Astr 1020 1st Edition Lecture 9Current LecturePrime of life, Main Sequence:GIANTS: luminous but cooler than the sunSUPERGIANTS: extremely luminous, massive starsWHITE DWARFS: very dim but hotRange of Stellar PropertiesMass: core must be hot enough to fuse, but not self-destruct. 0.1 MSun - 100 MSun Radius: depends on mass 0.1-10 Rsun (MS stars) 0.01 - 1000 Rsun (white dwarfs to red supergiants) Luminosity: depends on temperature and radius 10^-4 – 10^6 LSun Temperature: depends on mass 3,000 - 50,000 K Lifetime: depends on mass10^12 years – 10^6 yearsWhat is the possible mass range for stars?- Least massive star: governed by internal pressure, temp for fusion- M>.08 Msun for fusion (~80 Mjupiter)- M>.08 Msun: Brown Dwarf, no fusion, but still heat from grav. contractionChapter 16: Star BirthWe start with clouds of cold, interstellar gas (mostly H and He)- Molecular cloudso Cold enough to form molecules; T=10-30K- Often dusty- Cloud collapses due to gravity (must be triggered)Interstellar Matter ISM- Stars form out of interstellar gas at beginning of their lives- Stars return some of their gas to the ISM at the endSTARS OF ORIONBetelgeuse (red supergiant)Orion Belt Stars (massive OB-type)Orion Nebula (messier 42)Rigel (blue supergiant)What’s in the ISM? (Interstellar matter)- Linked to stars, so probably made of the same stuf- Hydrogen and Helium- ~2% “metals”- diferent forms depend on temperature- hot-atomic ionized gas- warm-molecules- warm –dust grainsDust- about half of the “metals” in the ISM are condensed into solid form- Dust grains about the size of smoke particles- .1-10 microns in sizeQ: Given that light waves tend to interact with objects of about the same size as their wavelength, how would you expect dust grains of ~10^-6 meters to afect light?A: Dust will block visible light, but let most infrared and radio waves passReddening due to Dust- Dust scatters short wavelength light more than longer wavelength light- Objects behind dust clouds appear redder because blue light is scattered- Depending on thickness of cloud, light may be blocked entirelyOrion Star Cluster –Trapeze Stars=brightest/hottest stars we knowInfrared light shines through dustPressure and Gravity- Pressure balances Gravity- Ideal Gas Law:P=nkTP=outward force n=density (particles of volume) k=constant Pressure changes if density or temperature change - Gravitational Collapseo Pressure pushes out, gravity pushes ino If pressure balances gravity nothing happenso Since dust blocks visible and UV light, molecular clouds tend to be quite cold (<100K)o Temperature drops –pressure dropso Gravity of cloud starts to dominate pressurecloud starts to contracto Shockwave from nearby supernova may also start gravitational collapse of ISM cloudsreminder: Conservation Laws- Energy- Momentum- Angular Momentum Collapse from Cloud to Protostar - First collapse from very large, cold molecular clouds- Fragments into star-sized massesNebular Theory- Gas cloud (nebula) starts to compress- Gravity causes nebula to contract, heat, and spin- Stars (and planets) form when nebula becomes dense and hot enough Collapse of the Solar Nebula - Why does it spin faster? Conservation of Angular MomentumM x V x RQ: IF angular momentum (m x v x r) is conserved (stays constant) what happens if r goesdown (no change in m)?A: velocity has to go upStar-Forming Cloud:SPINNING –conservation of angular momentumHOT –collapsecompressionDISK –collisions force common motionsMultiple Protostars Can Form From a Single Cloud- Why does it heat up? Conservation of Energy!- What does it flatten into a disk?Collisions between particles average out motions and orbits- How do collisions form a diskDirections average out –cloud flattensSpeeds average out –orbits become circularExtreme Conformism- Anything not going in the common direction at the common speed is eventually assimilated Collapsing cloud spins up, forms star, disk and jet WHY A JET?- Disk is due to conservation of angular momentum- Jet comes from rapidly-rotating magnetic fieldsRapidly- spinning magnetic fields launch charged particles from poles of star at very high speedsSUMMARY SLIDE OF HOW STARS ARE FORMED- Interstellar gas cloud starts to contract- It gets smaller and denser- And smaller…- Denser, Hotter until H-fusion beginso When core reaches at least 5 million