GENS4001 AstronomyStars and their Environment1 Part II: Stars and their Environment Dr Michael Burton

GENS4001 AstronomyStars and their Environment2 Fundamental Properties of Stars Parallax gives distance to closest stars. –Measured in Light Years. Luminosity from ,000 x Sun. Masses from binary star orbits (K3 rd L). –0.01 to 100 x Sun Colours give temperature. –blue=hot, yellow=tepid (6000K), red=cool.

GENS4001 AstronomyStars 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!

GENS4001 AstronomyStars and their Environment4 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 AstronomyStars and their Environment5 Nebulae Surrounding Star Birth Stars form from collapse of Molecular Clouds under gravity ( 1  10 6  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 AstronomyStars and their Environment6 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 AstronomyStars and their Environment7 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 AstronomyStars and their Environment8 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

9 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.

GENS4001 AstronomyStars and their Environment10 Globular Clusters Ancient star cities: –Contain up to 10 7 stars, 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 AstronomyStars and their Environment11 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 AstronomyStars and their Environment12 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 AstronomyStars and their Environment13 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 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!