Know that the Sun is a star of medium size, consisting mostly of hydrogen and helium, and that it radiates most of its energy in the infrared, visible light and ultraviolet regions of the electromagnetic spectrum

6.2.1 The Sun – A Medium‑Sized Star

1. Basic Physical Properties

  • Diameter: ≈ 1.39 × 10⁶ km (≈ 109 × Earth’s diameter)

  • Mass: ≈ 1.99 × 10³⁰ kg (≈ 330 000 × Earth’s mass)

  • Surface (photospheric) temperature: ≈ 5 800 K

  • Spectral class: G2 V – a yellow‑dwarf, i.e. a medium‑sized main‑sequence star

  • Average luminosity (total power output): ≈ 3.85 × 10²⁶ W

2. Composition (by mass)

ElementMass fraction
Hydrogen (H)≈ 73 %
Helium (He)≈ 25 %
Heavier elements (metals)≈ 2 %

The Sun is therefore almost entirely hydrogen and helium, the raw material for nuclear fusion.

3. Energy Production – Nuclear Fusion in the Core

  • The core (radius ≈ 0.25 R☉) is hot enough (≈ 1.5 × 10⁷ K) for hydrogen nuclei to fuse into helium via the proton‑proton chain.

  • Each complete fusion cycle releases about 4 × 10⁻¹² J of energy.

  • The energy is transported outward:

    • Radiative zone – photons diffuse outward.

    • Convective zone – hot plasma rises, cool plasma sinks.

  • At the surface the energy is emitted as electromagnetic radiation, described by the Stefan‑Boltzmann law:

    \$L = 4\pi R{\odot}^{2}\,\sigma\,T{\text{eff}}^{4}\$

    where σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴.

4. Solar Radiation – Spectrum

The Sun radiates across the whole electromagnetic spectrum, but the bulk of the energy that reaches Earth lies in three wavelength regions:

RegionWavelength rangeTypical exampleApprox. fraction of total solar output
Infrared (IR)700 nm – 1 mmHeat radiation≈ 50 %
Visible light400 nm – 700 nmYellow‑white light (peak ≈ 500 nm)≈ 40 %
Ultraviolet (UV)10 nm – 400 nmUV‑A, UV‑B, UV‑C≈ 10 %

The spectrum peaks in the visible region (around 500 nm), giving the Sun a white colour in space.

5. Practical Uses of Solar Energy

  • Photovoltaic (PV) cells – convert photons (mainly visible and near‑IR) into electricity.

    • Typical school‑level panel efficiency: 10–15 %.

    • Advantages: renewable, silent, no moving parts.

  • Solar‑thermal collectors – absorb IR radiation to heat a fluid (water, air, or oil).

    • Used for domestic hot‑water systems, solar‑powered steam turbines, and solar‑cooking.

    • Thermal conversion efficiencies can reach ≈ 60 %.

  • Both technologies exploit the Sun’s broad spectrum: PV cells are most responsive to visible/near‑IR photons, while solar‑thermal devices make best use of the larger IR component.

6. The Sun, Earth and Moon – Day, Night, Seasons & Phases

  • Rotation: Earth rotates once every 24 h, producing the day‑night cycle.

  • Axial tilt: ≈ 23.5° to the orbital plane.

    • During a year, different hemispheres receive sunlight at higher or lower angles, creating the four seasons.

  • Orbit: Earth travels around the Sun in ≈ 365 days. The Sun therefore appears to move east‑to‑west across the sky each day.

  • Moon’s motion:

    • Sidereal period ≈ 27.3 days (orbit relative to the stars).

    • Synodic period ≈ 29.5 days (time between successive full moons).

    • Changing Sun‑Earth‑Moon geometry produces the familiar lunar phases.

  • Because the Sun is effectively a point source at a distance of 1 AU (≈ 1.5 × 10⁸ km), its rays reach Earth as nearly parallel beams, allowing the simple geometry above to explain day‑night, seasons and lunar phases.

7. Overview of the Solar System (Core Syllabus)

CategoryMembers (in order from the Sun)
Planets (8)Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Dwarf planetsPluto, Eris, Haumea, Makemake, Ceres (asteroid belt)
Major moonsMoon (Earth); Phobos & Deimos (Mars); Io, Europa, Ganymede, Callisto (Jupiter); Titan, Enceladus (Saturn); etc.
Small bodiesAsteroids (main belt & Near‑Earth), comets (Kuiper Belt & Oort Cloud)

All of these objects orbit the Sun and are held in the Solar System by its gravitational field.

8. Why the Sun Appears Yellow from Earth

In space the Sun’s spectrum is essentially white because it contains all visible colours. In Earth’s atmosphere, shorter‑wavelength blue light is scattered more strongly by air molecules (Rayleigh scattering). The direct sunlight that reaches our eyes is therefore depleted in blue and looks yellow.

9. Suggested Diagram (for classroom use)

Cross‑section of the Sun (core → radiative zone → convective zone → photosphere) together with a simple bar‑chart showing the approximate IR, visible and UV fractions of the solar radiation.

10. Quick‑Check of Cambridge IGCSE 0625 Core Syllabus Coverage

Syllabus pointCovered?Notes
6.1.1 The Earth – rotation, tilt, day‑night, seasons✓Rotational period, axial tilt, seasonal explanation.
6.1.2 The Earth – orbit, lunar month, phases✓Orbital period, sidereal & synodic lunar months, phase geometry.
6.1.3 The Solar System – planets, dwarf planets, moons, asteroids, comets✓Table lists all major families in correct order.
6.2.1 The Sun – size, type, composition, energy production, spectrum✓All required facts, fusion description, spectral breakdown.
6.2.2 Solar energy – photovoltaic and solar‑thermal uses✓Efficiency, examples, link to spectrum.

11. Summary Checklist (for rapid revision)

  1. The Sun is a medium‑sized G2 V (yellow‑dwarf) star.

  2. Its mass is ≈ 73 % hydrogen, ≈ 25 % helium, ≈ 2 % heavier elements.

  3. Energy is generated by hydrogen‑fusion in the core (proton‑proton chain).

  4. Solar radiation reaching Earth is roughly 50 % IR, 40 % visible, 10 % UV.

  5. Solar energy can be harvested as electricity (photovoltaic cells) or heat (solar‑thermal collectors).

  6. Earth’s rotation (24 h) gives day‑night; axial tilt (23.5°) produces seasons; lunar orbital periods give phases.

  7. The Solar System contains 8 planets, several dwarf planets, many moons, asteroids and comets, all orbiting the Sun.

  8. Atmospheric Rayleigh scattering removes blue light, making the Sun appear yellow to us.