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Stellar Evolution

Stellar Evolution. From Protostars to Black Holes. Absolute magnitude/Spectral type. 0 – Hypergiants Ia , Ib – Super-giants II – Bright Giants III – Giants IV – Sub-giants V – Main Sequence VI – Sub-dwarfs VII – White dwarfs . Hertzsprung-Russel diagram. Young Stars.

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Stellar Evolution

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  1. Stellar Evolution From Protostars to Black Holes

  2. Absolute magnitude/Spectral type 0 – Hypergiants Ia, Ib – Super-giants II – Bright Giants III – Giants IV – Sub-giants V – Main Sequence VI – Sub-dwarfs VII – White dwarfs

  3. Hertzsprung-Russel diagram

  4. Young Stars • Protostars – Condensed, super-hot gas clouds • Hydrogen fusion • More massive stars reach about 10 million Kelvin thus allowing the fusion of Hydrogen • Star enters the Main Sequence • Brown dwarfs • Less massive stars never hot enough for nuclear fusion of Hydrogen to occur • Around 0.0125 solar masses • Stars below this mass are sub-brown dwarfs (classified as planets if orbiting a stellar object)

  5. Mature Stars • Once a star runs out of Hydrogen, it leaves the main sequence • Either • core becomes hot enough to fuse Helium • Electron degeneracy pressure balances gravitational forces causing stability Which process occurs depends upon the mass of the star.

  6. Low-mass stars • Red dwarfs – low temperature, low intensity • Post-fusion behaviour has not been observed due to the age of the universe • Models suggest • Red dwarfs of 0.1 solar mass could stay on main sequence for 6-12 trillion years • Could take hundreds of billions of years to collapse into White dwarfs

  7. Medium-size stars • 0.5-10 solar masses -> Red Giants • Accelerated fusion in outer layers causes expansion • Furthest out layers begin to cool so star becomes more red • Red-giant-branch phase – inert Helium core • Asymptotic-giant-branch phase – inert Carbon core

  8. Massive stars • Red Supergiants – brighter than red giants, hotter • Unlikely to survive -> Supernova • Extremely massive stars lose envelope gasses due to rapid stellar winds • Do not expand into super giants • Maintain very high surface temperatures (blue/white colour)

  9. Star Collapse Temperatures are high enough that the star can fuse elements up to Iron Once the process reaches Iron-56, it begins to consume energy If the core mass exceeds the Chandrasekhar limit (2.765 × 1030 kg), the star will collapse to form a Neutron star Exceeding the Tolman–Oppenheimer–Volkoff limit, (1.5-3.0 solar masses) leads to the formation of a Black Hole

  10. Supernova • A star collapse is accompanied by a supernova • Explosion brighter than a galaxy • Lasts for a few weeks • Radiation burst expels star’s material at about 30,000 km/s • Leaves behind a gas and dust cloud – Supernova Remnant

  11. Stellar Remnants • White and Black dwarfs • 0.6 solar mass compressed to the size of the Earth • Extremely hot (100,000 K at surface) • Once all material is burned, a cold, dark star is formed – Black dwarf (yet to be observed) • Neutron Stars • core collapse -> electron capture, protons -> neutrons • Radius ~10km, incredibly dense • Rotational period of less than a few seconds • Radiation pulses can be detected from each revolution, range from radio to gamma rays (pulsars) • Black Holes

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