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 as a star

What is the Sun?

Think of the Sun as a giant, glowing ball of fire that sits at the centre of our solar system. It is a star of medium size – not the biggest, but not the smallest either. Its diameter is about 109 times that of Earth, and it holds 99.86 % of the total mass of the solar system. 🌞

Composition of the Sun

The Sun is mainly made of two elements:

  • Hydrogen – about 74 % of the Sun’s mass.

  • Helium – about 24 % of the Sun’s mass.

The remaining 2 % is a mix of heavier elements (often called “metals” in astronomy). These heavy elements are crucial for the Sun’s nuclear reactions.

ElementMass Fraction
Hydrogen (H)74 %
Helium (He)24 %
Metals (Z)2 %

Energy output and spectrum

The Sun’s energy comes from nuclear fusion in its core, where hydrogen nuclei combine to form helium. This process releases a huge amount of energy, which travels outward as electromagnetic radiation.

The Sun emits most of its energy in three parts of the electromagnetic spectrum:

  1. Infrared (IR) – about 40 % of the total energy. Think of it as the Sun’s warm hug.

  2. Visible light – about 40 % as well. This is the light that lets us see the world.

  3. Ultraviolet (UV) – about 10 %. These are the invisible rays that can cause sunburn.

The remaining 10 % is split between X‑rays and radio waves, which are much less intense but still important for space weather studies.

Analogy: Imagine the Sun as a giant campfire. The infrared is the heat you feel, the visible light is the bright glow, and the ultraviolet is the invisible rays that can scorch your skin if you stay too close.

Exam Tips

Remember:

  • Use the percentages to answer questions about the Sun’s composition.

  • When asked about the Sun’s energy output, mention the three main spectral regions and their approximate percentages.

  • Use the analogy of a campfire or a glowing ball to explain why the Sun emits infrared, visible, and ultraviolet light.

  • Show that the Sun is a medium‑sized star by comparing its diameter to Earth’s and its mass to the solar system.

Good luck, and keep your curiosity shining bright! ✨