2. STARS & their life cycles

3. Like us, stars are born, grow older, become middle aged and eventually die!!! Unlike us, stars take billions of years to complete their life cycles, but we can observe different types of stars & piece the bits together!

4. Stars are formed when clouds of gas - usually hydrogen - collapse under the effect of gravity. As the cloud contracts, over millions of years, it gradually heats up and becomes denser. This is called a PROTOSTAR. STAGE 1a -BIRTH When it reaches a temperature of a few million degrees it ignites - thermonuclear fusion reactions begin, and the star begins to shine!

5. As the new stars begin to shine, they blow away any traces of remaining gas and dust that have not formed planets. STAGE 1b -BIRTH This pictures shows the Rosette Nebula - new, young stars have blown away an area of the gas cloud, about 12 light years across, allowing them to be seen clearly. Stars make the gas glow.

6. The star now begins to fuse atoms of hydrogen into helium at an amazing rate, giving out lots of energy. STAGE 2a -GROWING UP As it cools, it changes colour from white to blue to yellow. By this time, the star has become middle aged - we call it a MAIN SEQUENCE star.

7. STAGE 2b -GROWING UP The star is in equilibrium between 2 forces - gravity trying to collapse it & heat trying to make it expand due to pressure. Our Sun is a good example of this. The star stays balanced like this for several billion years.

8. As the star runs out of fuel the outer layers of gas expand up to 100 times and it cools, becoming redder. It is now called a RED GIANT. STAGE 3a -GETTING OLD Eventually, the star produces an unstable shell of heavier elements which it blows off into space. This is called a PLANETARY NEBULA. Energy from the star causes it to glow.

9. STAGE 3b -GETTING OLD The core shrinks & heats up, becoming a very dense WHITE DWARF star. 1cm 3 of one of these may have a mass of about 10,000kg! Finally, it fades away, radiating all its remaining energy to become a BROWN DWARF - a cold mass of gas in space. This will eventually happen to our Sun.

10. These are formed from large gas clouds, giving them masses several times that of the Sun, and several hundred times the diameter! They undergo the same early stages, but do so much faster, as they use fuel a lot quicker. HIGH MASS STARS - a They also undergo a much more drastic death!

11. Near the end of the RED SUPERGIANT phase, high mass stars become very unstable, and produce many envelopes of different elements. Eventually, the star can take no more and it explodes violently as a SUPERNOVA. HIGH MASS STARS - b As before, the core contracts, trying to become a WHITE DWARF, but things do not stop there!

12. The white dwarf may continue to acquire mass from a nearby companion star, becoming so dense that electrons and protons get squeezed together, leaving only neutrons. The results is a NEUTRON STAR - an object only 10-30km across, with a density of 300,000 million g/cm 3 ! HIGH MASS STARS - c Periodic bursts of radiation may be emitted from around the neutron star, if it is spinning, due to its magnetic fields. This is a PULSAR.

13. THE END A Mr H. production