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Lecture 3:
The Stars and their Environment
Dr Michael Burton
GENS4001-X1
The Stars and their Environment
1
Fundamental Properties of Stars
• Parallax gives distance to closest stars.
– Light years.
• Colours give temperature.
– blue=hot, yellow=tepid (6000K), red=cool.
• Luminosity from 0.001 -100,000 x Sun.
• Masses from binary star orbits (K3L).
– 0.01 to 100 x Sun
GENS4001 1999-X1
The Stars and their Environment
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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!
GENS4001 1999-X1
The Stars and their Environment
3
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.
GENS4001 1999-X1
The Stars and their Environment
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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).
GENS4001 1999-X1
The Stars and their Environment
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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.
GENS4001 1999-X1
The Stars and their Environment
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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.
GENS4001 1999-X1
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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.
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!
GENS4001 1999-X1
The Stars and their Environment
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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 <1s. Bounce off rigid
core detonates star - Supernova!
• Shines as brightly as galaxy for a few days!
GENS4001 1999-X1
The Stars and their Environment
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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?
GENS4001 1999-X1
The Stars and their Environment
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Black Holes
• Gravity wins, even light can’t 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!