Lecture 3: The Stars and their Environment

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Lecture 3The Stars and their Environment

• Dr Michael Burton

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Fundamental Properties of Stars

• Parallax gives distance to closest stars.
• Light years.
• Colours give temperature.
• bluehot, yellowtepid (6000K), redcool.
• Luminosity from 0.001 -100,000 x Sun.
• Masses from binary star orbits (K3L).
• 0.01 to 100 x Sun

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Mass of the Sun

• 2 x 1030 kilograms
• 2 million, million, million, million, million kg
• 2,000,000,000,000,000,000,000,000,000,000 kg
• But not 2,000,000,000,000,000,000,000,000,000 kg
• Or 2,000,000,000,000,000,000,000,000,000,000,000
  kg!

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Hertzsprung-Russell Diagram

• Fundamental tool for understanding stars.
• Luminosity (or magnitude) vs
• Temperature (or colour or spectral type).
• Main Sequence
• Red Giants
• White Dwarfs
• Position on MS determined by mass.

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Nebulae Surrounding Star Birth

• Collapse from Molecular Clouds under gravity (1
  106 1019 particles per cm-3).
• Dark Nebulae (100 K).
• Shine through fluorescing hydrogen gas.
• Red Nebulae (HII regions) (10,000K).
• Reflect starlight by dust scattering.
• Blue Nebulae (cf the sky).

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Star Birth

• Protostar - collapsing core of molecular cloud.
  Pressure builds till nuclear fusion ignites in
  centre, becoming a star.
• Associated with disks (planetary systems),
  outflows and jets.
• Disperse their cocoon to become visible.
• Typically form in clusters, dominated by light
  from 1-2 brightest members.

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Stellar Evolution Main Sequence Life

• Main Sequence stars gravity balances nuclear
  fusion of H to He at 15 million K.
• More massive stars burn fuel more quickly.
• Hydrogen shell burning when fuel exhausted in the
  core.
• Star swells to a cool, extended Red Giant.
• 3000K, Radius 1 AU.

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Stellar Evolution Life on the HR Diagram

• Leaves MS, climbs Giant Branch.
• Turn-off point on HR diagram gives age.
• Fusion of helium begins in core (at 100 million
  K), descends and contracts.
• Helium shell ignites, sheds outer layers.
• Globular clusters ancient star cities
• Horizontal Branch stars burning helium.
• HR diagram evolution as function of mass.

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Star Death Low Mass Stars

• Main Sequence to Red Giant to Planetary Nebula
  White Dwarf.
• PN ejected envelope, forms expanding shell.
• WD burnt out stellar core. Mass of star but size
  of Earth.
• Teaspoon weighs 5 tons!

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Star Death High Mass Stars

• MS to Red Giant to Supergiant to Supernova to
  Neutron Star or Black Hole.
• Nuclear fusion continues in shells to iron.
• Protons electrons fuse to neutrons.
• Unstable, collapses in lt1s. Bounce off rigid
  core detonates star - Supernova!
• Shines as brightly as galaxy for a few days!

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Stellar Remnants

• Low mass stars White Dwarfs
• High mass stars
• supernova remnants, expanding at 10,000 km/s
• may trigger future star formation?
• Neutron stars mass star but just 10 km across.
• Teaspoon weighs 100 million tons!
• Seen as Pulsars, flashing beacons in space.
• or Black Holes?

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Black Holes

• Gravity wins, even light cant escape!
• Collapse to a Singularity with an Event
  Horizon (R 2GM/c2).
• Mass, angular momentum and charge only.
• Cosmic censorship, time slows down.
• Supermassive Black Holes in galaxy cores.
• Primordial Black Holes in Big Bang.
• Black Holes evaporate through production of
  virtual particles at event horizon!