The Sun as a Star (Cambridge International IGCSE Physics)

Spec point — 6.2.1 · 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 and ultraviolet regions of the electromagnetic spectrum.

The Sun as a star

• The Sun is a medium-sized star which lies at the centre of the Solar System
• It consists mostly of the two elements hydrogen and helium
• It radiates most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum

The Sun is a medium-sized star consisting of mostly hydrogen and helium

Nuclear fusion in stars

Spec point — 6.2.1 · Know that stars are powered by nuclear reactions that release energy and that in stable stars the nuclear reactions involve the fusion of hydrogen into helium

Extended Tier Only

• Stars are huge balls of (mostly) hydrogen gas
• In the centre of a star, hydrogen nuclei undergo nuclear fusion to form helium nuclei
• An equation for a possible fusion reaction is:

H _ 1 ^ 2 + H _ 1 ^ 3 -> He _ 2 ^ 4 + n _ 0 ^ 1

• Where ${}_{1}^{2}\mathrm{H}$ (deuterium) and H _ 1 ^ 3 (tritium) are both isotopes of hydrogen

  • These are formed through other fusion reactions in the star

• A huge amount of energy is released in the reaction

• All stable stars are powered by nuclear fusion reactions

Exam Hint It is useful to remember that hydrogen is the fuel within stars, but the details of the reaction between deuterium and tritium is not required at this level.