📚 Astrophysics | 天体物理学
Astrophysics is a fascinating branch of physics that studies the universe beyond Earth, including stars, galaxies, planets and the large-scale structure of space itself. In the Edexcel IGCSE Physics syllabus, this topic brings together ideas about gravity, energy and radiation to explain how celestial objects form, live and die.
天体物理学是物理学中令人着迷的分支，它研究地球之外的宇宙，包括恒星、星系、行星以及空间本身的大尺度结构。在 Edexcel IGCSE 物理考纲中，这一主题将重力、能量和辐射等概念结合起来，用于解释天体如何形成、演化和消亡。
1. The Solar System | 太阳系
The Solar System consists of the Sun at its centre, eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), dwarf planets such as Pluto, and many smaller objects. The Sun is a star that provides light and heat to the planets through nuclear fusion.
太阳系以太阳为中心，包括八大行星（水星、金星、地球、火星、木星、土星、天王星和海王星）、矮行星（如冥王星），以及许多更小的天体。太阳是一颗恒星，通过核聚变为行星提供光和热。
Between Mars and Jupiter lies the main asteroid belt, a region filled with rocky fragments. Comets are icy bodies that develop glowing tails when they approach the Sun. Meteoroids are smaller rocky or metallic chunks that may enter Earth’s atmosphere as meteors, or “shooting stars”.
在火星和木星之间是主小行星带，那里充满了岩石碎片。彗星是冰质天体，在靠近太阳时会形成发光的彗尾。流星体是更小的岩石或金属碎块，进入地球大气层时便成为我们俗称的”流星”。
Astronomers use the astronomical unit (AU) for distances within the Solar System, where 1 AU is the average Earth–Sun distance. For distances to stars and galaxies, they use the light-year, the distance light travels in one year.
天文学家在天文学中使用天文单位（AU）来衡量太阳系内的距离，1 AU 约为地球到太阳的平均距离。对于恒星和星系之间的距离，则使用光年，即光在一年内传播的距离。
2. Gravity and Orbital Motion | 引力与轨道运动
Every object with mass attracts every other object with a force called gravity. For a planet moving around the Sun, the gravitational pull of the Sun provides the centripetal force needed to keep the planet moving in a circular path. Without this force, the planet would travel in a straight line into space.
任何有质量的物体都会通过一种称为引力的力吸引其他物体。对于绕太阳运行的行星，太阳的引力提供了使行星沿圆形轨道运动所需的向心力。如果没有这个力，行星就会沿直线飞入太空。
For a stable circular orbit, the gravitational force must exactly equal the required centripetal force. This relationship is written as:
对于稳定的圆形轨道，引力必须恰好等于所需的向心力。这一关系可以写成：
G × M × m / r² = m × v² / r
Here, M is the mass of the central body, m is the mass of the orbiting body, r is the orbital radius and v is the orbital speed. Rearranging gives the orbital speed:
其中 M 是中心天体的质量，m 是轨道上天体的质量，r 是轨道半径，v 是轨道速度。整理后可以得到轨道速度：
v = √(G × M / r)
This shows that satellites closer to a planet move faster, and that the orbital speed does not depend on the mass of the satellite itself. Artificial satellites and the Moon follow exactly this same principle.
由此可见，离行星越近的卫星运动得越快，而且轨道速度与卫星自身的质量无关。人造卫星和月球遵循的正是同样的原理。
3. Star Formation and the Main Sequence | 恒星的诞生与主序星阶段
Stars are born inside vast clouds of gas and dust called nebulae. A force such as a nearby supernova explosion can disturb the cloud, causing part of it to contract under gravity. The shrinking region becomes warmer and denser, forming a protostar.
恒星诞生于巨大而稀薄的气体和尘埃云中，称为星云。附近的超新星爆发等力量可以扰动星云，使其一部分在引力作用下收缩。收缩的区域越来越热、越来越密，形成原恒星。
When the core temperature reaches about 10 million kelvin, hydrogen nuclei fuse to form helium nuclei, releasing enormous energy. The outward radiation pressure now balances the inward gravitational pull, and the star reaches a stable state called the main sequence. Our Sun is a typical main-sequence star and has been stable for about 4.6 billion years.
当核心温度达到约一千万开尔文时，氢原子核聚变为氦原子核，释放出巨大能量。向外的辐射压与向内的引力达到平衡，恒星进入称为主序星的稳定状态。太阳就是一颗典型的主序星，已经稳定存在了约46亿年。
During the main-sequence phase, the star is in a perfect balance called hydrostatic equilibrium. The energy produced by fusion at the core keeps the star from collapsing, while gravity keeps the star from blowing apart.
在主序星阶段，恒星处于一种称为流体静力平衡的完美平衡状态。核心核聚变产生的能量阻止恒星坍缩，而引力则阻止恒星彻底散开。
4. Life Cycle of Low-Mass Stars | 低质量恒星的生命周期
Stars with a mass similar to or less than that of the Sun eventually exhaust the hydrogen in their cores. The core contracts and heats up, causing the outer layers to expand enormously. The star becomes a red giant, large and cool in colour but very bright.
质量与太阳相近或更小的恒星最终会耗尽核心中的氢。核心收缩并升温，导致外层急剧膨胀。恒星变成红巨星，体积庞大、表面温度较低，但光度很高。
In the red giant phase, helium nuclei fuse to form heavier elements such as carbon and oxygen. The outer layers are then ejected into space as a glowing shell of gas called a planetary nebula, despite having nothing to do with planets. What remains is the hot, dense core called a white dwarf.
在红巨星阶段，氦核聚变形成碳、氧等更重的元素。随后外层气体被抛入太空，形成发光的壳层，称为行星状星云（虽然它和行星毫无关系）。剩余下来的炽热而致密的核心称为白矮星。
A white dwarf is roughly the size of the Earth but contains the mass of a star. Over billions of years it will slowly cool and fade, eventually becoming a black dwarf. Low-mass stars like the Sun do not explode as supernovae.
白矮星的体积与地球相仿，却拥有接近恒星的质量。经过数百亿年，它会逐渐冷却和暗淡，最终成为黑矮星。像太阳这样的低质量恒星不会以超新星爆发的方式结束生命。
The whole sequence for a low-mass star is: nebula → protostar → main sequence star → red giant → planetary nebula → white dwarf.
低质量恒星完整的一生为：星云 → 原恒星 → 主序星 → 红巨星 → 行星状星云 → 白矮星。
5. Life Cycle of High-Mass Stars | 大质量恒星的生命周期
High-mass stars, those many times heavier than the Sun, follow a very different path. After leaving the main sequence, they become red supergiants, which are even larger and hotter than red giants. In these stars, fusion continues to create elements as heavy as iron.
大质量恒星的质量是太阳的很多倍，它们的演化路径完全不同。离开主序星阶段后，它们会变成红超巨星，比红巨星更大、更热。在这些恒星内部，核聚变可以一直进行到生成铁等较重元素。
Iron is the most stable nucleus, and fusing iron does not release energy; instead it absorbs energy. When the core forms iron, fusion can no longer support the star against gravity. The core collapses in less than a second, and the outer layers are blown violently outward in a supernova explosion.
铁是最稳定的原子核，聚变铁不会释放能量，反而会吸收能量。当核心形成铁时，核聚变无法再支撑恒星抵抗引力。核心在不到一秒内坍缩，外层物质以超新星爆发的形式被猛烈抛射出去。
A supernova can briefly outshine an entire galaxy and releases elements heavier than iron, such as gold and uranium, into space. These elements become part of future star systems and planets. What remains after the explosion depends on the mass of the original core.
超新星爆发的亮度可以在短时间内盖过整个星系，并将比铁更重的元素（比如金和铀）抛洒到太空中。这些元素会成为未来恒星和行星的一部分。爆发后留下的天体取决于核心原本的质量。
The life cycle of a high-mass star is: nebula → protostar → main sequence star → red supergiant → supernova → neutron star or black hole.
大质量恒星的一生为：星云 → 原恒星 → 主序星 → 红超巨星 → 超新星 → 中子星或黑洞。
6. Stellar Remnants: White Dwarfs, Neutron Stars, Black Holes | 恒星遗迹：白矮星、中子星与黑洞
The end state of a star depends on the mass of its leftover core after shedding or exploding. For low-mass stars, a white dwarf forms. Its density is extraordinarily high: a teaspoon of white dwarf material would weigh several tonnes on Earth.
恒星的最终状态取决于其残核在失去外层或爆发之后的质量。对于低质量恒星，会形成白矮星。它的密度高得惊人：一茶匙白矮星物质在地球上可能重达数吨。
If the core left behind after a supernova has a mass between about 1.4 and 3 solar masses, it collapses into a neutron star. Neutron stars are made almost entirely of neutrons and are even denser than white dwarfs. Some neutron stars, called pulsars, emit regular beams of radio waves as they rotate rapidly.
如果超新星爆发后留下的核心质量介于约1.4到3个太阳质量之间，它会坍缩成中子星。中子星几乎完全由中子组成，密度甚至超过白矮星。有些中子星称为脉冲星，在快速旋转时发出有规律的射电波束。
If the core mass exceeds about 3 solar masses, nothing can stop the collapse. The object becomes a black hole, where gravity is so strong that not even light can escape. We can detect black holes only by their effect on nearby matter, such as accretion disks of hot gas or the motion of companion stars.
如果核心质量超过约3个太阳质量，任何力量都无法阻止坍缩。天体变成黑洞，其引力之强连光都无法逃脱。我们只能通过黑洞对周围物质的影响来探测它们，例如炽热的气体吸积盘或伴星的运动。
All three remnants are extremely dense and compact. The different outcomes reveal how gravity and nuclear physics work together at the most extreme scales in the universe.
这三种遗迹都极其致密而紧凑。它们的不同命运揭示了引力和核物理在宇宙最极端尺度上如何共同作用。
7. The Doppler Effect | 多普勒效应
When a source of waves moves towards or away from an observer, the observed wavelength changes. If the source approaches, the waves are compressed, so the wavelength becomes shorter and the frequency higher. If the source moves away, the waves are stretched, so the wavelength becomes longer and the frequency lower. This is called the Doppler effect.
当波源朝向或远离观察者运动时，观察者接收到的波长会发生变化。如果波源靠近，波被压缩，波长变短、频率升高；如果波源远离，波被拉伸，波长变长、频率降低。这就是多普勒效应。
The same effect applies to light. When a star or galaxy moves away from us, its spectral lines shift towards the red end of the spectrum, known as redshift. When an object moves towards us, its spectral lines shift towards the blue end, called blueshift.
这个效应同样适用于光。当恒星或星系远离我们时，它的谱线向光谱红端移动，称为红移；当物体靠近我们时，谱线向蓝端移动，称为蓝移。
By examining the absorption lines in a star’s spectrum, astronomers can measure how fast the star is moving along our line of sight. This technique is widely used to study binary stars, rotating galaxies and the expansion of the universe.
通过分析恒星光谱中的吸收线，天文学家可以测量恒星沿我们视线方向运动的速度。这一技术广泛用于研究双星、旋转星系以及宇宙的膨胀。
8. Redshift and the Expanding Universe | 红移与膨胀的宇宙
In the 1920s, Edwin Hubble observed that almost all distant galaxies show a redshift in their spectra. This means that they are moving away from us. He also discovered that the farther away a galaxy is, the faster it is receding. This relationship is known as Hubble’s law.
20世纪20年代，埃德温·哈勃观察到几乎所有遥远星系的谱线都发生红移，这意味着它们正在远离我们。他还发现，星系距离我们越远，退行速度越快。这个关系被称为哈勃定律。
Hubble’s law can be written as:
哈勃定律可以写成：
v = H₀ × d
Here, v is the recession velocity, d is the distance to the galaxy and H₀ is the Hubble constant. This linear relation is exactly what we would expect if the whole universe is expanding uniformly.
其中 v 是退行速度，d 是到星系的距离，H₀ 是哈勃常数。这种线性关系正是我们预期整个宇宙均匀膨胀时看到的结果。
An expanding universe implies that in the past all galaxies were closer together. If you run the expansion backwards, the entire universe was once concentrated at a single point. This idea lies at the heart of the Big Bang theory.
膨胀的宇宙意味着在过去所有星系都靠得更近。如果把膨胀倒推回去，整个宇宙曾集中在一个点上。这个想法正是大爆炸理论的核心。
It is important to note that galaxies do not all move through space from us; rather, the space between galaxies stretches, carrying the galaxies apart. Redshift from expanding space differs slightly from a simple Doppler shift caused by motion through a fixed space.
需要特别指出的是，星系并非都在空间中远离我们，而是星系之间的空间本身在膨胀，将星系彼此拉开。空间膨胀引起的红移与物体在静止空间中运动产生的简单多普勒红移略有不同。
9. Cosmic Microwave Background Radiation | 宇宙微波背景辐射
If the universe began in a hot, dense state, it should have released huge amounts of radiation. As the universe expanded and cooled, that ancient radiation would now be stretched into the microwave part of the electromagnetic spectrum. This faint glow is called the cosmic microwave background (CMB).
如果宇宙起源于一个炽热而致密的状态，它应当释放出大量辐射。随着宇宙膨胀和冷却，这种古老辐射现在应当被拉伸到电磁波谱的微波部分。这种微弱的背景光辉被称为宇宙微波背景辐射（CMB）。
The CMB was accidentally discovered in 1965 by Penzias and Wilson as a uniform microwave signal coming from all directions in the sky. It has a nearly perfect black-body spectrum with a temperature of about 2.7 K, just a few degrees above absolute zero.
宇宙微波背景辐射于1965年被彭齐亚斯和威尔逊意外发现，它表现为从天空四面八方传来的均匀微波信号。它具有接近完美的黑体辐射谱，温度约为2.7 K，仅比绝对零度高几度。
The existence of the CMB was predicted in advance by the Big Bang theory, and its discovery provides extremely strong evidence for that theory. A competing theory called the Steady State model could not easily explain why such radiation should exist everywhere.
宇宙微波背景辐射的出现是大爆炸理论事先预言过的，因此它的发现为该理论提供了极强力的证据。与之竞争的稳恒态模型很难解释为什么这样的辐射会普遍存在。
Tiny fluctuations in the CMB temperature reveal the seeds of galaxy formation. These small variations in density, measured by satellites such as COBE, WMAP and Planck, match the predictions of the Big Bang model beautifully.
CMB温度中的微小涨落揭示了星系形成的种子。这些由COBE、WMAP和普朗克等卫星测量到的微小密度差异与大爆炸模型的预测高度吻合。
10. The Big Bang Theory | 大爆炸理论
The Big Bang theory is the leading scientific model for the origin of the universe. It states that the universe began about 13.8 billion years ago from an extremely hot and dense singularity, and has been expanding and cooling ever since.
大爆炸理论是关于宇宙起源的主流科学模型。它认为宇宙大约在138亿年前从一个极热、极端致密的奇点开始，此后一直在膨胀和冷却。
During the first few minutes after the Big Bang, conditions were so extreme that protons and neutrons formed hydrogen and helium nuclei. This process, called primordial nucleosynthesis, explains why the universe today is roughly 75% hydrogen and 25% helium by mass.
在大爆炸后的最初几分钟内，条件极其极端，质子和中子形成了氢和氦的原子核。这一过程称为原初核合成，解释了今天宇宙中按质量约有75%的氢和25%的氦。
After about 380,000 years, the universe had cooled enough for electrons to combine with nuclei, forming neutral atoms. Light could finally travel freely, and that ancient light is the CMB we detect today.
大约38万年后，宇宙冷却到电子可以与原子核结合形成中性原子的程度。光终于可以自由传播，那种古老的光就是我们今天探测到的CMB。
The main evidence for the Big Bang theory comes from three sources: the observed expansion of the universe, the cosmic microwave background radiation, and the measured abundances of light elements. Together these create a coherent picture of a universe that began in a fiery explosion and has evolved into the galaxies, stars and planets we see around us.
大爆炸理论的主要证据来自三方面：观测到的宇宙膨胀、宇宙微波背景辐射、以及轻元素丰度的测量结果。它们共同描绘出一幅连贯的图景：宇宙始于一场炽热的爆发，逐渐演化成我们今天看到的星系、恒星和行星。
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