The Sun as a Star (Cambridge (CIE) O Level Physics): Revision Note

The Milky Way

• Galaxies are made up of billions of stars

• The Universe is made up of many different galaxies

• The Sun is one of the billions of stars in a galaxy called the Milky Way

• Other stars in the Milky Way galaxy are much further away from Earth than the Sun is

• Some of these stars also have planets which orbit them

Our Place in Space

Our solar system is just one out of potentially billions in our galactic neighbourhood, the Milky Way. There are estimated to be more than 100 billion galaxies in the entire universe

The Sun

• The Sun lies at the centre of the Solar System

  • The Sun is a star which makes up over 99% of the mass of the solar system

  • The fact that most of the mass of the Solar System is concentrated in the Sun is the reason the smaller planets orbit the Sun

• The planets are kept in orbit due to the gravitational pull of the Sun

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

• It radiates most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum

The Sun

Our Sun (Image courtesy of NASA)

• Stars come in a wide range of sizes and colours, from yellow stars to red dwarfs, from blue giants to red supergiants

  • These can be classified according to their colour

• Warm objects emit infrared and extremely hot objects emit visible light as well

  • Therefore, the colour they emit depends on how hot they are

• A star's colour is related to its surface temperature

  • A red star is the coolest (at around 3000 K)

  • A blue star is the hottest (at around 30 000 K)

Temperature & Colour of Stars

The colour of a star correlates to its temperature

Nuclear Fusion in Stars

• In the centre of a stable star, hydrogen atoms undergo nuclear fusion to form helium

• The equation for the reaction is shown here:

• The equation shows  (deuterium) and (tritium) which are both isotopes of hydrogen

  • They can be formed through other fusion reactions in the star

• A huge amount of energy is released in the reaction

• This provides a pressure that prevents the star from collapsing under its gravity

Nuclear Fusion in Stars

The fusion of deuterium and tritium to form helium with the release of energy