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Chapter 9 — Chapter 9 — Space and Environment
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📘Easy summary
Space refers to a region beyond Earth's atmosphere in which planets, stars and galaxies exist and interact through gravity and electromagnetic radiation. Luminosity is the total amount of energy emitted by a star per unit time in the form of electromagnetic radiations; symbol L, unit watt. It depends primarily on the star's radius and surface temperature: of two stars of equal size, the hotter one is more luminous. Stefan–Boltzmann law: the luminosity of a star is proportional to the product of its surface area and the fourth power of its surface temperature: L = 4πr²σT⁴, σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴. It helps estimate the size and brightness of stars from their temperature.
⭐Key points
- L = total power radiated (W).
- L = 4πr²σT⁴.
- L ∝ r² and ∝ T⁴.
🔬Diagram
🌿Real-life example
r = 7.0 × 10⁸ m, T = 6000 K: L = 4π(7 × 10⁸)² × 5.67 × 10⁻⁸ × 6000⁴ ≈ 4.5 × 10²⁶ W.
💡Exam tip
- Doubling T multiplies L by 16.
✅Quick check
1. Luminosity is:
2. Stefan–Boltzmann law:
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