The Sun as a Star (CIE IGCSE Physics)

• 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 gravitational pull of the Sun on the planets keeps them in orbit

• 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

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)

The colour of a star correlates to its temperature

EXTENDED

• In the centre of a stable star, hydrogen nuclei undergo nuclear fusion to form helium
• The equation for the reaction is shown here:

Deuterium and tritium 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

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

ANSWER:  B

• • Hydrogen nuclei have positive charges
  • So two hydrogen nuclei would have a repulsive force between them
  • High temperatures are required to give the nuclei enough energy to overcome the repulsive force
  • The answer is not A because the products of the hydrogen fusion shown in the reaction is helium
    • Helium is an inert gas

  • The answer is not C because hydrogen is a very abundant element
    • It is the most common element in the universe

  • The answer is not D because hydrogen fusion would produce a huge amount of energy