📚 8.01 The Universe and Solar System | 8.01 宇宙与太阳系
In this revision guide, we will explore the key concepts of the universe and our solar system as required by the Edexcel IGCSE Science syllabus. You will learn about the structure of the solar system, the forces that keep objects in orbit, the life cycles of stars, and the evidence for the Big Bang.
本复习指南将涵盖 Edexcel IGCSE 科学大纲中关于宇宙和太阳系的重要概念。你将学习太阳系的结构、使天体保持轨道的力、恒星的演化过程，以及支持大爆炸理论的证据。
1. Structure of the Solar System | 太阳系的结构
The Solar System consists of the Sun at its centre, eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), dwarf planets, moons, asteroids and comets. The Sun is a star made mostly of hydrogen and helium.
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、卫星、小行星和彗星。太阳是一颗主要由氢和氦组成的恒星。
- The four inner planets are small and rocky: Mercury, Venus, Earth and Mars.
- 四颗内行星是小型岩质行星：水星、金星、地球和火星。
- The four outer planets are large and mostly gaseous: Jupiter, Saturn, Uranus and Neptune.
- 四颗外行星是大型气态行星：木星、土星、天王星和海王星。
Between Mars and Jupiter lies the asteroid belt, and beyond Neptune is the Kuiper Belt.
火星和木星之间有小行星带，海王星之外是柯伊伯带。
2. The Sun and Other Stars | 太阳与其他恒星
Stars are enormous spheres of hot gas that emit light and heat. The Sun is the closest star to Earth and provides the energy that supports life. Other stars appear as points of light because they are extremely far away.
恒星是发出光和热的巨大炽热气态球体。太阳离地球最近，为生命提供能量。其他恒星看起来只是光点，是因为它们距离极远。
- Blue-white stars are hotter than red stars.
- 蓝白色恒星比红色恒星温度更高。
- The Sun is a medium-sized, yellow main sequence star.
- 太阳是一颗中等大小的黄色主序星。
3. Gravity and Orbits | 引力与轨道
Gravity is the force of attraction between any two objects with mass. The gravitational force of the Sun pulls the planets towards it, while the planets’ forward motion keeps them in orbit. A stable orbit is maintained when the gravitational pull provides the correct inward force for the object’s speed.
引力是两个有质量的物体之间的吸引力。太阳的引力把行星拉向它，而行星的前进运动使其保持在轨道上。当引力为物体的速度提供合适的向心力时，物体就能维持稳定轨道。
orbital speed = circumference / time period
轨道速度 = 轨道周长 / 周期
Planets closer to the Sun travel faster in their orbits than planets further away.
离太阳更近的行星比离太阳更远的行星运行速度更快。
4. The Earth’s Orbit and Seasons | 地球公转与四季
The Earth takes about 365.25 days to complete one orbit around the Sun. The Earth’s axis is tilted at about 23.5 degrees, and this tilt causes the seasons as the Earth travels around the Sun.
地球绕太阳公转一周约需 365.25 天。地轴倾斜约 23.5 度，这种倾斜使地球在绕太阳运行的过程中产生四季变化。
- When the Northern Hemisphere tilts towards the Sun, the north experiences summer and longer days.
- 当北半球倾向太阳时，北半球处于夏季且白昼较长。
- When the Northern Hemisphere tilts away from the Sun, the north experiences winter and shorter days.
- 当北半球背离太阳时，北半球处于冬季且白昼较短。
5. The Moon and Tides | 月球与潮汐
The Moon is the Earth’s natural satellite. It takes about 27.3 days to orbit the Earth. The gravitational pull of the Moon, and to a smaller extent the Sun, causes the ocean tides on Earth.
月球是地球的天然卫星，绕地球一周约需 27.3 天。月球和太阳（影响较小）的引力引起了地球上的海洋潮汐。
When the Sun, Earth and Moon are aligned, spring tides occur with a larger tidal range. When the Sun and Moon are at right angles with respect to Earth, neap tides occur with a smaller tidal range.
当太阳、地球和月球排成一线时，出现大潮，潮差较大；当太阳和月球相对于地球夹角成直角时，出现小潮，潮差较小。
6. Life Cycle of a Star | 恒星的演化
Stars form from giant clouds of dust and gas called nebulae. Gravity pulls the material together until the temperature and pressure in the core are high enough for nuclear fusion to begin. Fusion releases enormous amounts of energy and makes the star shine.
恒星由巨大的尘埃和气体云（星云）形成。引力使物质聚集，直到核心温度和压力足够高，核聚变开始发生。核聚变释放巨大能量并使恒星发光。
Nebula → Main sequence star → Red giant → White dwarf → Black dwarf
星云 → 主序星 → 红巨星 → 白矮星 → 黑矮星
For much more massive stars, the sequence becomes: nebula → main sequence → red supergiant → supernova → neutron star or black hole.
对于质量更大的恒星，演化过程为：星云 → 主序星 → 红超巨星 → 超新星 → 中子星或黑洞。
7. The Expanding Universe and Red Shift | 膨胀的宇宙与红移
Astronomers observe that light from distant galaxies is shifted towards longer wavelengths. This is called red shift. It suggests that galaxies are moving away from us and that the universe is expanding.
天文学家观察到来自遥远星系的光向长波方向移动，这称为红移。这表明星系正在离我们而去，宇宙正在膨胀。
- The further away a galaxy is, the larger its red shift.
- 星系离我们越远，它的红移越大。
- The larger the red shift, the faster the galaxy is moving away.
- 红移越大，星系远离的速度就越快。
8. Cosmic Microwave Background | 宇宙微波背景辐射
The cosmic microwave background (CMB) is a faint radiation that fills the entire sky. It is the afterglow of the hot, early universe and is observed at microwave wavelengths. Its existence is a major piece of evidence for the Big Bang theory.
宇宙微波背景（CMB）是充满整个天空的微弱辐射，是早期炽热宇宙的余辉，在微波波段被观测到。它的存在是大爆炸理论的重要证据。
CMB temperature ≈ 2.7 K (-270.45 °C)
CMB 温度 ≈ 2.7 K (-270.45 °C)
9. The Big Bang Theory | 大爆炸理论
The Big Bang theory states that the universe began from an extremely hot and dense point about 13.8 billion years ago and has been expanding ever since. The two main evidence are the red shift of distant galaxies and the cosmic microwave background.
大爆炸理论认为宇宙起源于约 138 亿年前一个极热、极密的点，并一直在膨胀。两个主要证据是遥远星系的红移和宇宙微波背景辐射。
The theory also explains the abundance of hydrogen and helium in the universe, which matches observations.
该理论还能解释宇宙中氢和氦的丰度，与观测相符。
10. Units of Distance in Space | 天文距离单位
Distances in space are enormous, so astronomers use the light-year as a convenient unit. One light-year is the distance that light travels in one year.
太空中的距离非常巨大，因此天文学家使用光年作为方便的单位。一光年是光在一年内传播的距离。
1 light-year = 9.46 × 10¹² km
1 光年 = 9.46 × 10¹² 千米
The nearest star to Earth after the Sun, Proxima Centauri, is about 4.24 light-years away.
除太阳外离地球最近的恒星比邻星距离我们约 4.24 光年。
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