Stars

The early universe contained only hydrogen

• Seconds after the big bang there was only hydrogen (with some helium forming shortly after), as the universe expanded these atoms clumped together to form stars
• In the core of the stars hydrogen nuclei smash together to form helium nuclei. This is called nuclear fusion. As stars grow older all the hydrogen in the core turn to helium
• Once the hydrogen has run out the helium nuclei fuse to form other, heavier elements
• 3 helium nuclei combine to make 1 carbon nucleus. More helium combine with carbon nuclei to make oxygen and neon
• When the helium runs out some of the carbon, oxygen and neon combine to make silicon
• In the biggest stars nuclei keep combining by fusion until iron is formed
• At the end of their lives, massive stars explode, flinging gas out into space
• In these explosions, heavy nuclei combine with each other and with neutrons to make most of the elements in the universe 
• The dust and gas from these supernova explosions can form new stars and planets. These second (3rd, 4th....) generation star systems contain heavier elements as well as hydrogen  

1) Stars initially form form clouds of gas and dust.

2) The force of gravity makes the gas clouds come spiralling in and as they do gravitational energy is converted int heat energy and the temperature rises.

3) When the temperature gets high enough, hydrogen nuclei undergo nuclear fusion, form helium nuclei and give out massive amounts of heat and light. A star has been born. It immediately enters a long stable period where the heat created by nuclear fusion provides an outward pressure to balance the force of gravity pulling everything inwards.

In this stable period it's called a MAIN SEQUENCE STAR. Because of of the huge amount of hydrogen stars contain the stable period can last millions of years.

4) Eventually the hydrogen begins to run out and the star swells into a RED GIANT. It becomes red because the surface cools.

5) A small star like our Sun will then begin to cool and contract into a WHITE DWARF and then finally as the light fades completely, it becomes a BLACK DWARF.

6) Big stars though start to glow brightly again as they undergo more fusion and expand and contract several times forming heavier elements in various nuclear reactions. Eventually they explode in a SUPERNOVA

7) The exploding supernova throws the outer layers of dust and gas into space, leaving a very dense core called a NEUTRON STAR, or if it's big enough a BLACK HOLE !!!!!!!!!!!!!!!!!!!

8) The dust and gas thrown off by the supernova will form into SECOND GENERATION STARS like our Sun. The heavier elements are only made in the final stages of a big star, many in the supernova itself, so the presence of heavier elements in the Sun and the inner planets is clear evidence that our world has been made up out of these elements.

9) The matter from which neutron stars and white and black dwarfs are made is millions of times denser than any matter on Earth because gravity is so strong it even crushes atoms.