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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 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 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 4 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 5 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 6 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 The Stars and their Environment 7 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 9 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 10 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 11 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!