Chapter 21 — Chapter 21 — Space and Environment

The Sun is a medium-sized star of mainly hydrogen (~73%) and helium (~25%). In its core (~15 million K, enormous pressure) nuclear fusion converts hydrogen into helium (proton–proton chain: 4 ¹H → ⁴He + 2e⁺ + neutrinos + energy); the mass defect (~0.7%) becomes energy by E = mc², which travels outward by radiation and convection and leaves as light and heat. Planets are compared by distance from Sun, diameter/mass, density, surface temperature, gravity, orbital and rotation periods, atmosphere, number of moons. Inner rocky planets (Mercury, Venus, Earth, Mars) are dense; gas giants (Jupiter, Saturn) and ice giants (Uranus, Neptune) are large but low density (mostly H, He; small rocky core). Surface temperature depends mainly on orbital distance (inverse-square law of sunlight intensity) and also on atmosphere (Venus hotter than Mercury due to thick CO₂ greenhouse).