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Part II: Stars and their Environment Dr Michael Burton GENS4001 Astronomy Stars and their Environment 1 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. – blue=hot, yellow=tepid (6000K), red=cool. GENS4001 Astronomy Stars and their Environment 2 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 Astronomy Stars and their Environment 3 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. GENS4001 Astronomy Stars and their Environment 4 Nebulae Surrounding Star Birth • Stars form from collapse of Molecular Clouds under gravity (1106 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). GENS4001 Astronomy Stars and their Environment 5 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 1–2 brightest members. GENS4001 Astronomy Stars and their Environment 6 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? GENS4001 Astronomy Stars and their Environment 7 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 GENS4001 Astronomy Stars and their Environment 8 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. 9 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. GENS4001 Astronomy Stars and their Environment 10 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! GENS4001 Astronomy Stars and their Environment 11 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 <1s. Bounce off rigid core detonates star – Supernova! • Shines as bright as a galaxy for a few days! We are Stardust from Supernovae! GENS4001 Astronomy Stars and their Environment 12 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 Astronomy Stars and their Environment 13 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 14 of virtual particles at event horizon!