unit 5 stars

  • Created by: megan
  • Created on: 24-03-13 17:47
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  • Stars
    • star classes
      • Astronomers have classified stars into groups according to their teperature.
      • Hotter stars have more mass and are more luminous.
        • The larger gravitational pressure at the centre of a massive star makes nuclear fusion reactions very fast, and produce lots of energy.
      • Spectral class goes from hottest to coolest. OBAGFKM
        • The colour goes from blue to red, mass, radius, and luminosity  decreases but main sequence life span increases.
    • The hertz sprung Russel Diagram
      • A graph of luminosity against star temperature shows a general negative correlation
      • The main sequence are stable stars which will exist in this state for the majority of their lifetime.
    • Stellar Evolution
      • As a star under goes nuclear fusion, the binding energy differences of the nuclei before and after the fusion mean that the process releases energy
      • The pressure of from the vibration of its particles, and the EM radiation trying to escape holds the strucure of the star against gravitational collapse
      • The initial mass of the star is a critical factor in determining how the battle between gravity and pressure wages.
    • Low mass stars.
      • Once it has accreted about the mass of our sun, a low mass star under goes nuclear fusion of hydrogen, converting it to helium
        • radiation pressure= gravity -> stable star
        • The star will run low on fuel so it expands a little due to all the energy produced. The temperature falls -> Red giant
          • Once most of the hydrogen is used, helium will form. This causes an explosion that throws material into space -> planetary nebula.
            • As the fuel runs out, the outward pressure from fusion drops and gravity takes hold. The star contracts, heats up -> white dwarf.
              • When the white dwarf cools down it will leave a black dwarf.
    • Massive stars
      • If a protostar is more than 4 times the mass of our sun, the star begins life as a blue supergiant.
        • Radiation Pressure = Gravity
        • The fusion processes happen at higher temperatures, so it burns quickly and further fusion of some large atoms occur
          • When the material has been fused to where it is mostly iron, it can no longer undergo nuclear fusion and stops producing energy
            • The star will collapse, this increase in density produces a huge burst of energy -> type II supernova
              • Within a supernova there is so much energy that nuclear reactions occur which produce the elements above iron.
              • The entire star may be completely shattered after this. if there remains a central core this will either be a neutron star or black hole
                • a neutron star consists almost entirely of neutrons packed densely together.
                • Black  holes are even smaller than neutron stars, the gravitational pull of a black hole is so strong that things travelling at the speed of light cannot escape.


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