6.2.1 The Sun as a star

Learning Objectives

Core

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

Supplement

2. 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

Contents to learn

The Sun

• The sun lies at the center of the solar system.
• It is the most massive object in the solar system, consisting of over 99% of its total mass.
• All other celestial bodies in the solar system, including planets, moons, asteroids, comets, and dwarf planets, orbit around the Sun due to its strong gravitational pull.
• Our sun is a medium-sized star composed primarily of Hydrogen(H) and Helium(He).
• It radiates energy in the infrared (IR), visible light and Ultraviolet(UV) region of the electromagnetic spectrum.
• Stars exist in a wide range of sizes and colours from yellow stars like our sun to red dwarfs, and from blue giants to red super giants depending on their colours.
• A star’s colour is correlated to its surface temperature.
•                   Blue stars are the hottest with temperatures around 30,000 K.
•                   Red stars are the coolest with temperatures around 3,000 K.

Nuclear Fusion Reaction in a Star

• In the centre of a stable star where immense pressure and temperature exist, hydrogen nuclei fuse to form helium.
• The nuclear fusion reaction is represented by the equation.

• Deuterium and tritium, both isotopes of hydrogen, can be fused to form a helium nucleus and a neutron.
• A huge amount of energy is released in this reaction.
• This energy exerts a pressure that prevents the star from collapsing under its own gravity.