GENS4001-X1The Stars and their Environment1 Lecture 3: The Stars and their Environment Dr Michael Burton

GENS X1The Stars and their Environment2 Fundamental Properties of Stars Parallax gives distance to closest stars. –Light years. Colours give temperature. –blue=hot, yellow=tepid (6000K), red=cool. Luminosity from ,000 x Sun. Masses from binary star orbits (K3L). –0.01 to 100 x Sun

GENS X1The Stars and their Environment3 Mass of the Sun 2 x 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!

GENS X1The Stars and their Environment4 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.

GENS X1The Stars and their Environment5 Nebulae Surrounding Star Birth Collapse from Molecular Clouds under gravity ( 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).

GENS X1The Stars and their Environment6 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.

GENS X1The Stars and their Environment7 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.

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.

GENS X1The Stars and their Environment9 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!

GENS X1The Stars and their Environment10 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!

GENS X1The Stars and their Environment11 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?

Black Holes Gravity wins, even light can’t escape! Collapse to a ‘Singularity’ with an ‘Event Horizon’ (R = 2GM/c 2 ). 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!