2. THE BIG BANG This model suggests that somewhere around 13.7 billion years ago all matter in the Universe was contained in a hot, dense particle. The temperature of the ball was incredibly high - billions and billions of degrees.

3. THE BIG BANG Such an incredible amount of energy made the particle explode; as it expanded it immediately started to cool, forming all the bits that are found in atoms. Eventually, atoms themselves were formed.

5. Evidence for the Big Bang The majority of the Universe should still be made up of hydrogen and helium; these are the simplest types of atoms possible because they contain the least number of protons, neutrons and electrons. In other words, there hasn’t been enough time yet for lots of big atoms to be made so that the Universe becomes filled with them.

6. Evidence for the Big Bang Such a “Big Bang” must have made an incredible amount of radiation, so much that there should still be some left spreading throughout the Universe. This has been detected; all hot things give off radiation (eg, infra-red, a long wavelength type). There are definitely microwaves spreading throughout the Universe from no apparent source!

7. Evidence for the Big Bang Analysis of light coming from other extremely distant stars in the Universe shows a “Red Shift”; this means that the light appears to be at a slightly lower frequency than it should be. This indicates that the stars are traveling away from us. Here’s an example of Doppler effect on sound

8. Evidence for the Big Bang Did you notice how the sound changed? As the siren was coming closer, the pitch of the sound was higher (this corresponds to shorter wavelengths). As the siren went further away, the pitch of the sound lowered (this corresponds to longer wavelengths). Here’s an example of Doppler effect on sound

9. Evidence for the Big Bang If you pass normal white light through a glass prism, the light separates and looks like a “rainbow”. This is known as the VISIBLE SPECTRUM OF LIGHT.

10. Evidence for the Big Bang But light analysed from stars at different distances from us shows that the light appears to be slightly different each time. Very distant galaxy Our sun

11. Life of a Star Stars start off as a cloud of dust and hydrogen called a nebula. As the cloud collapses on itself due to its own gravity, it becomes hotter and very dense. At this stage, we call it a protostar. As the gas becomes even hotter, Nuclear Fusion processes begin and the star begins to glow.

12. Life of a Star The fusion process involves hydrogen nuclei being forced together to form helium nuclei. This process produces more energy, so more of these reactions can occur. Of course this makes even more energy, and so on. This is a type of CHAIN REACTION.

13. Life of a Star What happens next depends of the mass of the star. If the protostar has a mass less than one tenth of our Sun, it will not get hot enough to sustain the fusion reactions. It will fade to form a small red star before turning cold and then dying.

14. Life of a Star But a star about the size of the Sun will initially glow very brightly. It then settles to have a long stable middle life period of about 10 billion years. Our Sun is about half way into its midlife.

15. Life of a Star Once all of the hydrogen runs out, the core of the star will shrink and different fusion reactions will occur. The outer layers will expand and cool forming a RED GIANT. Eventually the outer parts of the star will drift into space leaving a very small and dense object called a WHITE DWARF.

16. Life of a Star Eventually, the white dwarf cools down and fades away leaving a mass of gases in space.

17. Life of a Star Stars about 2 to 6 times the size of the Sun only survive for about 1 million years, but are much more spectacular to look at. Their gravitational pull on themselves is huge, as is their energy output. They appear to be very hot bright stars that glow blue in the night sky.

18. Life of a Star When these stars run out of fuel they explode, producing what we call a SUPERNOVA. Much of the star’s matter is blown into space, leaving an expanding cloud which is called a NEBULA. This explosion increases the brightness of the star by about a billion times!

19. Life of a Star After the supernova explosion, the original protons and electron in the core may be squeezed together to form a NEUTRON STAR. These are so dense that a millilitre of this star would weigh 10 18 kg! These stars do not glow, but radio-telescopes can detect signals from them. A bigger star will collapse to become a black hole.

20. Hertzsprung-Russell Diagram This graph plots information about stars. It relates their luminosity (this is the power output of the star, ie, energy emitted per second; this affects its brightness) on the y- axis to its surface temperature on the x-axis (this is determined by its colour - the cooler it is, the more red the colour; the hotter it is, the bluer its colour).

21. Hertzsprung-Russell Diagram In this example, there are 2 stars of the same size. The blue star is twice as hot as the red star. Therefore, the blue star must be giving off more energy per second, ie, it is more LUMINOUS.

22. Hertzsprung-Russell Diagram

23. Relating Position on Diagram to Characteristics of Star Astronomers reasoned that if a star were hotter, it should have a higher luminosity, and a cooler star would be dimmer. As it turns out, most stars fit this pattern. They can be found on the HR Diagram in the large group that stretches across the middle of the diagram. These are called the Main Sequence Stars.

24. Hertzsprung-Russell Diagram

25. Our sun is a main sequence star, as are many stars near our solar system.

26. Hertzsprung-Russell Diagram Other stars have characteristics that place them in other groups. Stars that are cool but very luminous must be very large. We call these red giants or supergiants. Finally, there are stars that seem very hot, but dim. These are white dwarf stars. They shine with great intensity but are very small so they do not appear very bright to us.

27. Hertzsprung-Russell Diagram If stars of different sizes have the same surface temperature (and therefore appear to be the same colour), the larger star is more LUMINOUS (ie, gives off more energy per second).