Star FormationGravity in action; What it’s all about:A toy model:Stars form in interstellar space, form from interstellar matter: gas and dustLocally the stored energy in gravity (the potential energy) must be able to over comeall sources of dissipation (energies): thermal, magnetic, rotational.Clumps collapse under self-gravity. Angular momentum conserved. Form diskClumps continue to fragment: One cloud forms many starsCentral core of clump grows within dusty cocoon.Central core becomes proto-star.Gravitational energy fuels the system.Disk perhaps forms planetary system.Instabilities clear away cocoon: : Stellar winds: jets.Temperature of central core reaches a point at which nuclear fusion commences (tenmillion Kelvins)(hydrogen into helium with the release of energy).Star becomes stable and is on the main sequence of the H-R diagram.the detailsInitial cloud of gas and dust much larger than solar system and more massive than the sun(10pc, 10Msol).Under density fluctuations, cloud fragments into smaller clumpsA scenario for the formation of associations/clusters of starsSingle stars a rarity.Fragment collapses under its self-gravity.Central, dense core grows.Temperature increases. Conversion of gravitational energyinto thermal and luminous energies. Falling matter heats proto-starWhen core becomes opaque, protostar becomes unstable and violent convections:mix material throughout the starstrong stellar winds and jets (T-Tauri stars)Cocoon cleared away. Star appearsTime scale for the formation of a sun-like star ( A G2 star) is about 30 million years.Time scale for collapse is strongly dependent on mass of fragments; mass of protostar.Most massive stars collapse over the shortest periodtens to hundreds of thousands of years for O and B starsHundreds of millions to billions of years for K and M1 November 9, 2003stars.Maximum stellar mass: about 100.Minimum stellar mass: about 0.01: Brown dwarfs: Too big to be planets, too smallto be stars (no proton-proton cycle).observational testsThe formation of disks: conservation of angular momentumobserve: Pictoris, star + diskobserve: Instabilities and jets: HH (Herbig-Haro) Objects; Radio jetsFormation Time scales: Effects of mass:Star clusters: all stars form at same time: stars with many masses:NGC 2264: massive stars on the main sequence; low mass stars still collapsingand unstable.Formation of stars in dense Giant Molecular Clouds (GMC):Bursts of star formation: Forces of compressionHidden proto-stars: IR sources: Hot, dusty cocoonsStructure of Orion complex of star formationHistory of star formation: Several generations of activityThe GMCThe Orion nebulaThe IR sourcesThe collapsing fragmentsThe Orion I Association.2 November 9,