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STARS & their life cycles 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! STAR EVOLUTION LOW MASS = Sun HIGH MASS = Several times the mass of the Sun STAGE 1a - BIRTH 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. When it reaches a temperature of a few million degrees it ignites - thermonuclear fusion reactions begin, and the star begins to shine! STAGE 1b - BIRTH As the new stars begin to shine, they blow away any traces of remaining gas and dust that have not formed planets. 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. STAGE 2a - GROWING UP The star now begins to fuse atoms of hydrogen into helium at an amazing rate, giving out lots of energy. 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. 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. Most stars staysbalanced like this for several billion years. STAGE 3a - GETTING OLD 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. 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. STAGE 3b - GETTING OLD The core shrinks & heats up, becoming a very dense WHITE DWARF star. 1cm3 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, cooling down. This will eventually happen to our Sun!!! HIGH MASS STARS - a 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. They also undergo a much more drastic death! HIGH MASS STARS - b 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. As before, the core contracts, trying to become a WHITE DWARF, but things do not stop there! HIGH MASS STARS - c 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/cm3! 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. HIGH MASS STARS - d The neutron star may continue to gain mass from nearby stars. At a critical moment, it becomes so dense it collapses in on itself, becoming a single point of zero size! Its gravity is so strong that even light cannot escape from inside a certain boundary - the EVENT HORIZON. The star is now a BLACK HOLE ! Although we will never actually SEE a black hole, we can see their effects. A number of objects do show BLACK HOLE properties. COSMIC RECYCLING Supernovae explosions fill the space nearby with gas and new heavy elements - in fact, elements more massive than iron in the periodic table can only be made this way. These nebulae may then lead to the formation of a new generation of stars - this has happened to our Sun. Some atoms in our bodies where formed this way! We are all made from stardust..or all the leftover ‘crud’……!?! SUMMARY THE END PHYSAROONIE!!! A Mr H. production