Stars and the Universe

2026 Syllabus Objectives

By the end of this topic, you should be able to:

The Sun as a Star:

  • Understand that the Sun is a medium-sized star made mostly of hydrogen and helium
  • Know that the Sun radiates energy mainly as infrared, visible light, and ultraviolet radiation
  • (Extended only) Understand that stars produce energy through nuclear fusion, where hydrogen nuclei join together to form helium

Stars:

  • Know that galaxies contain billions of stars
  • Understand that the Sun is a star in the Milky Way galaxy
  • Know that other stars in the Milky Way are much farther from Earth than the Sun is
  • Understand that astronomical distances are measured in light-years
  • (Extended only) Know that one light-year equals 9.5 × 10¹⁵ m
  • (Extended only) Describe the complete life cycle of stars, from formation to their final stages

The Universe:

  • Know that the Milky Way is one of billions of galaxies in the Universe
  • Understand that the Milky Way's diameter is approximately 100,000 light-years
  • Describe redshift and understand why it occurs
  • Know that redshift provides evidence for the expanding Universe and the Big Bang Theory
  • (Extended only) Understand cosmic microwave background radiation (CMBR) and what it tells us about the Universe
  • (Extended only) Use Hubble's Law to calculate recession speeds and distances of galaxies
  • (Extended only) Know that the Hubble constant H₀ = 2.2 × 10⁻¹⁸ per second
  • (Extended only) Understand how the age of the Universe can be estimated

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The Sun as a Star

What is the Sun?

The Sun is a star – a giant ball of glowing gas that produces its own light and heat. It's not the biggest or smallest star in the Universe; it's actually a medium-sized star.

What is the Sun made of?

The Sun consists mostly of two elements:

  • Hydrogen (about 70% of the Sun's mass)
  • Helium (about 28% of the Sun's mass)

What type of energy does the Sun emit?

The Sun radiates (gives off) energy in different forms. Most of this energy is in three regions of the electromagnetic spectrum:

  • Infrared radiation (heat we can feel)
  • Visible light (light we can see)
  • Ultraviolet radiation (UV rays that can cause sunburn)

How does the Sun produce energy? (Extended only)

The Sun produces energy through a process called nuclear fusion. Here's what happens:

  1. In the Sun's core, the temperature is incredibly hot – around 15 million degrees Celsius
  2. At these extreme temperatures, hydrogen nuclei (the centers of hydrogen atoms) smash into each other with enormous force
  3. When they collide, they join together (fuse) to form helium nuclei
  4. This fusion reaction releases a huge amount of energy in the form of heat and light

A simple fusion reaction can be written as:

²₁H + ³₁H → ⁴₂He + ¹₀n + energy

Where:

  • ²₁H (deuterium) and ³₁H (tritium) are both types (isotopes) of hydrogen
  • ⁴₂He is helium
  • ¹₀n is a neutron
  • Energy is released in the reaction

Important point: Hydrogen is the fuel that powers stars. As long as a star has hydrogen to fuse, it can keep shining.

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