Part II: Stars and their Environment

1
Part IIStars and their Environment

• Dr Michael Burton

2
Fundamental Properties of Stars

• Parallax gives distance to closest stars.
• Measured in Light Years.
• Luminosity from 0.001 -100,000 x Sun.
• Masses from binary star orbits (K3rdL).
• 0.01 to 100 x Sun
• Colours give temperature.
• bluehot, yellowtepid (6000K), redcool.

3
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!

4
Hertzsprung-Russell Diagram

• Fundamental tool for understanding stars.
• Graph of Luminosity (or magnitude) vs
• Temperature (or colour or spectral type).
• Main Sequence
• Red Giants
• White Dwarfs
• Mass determines Main Sequence position.

5
Nebulae Surrounding Star Birth

• Stars form from collapse of Molecular Clouds
  under gravity (1?106 ?1019 atoms per cc)
• Dark Nebulae (100 K).
• Absorb light through extinction.
• Shine through fluorescing hydrogen gas.
• Red Nebulae (HII regions) (10,000K).
• Reflect starlight by dust scattering.
• Blue Nebulae (cf daytime sky).

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Star Birth

• Protostar - collapsing core of molecular cloud.
  Pressure builds till heat ignites nuclear fusion
  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 12 brightest members.

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Extra-Solar Planetary Systems

• Over 35 Planetary systems now detected
• Through wobble caused by orbit around star
• Find massive planets close to parent star
• Numerous Proto-planetary disks also found
• An inevitable by-product of Star Formation?

8
Stellar Evolution Main Sequence Life

• Main Sequence stars
• gravity balances nuclear fusion,
• hydrogen to helium at 15 million K.
• More massive stars burn fuel more quickly
• Have shorter lifetimes!
• Hydrogen shell burning when core all converted to
  helium.
• Leaves Main Sequence

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Stellar Evolution Post Main Sequence

• Star ascends Giant Branch
• swells to a cool, extended Red Giant.
• 3000K, Radius 1 AU.
• Helium Flash when fusion of helium begins in
  core (at 100 million K)
• Helium burning core
• Hydrogen burning shell
• Descends Horizontal Branch and contracts.
• Helium shell ignites, sheds outer layers.

10
Globular Clusters

• Ancient star cities
• Contain up to 107 stars, 1010 years old.
• Full range of stellar evolution displayed
• Position on HR diagram determined by Mass.
• Turn-off point gives age.
• Horizontal Branch stars burning helium.

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Star Death Low Mass Stars

• Main Sequence ? Red Giant
• ? Planetary Nebula White Dwarf.
• Planetary Nebula ejected envelope,
• forms expanding shell.
• White Dwarf 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 ? Red Giant ? Supergiant ? Supernova ? Neutron
  Star or Black Hole.
• Nuclear fusion continues in shells to iron.
• Unstable, collapses in lt1s. Bounce off rigid
  core detonates star Supernova!
• Shines as bright as a galaxy for a few days!
• We are Stardust from Supernovae!

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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?

14
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!