The Sun as a Star (Cambridge O Level Physics)

Revision Note

Test yourself
Katie M

Author

Katie M

Last updated

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 Place in Space, downloadable IGCSE & GCSE Physics revision notes

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

The Sun, downloadable IGCSE & GCSE Physics revision notes

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

Colour of Stars, downloadable IGCSE & GCSE Physics revision notes

The colour of a star correlates to its temperature

Did this video help you?

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:

straight H presubscript 1 presuperscript 2 space plus space straight H presubscript 1 presuperscript 3 space rightwards arrow space He presubscript 2 presuperscript 4 space plus space straight n presubscript 0 presuperscript 1

  • The equation shows straight H presubscript 1 presuperscript 2 (deuterium) and straight H presubscript 1 presuperscript 3 (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

Nuclear Fusion, downloadable AS & A Level Physics revision notes

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

Worked example

An example of a hydrogen fusion reaction which takes place in stars is shown here.

straight H presubscript 1 presuperscript 2 space plus space straight H presubscript 1 presuperscript 1 space rightwards arrow space He presubscript 2 presuperscript 3

Which of the following is a valid reason as to why hydrogen fusion is not currently possible on Earth?

A   Hydrogen fusion produces dangerous radioactive waste

B   Hydrogen nuclei require very high temperatures to fuse together

C   Hydrogen is a rare element that would be difficult to get large amounts of

D   Hydrogen fusion does not produce enough energy to be commercially viable

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 product 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

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.