IGCSE OCR Physics: Astrophysics Key Concepts | IGCSE OCR 物理：天体物理考点精讲

📚 IGCSE OCR Physics: Astrophysics Key Concepts | IGCSE OCR 物理：天体物理考点精讲

Astrophysics uses physical laws to explain how stars, planets, and galaxies form, evolve, and interact. In the OCR IGCSE Physics specification, this topic covers the Solar System, stellar life cycles, redshift, the Big Bang, and the observational evidence that supports our current understanding of the Universe.

天体物理学运用物理定律来解释恒星、行星和星系的形成、演化与相互作用。在 OCR IGCSE 物理考纲中，本专题涵盖太阳系、恒星生命周期、红移、大爆炸以及支持我们当前对宇宙理解的观测证据。

1. Structure of the Solar System | 太阳系的结构

The Solar System consists of the Sun, eight planets, dwarf planets, moons, asteroids, and comets. The four inner planets (Mercury, Venus, Earth, Mars) are rocky and relatively small, while the four outer planets (Jupiter, Saturn, Uranus, Neptune) are gas giants, much larger and composed mainly of hydrogen and helium. Between Mars and Jupiter lies the asteroid belt, a region of rocky debris. All planets orbit the Sun in nearly circular elliptical paths, in the same direction, and roughly in the same plane (the ecliptic).

太阳系由太阳、八颗行星、矮行星、卫星、小行星和彗星组成。四颗内行星（水星、金星、地球、火星）为岩质、相对较小，而四颗外行星（木星、土星、天王星、海王星）为气态巨行星，体积更大，主要由氢和氦构成。在火星与木星之间是小行星带，由岩屑组成。所有行星都在同一方向上以近似圆形的椭圆轨道绕太阳运行，且大体位于同一平面（黄道面）上。

2. Orbital Motion and Gravity | 轨道运动与引力

Gravity provides the centripetal force that keeps planets in orbit around the Sun, and moons around planets. For a body moving in a circular orbit, the gravitational force equals the centripetal force: GMm/r² = mv²/r. This simplifies to v² = GM/r, meaning the orbital speed v is inversely proportional to the square root of the orbital radius (v ∝ 1/√r). Therefore, planets closer to the Sun move faster. The orbital period T can be found from v = 2πr/T, leading to T² ∝ r³ (Kepler’s third law).

引力提供了使行星绕太阳运行、卫星绕行星运行的向心力。对于沿圆轨道运动的天体，万有引力等于向心力：GMm/r

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