study of the Universe, its origins, and evolution is called cosmology (宇宙学宇宙学). The Universe contains billions of galaxies. Their average separation is 106 light-years. Together, they forma network of long, clumpy (丛,簇) filaments(细丝) with huge voids spacesin between. Despite their local irregularities, the galaxies are, on a large scale, evenly (均匀地) distributedin all directions. The motion of galaxiesindicates that they are surrounded bymassive amount of thinly-spread, invisible material. This is called dark matter (暗物质暗物质).Its nature is not yet known.专 业 英 语Physics English6 Hubbles law (哈勃定律哈勃定律)Measurements of Doppler red shifts indicate that, in general, the galaxies are receding (后退) from each other. The further away the galaxy, the greater is its red shift and, therefore, the greater its recession velocity.According to Hubbles law, the distance d of a galaxy and its recession velocity vilsiti v are linked by this equation:400800120002468approximate distance/Mpcrecession velocity/104 km s-1(2) 专 业 英 语Physics EnglishH0 is called the Hubble constant. Large distances are difficult to estimate accurately, so the value of H0 has a high uncertainty. However, it is thought to lie in the range 50-100 km s-1 Mpc-1. Its value is important for several reasons: It enables the distances of the most remote galaxies to beestimated from their red shifts. The age of the Universe can be estimated from it (H0 has dimensions of 1/time). The fate of the Universe depends on it.专 业 英 语Physics English7 The expanding UniverseThe most generally accepted explanation of galactic red shifts is that the Universe is expanding . At zero time, all its matter and energy was together in a highly concentrated state.Estimating the age of the Universe If a galaxy is d from our own, and has a steady recession velocity v, then separation of the galaxies must have occurred at a time d/v ago. This represents the approximate age of the Universe. From equation (2) d/v = 1/H0, so:The age of the Universe 1/H0专 业 英 语Physics EnglishThis gives an age in the range 1-21010 years (10-20 billion years). Note: The above calculation assumes constant v. In reality, v is thought to have increased with time. This gives a reduced age estimate.Olbers paradox (奥伯斯佯谬) In the 17th century, it was pointed out that, if the stars continued out to infinity, the night sky should be white, not dark because light must be coming from every possible direction in the sky. This became known as Olbers paradox prdks.Two reasons for the dark night sky have been suggested: In an expanding Universe, red-shifted wavelengths mean reduced photon energies (光子能), so the intensity of the light from distant stars is reduced. There is a limit to our observable Universe. If, say, the Universe is 15 billion years old, then we have yet to receive light from stars more than 15 billion light-years away. So everything beyond that distance looks dark.The cosmological principle (宇宙学原理) This says that, apart from small-scale irregularities, the Universe should appear the same from all points within it (i.e. the distribution of galaxies and their recession velocities should appear the same from all points).专 业 英 语Physics English8 The hot big bang theory (热大爆炸理论) According to this theory, sometimes called the standard model, the Universe (and time) began about 10-20 billion years ago when a single, hot “superatom” erupted in a burst of energy called the big bang (宇宇宙宙大大爆爆炸炸). As expansion and cooling took place, particles and antiparticles formed. Further cooling meant that combinations were possible, so nuclei and then atoms formed and eventually galaxies.专 业 英 语Physics EnglishFundamental forces In the instant after the big bang, the fundamental forces existed as one superforce (超力). But within 10-11s, they had separated from each other.Cosmic background (宇宙本底) There is a steady background radiation (背景辐射,本底辐射) which comes from every direction in space. It peaks in the microwave region, and corresponds to the radiation from a black body at 2.7 K. It is thought to be the red-shifted remnant (残迹) of radiation from the big bang. Its presence is predicted by the big bang theory.专 业 英 语Physics EnglishInflation (膨胀) The standard model cannot satisfactorily explain why, on a large scale, the Universe and its microwave background radiation are so uniform. Mathematically, it is possible to overcome this difficulty by assuming that the early Universe went through a brief period of very rapid inflation (胀大), when its volume increased by a factor 1050.9 The fate of the UniverseGravity is slowing the expansion of the Universe. The fate of the Universe depends on how its average density compares with a certain critical density (临界密度): If the expansion reaches a maximum, and is followed by contraction (收缩). If the expansion continues, but the rate falls to zero after infinite time. If the expansion continues indefinitely.The average density of the Universe is thought to be close to the critical density.Linking and H0 The critical density depends on the value of the Hubble constant. A higher H0 means a higher recession velocity per unit separation. So a higher density is needed to stop the expansion. It can be shown that:where G is the gravitational constant. This gives in the range 5-2010-27 kg m-3.专 业 英 语Physics English10 Models of the UniverseThe big bang was not an explosion into existing space. Space itself started to expand. The galaxies are separating because the space between them is increasing. Space has three dimensions of distance (represented by x, y, and z co-ordinates) and one of time. According to Einsteins theory of general relativity, gravity causes a curvature (弯曲) of space -time. If gravity is sufficiently strong, it may produce a “closed” Universe (封闭宇宙), as shown right.Closed Universe( ). The surface is finite and bounded. To visualize the expansion of the Universe, it is simpler to use models with only two of the distance dimensions. Imagine that the Universe is on an expanding, elastic surface. Three possible models are shown below. In each case, the galaxies move apart as the surface stretches. From any position on the surface, each galaxy recedes at a velocity that is proportional to its distance away. Note: It is the value of the critical density, and therefore of the Hubble constant, which decides whether we live in an open, flat, or closed Universe.专 业 英 语Physics EnglishClosed Universe ( ) The surface is finite and bounded.Flat Universe ( ) The surface is infinite and unbounded.Open Universe ( ) The surface is infiniteand unbounded.