Stellar Physics

• The intensity of energy emitted from a star depends on the surface temperature of the star and the wavelength of light emitted. The peak wavelength for each temperature is where the intensity of light emitted is greatest.
• Which color of star has the greatest temperature

  Blue has the highest temperature and red has the least power of a star
• What is the stellar luminosity 


  The luminosity of a star is the total energy radiated per second measured in watts (W)
• Apparent brightness


  The apparent brightness of a star is the energy per second landing on an area of one square meter at the surface of the earth. As you move away from the star the radiation spread out to cover the surface of a bigger sphere.
• Astronomical units AU


  The AU is the average distance between earth and the sun 1AU=1.5x10^11
• Formation of the star

  1. stars form in huge clouds of gas and dust called nebulae.
  2. these clouds are very cold and dense relative to the rest of space.
  3. stars are formed interstellar clouds
  4. gravitational force cause the clouds of gas and dust to contract and become more dense
  5. this in turns produces an outwards force from the thermal pressure however this force is smaller then the increasing gravitational force
  6. the cloud of gas and dust increase in temperature becoming hot enough to under go fusion
• First stage of the proton-proton chain


  Two hydrogen nuclei undergo fusion to form a deuterium nuclear (one proton and one neutron known as heavy hydrogen). A positron (anitelectron) and a neutrino are released at the same time.

  
• Second stage of the proton-proton chain


  The deuterium nucleus fuses with a proton to form a helium 3 nucleus (2 protons and 1 neutron) a gamma ray is released at the same time.

  
• Third stage of the proton-proton chain

  Two helium 3 nuclei fuse to form a helium 4 nucleus two protons are released at the same time.

  
• How is mass lost during the proton-proton chain 


  The lost mass is converted into energy by E=mc^2
• Hertzsprung-Russel Diagram

  relates the luminosity of a star to its surface temperature

  
• Red Giant

  1. Hydrogen used up

  2. Nuclear fusion stops

  3. Core and hydrogen shell contract and heat up

  4. Hydrogen shell becomes hot enough for fusion

  5. Thermal pressure greater than gravitational force

  6. Surface pushed outwards and cools

  7. New equilibrium reached

  8. Core shrinks and reaches high temperatures to fuse helium
• White dawf

  1. helium is used up
  2. core contracts
  3. layers become more unstable
  4. star pulsates and ejects the outer layers into space
  5. as a nebula
• Lifetime of a star is determined by

  star with a similar mass as the sun follow the stages:

  main sequence star - red giant - white dawf

  stars with a larger mass than our sun follow the stages:

  main sequence star - red super giant - supernova -neutron star or black hole