the star

• Brightness of a star depends on both distance and luminosity.
• Luminosity is the amount of power a star radiates.
• Apparent brightness : amount of starlight that reaches Earth.
• Parallax: The apparent shift in the position of a celestial object as a result of the Earth's rotation
• Properties of thermal radiation:

  • hotter objects emit more light per unit area at all frequencies.
  • hotter objects emit photons with a higher average energy.
• The wavelength of the emission peak varies as 1/T
• Higher temperature blackbodies are brighter at shorter wavelengths.
• Shape of emission curve is given by Planck's law
• Level of ionizaton also reveals a star's temperature. The higher the temperature, the higher the level of ionization.
• Absorption lines in star's spectrum tell us its ionization level.
• Spectral types: OBAFGKM

  O is the hottest and M is the coolest
• Parallax tells us distances to the nearest stars.
• 
  

  Most stars fall somewhere on the main sequence of the H-R diagram
• Giants and supergiants: stars with lower Temperatures and higher Luminosity than main-sequence stars (must have larger radii)
• white dwarfs: Stars with higher Temperature and lower luminosity compared to main-sequence stars (must have smaller radii)
• Hertzsprung- Russel (H-R) dirgram depicts:

  1. Temperature
  2. Colour
  3. spectral type
  4. Luminosity
  5. Radius
• Main sequence stars are fusing hydrogen into helium in their cores like the sun.
• Luminous main sequence star: A star that has used up its hydrogen fuel and is now fusing helium

  • they are hot and Blue
  • Massive
• Less luminous stars are cool and yellow/red
• Stars form in dark clouds of dusty gas in interstellar space.
• Interstellar space: gas between stars
• Most of the matter in star-forming clouds is in the form of molecules.
• Molecular clouds have temperatures of 10 – 30 K and a density of about 300 molecules per cubic centimetre
• Stars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter - lived.
• Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes
• High mass star:

  • high luminosity
  • short-lived
  • Larger radius
  • Blue
• Low-mass stars:

  • Low luminosity
  • long-lived
  • small radius
  • red
• Observations of star clusters show that athe star becomes larger, redder, and more luminous after its time on the main sequence is over.
• Luminosity increases because the core thermostat is broken.
• Increasing the fusion rate in the shell does not stop the core from contracting.
• Red Giants: Broken Thermostat

  A) stellar

  B) hydrogen

  C) helium

  D) hydrogen

  
• Helium fusion requires higher temperatures than hydrogen fusion.
• larger charge leads to greater repulsion.
• Core temperature rises rapidly when helium fusion begins.
• Helium fusion rate skyrockets until thermal pressure takes over and expands the core again.
• Helium-burning stars neither shrink nor grow because core thermostat is (temporarily) fixed.
• Models show that a red giant should shrink and become less luminous after helium fusion beginsin the core.
• Double shell burning:

  After core helium fusion stops, helium fuses into carbon in a shell around the carbon core, and hydrogen fuses to helium in a shell around the helium layer.
• Double shell–burning stage never reaches equilibrium—fusion rate periodically spikes upward a series of thermal pulses.
• Double shell burning ends with a pulse that ejects the H and He into space as a planetary nebula.

  -The core left behind becomes a white dwarf