The Interstellar Medium Dust in the ISM (ISM) - solid grains of heavier elements -silicon, carbon, ice Material found in space between stars - 2% of ISM - space almost a perfect vacuum Dust absorbs (blocks) light and heats up - large amounts of material are present Interstellar Extinction visible in Infrared Two Components Interstellar Dust Interstellar Gas Dust can also scatter light - blue scatters more than red Reflection Nebula (blue) Sombrero Galaxy, visible Sombrero Galaxy, infrared Gas in the ISM HI map of Milky Way galaxy - atoms, molecules of H, He - 98% of ISM (mostly H) - well mixed with dust Hydrogen gas can exist in three states - depending on conditions 1. Atomic Hydrogen (H(H(H I))) - most common form of Hydrogen - in most regions (except hottest/coldest) - low density: 1 atom/cm 3 (air = 3 x 10 19 ) - gives off energy at λ = 21 cm (radio) HI map of galaxies M81 and NGC 3077 2. Ionized Hydrogen (H(H(H II ))) - occur in the hottest regions in space (T > 10,000 K) - surrounding very hot stars (O, B) - gives off H emission lines (red) Emission Nebula Orion Nebula ↑ Horsehead Nebula (Orion) ↓ Rosette Nebula 3. Molecular Hydrogen (H(H(H 222))) - occur only in coldest, densest regions (T < 50 K) - completely invisible North American Nebula - can only find with “tracers” - other molecules in same region that are visible (CO, H 2O, etc) 5 - estimated 10 H2 for every 1 CO IC 5067, The Pelican Nebula CO map of Orion H2O in Orion Nebula a little bit of everything: Life of a Star The “Battle”: Gravity vs. Gas Pressure Gravity depends on mass and distance Pressure depends on temperature Rho Ophiuchus Keyhole Nebula Gas Pressure Gravity If Gravity < Pressure, - will expand (an H II region) Thackeray’s Globules If Gravity = Pressure Trifid Nebula - Equilibrium/stable (a star) If Gravity > Pressure - will contract (molecular cloud) To form stars and planets: - best locations are Giant Molecular Clouds - cold and dense Two Important Principles Gravitational ContractionContraction:::: 1. Release of grav. potential energy - contraction rising temperature 2. Contraction of an “Ideal” (normal) Gas - will continue indefinitely - until alternate energy source is available - or until it is no longer an ideal gas Forming stars: Giant Molecular Cloud - starts collapse and fragments into smaller and smaller regions - one large cloud can form up to 1 million stars http://www.astro.ex.ac.uk/people/mbate/Cluster/cluster3d.html PROTOSTAR: - Smallest fragment of a cool, dense gas cloud What happens: • Protostar shrinks due to gravity • Internal temperature increasing Why? • Gravity >>> Gas Pressure • Grav. Pot. Energy converted to heat >> Hydrogen Fusion begins << What happens: • H Fusion starts at center of protostar Why? • Temperature at center reaches 10 million K Questions & Answers: MAIN SEQUENCE STARSTAR:::: Q: Why are many stars (~50%) in binary or multiple What happens: star systems? • Becomes stable (normal) star, H fusion in A: Fragmentation of Cloud. core - Many stars are formed together in Why? the same region • Core and envelope are 75% Hydrogen • Energy source stops collapse Q: Why are 90% of all stars on the Main Sequence? • Establishes Hydrostatic & Thermal Equil. A: Main Sequence = H fusion. - Every star starts with H fusion - Stars undergo H fusion for most of their life Q: Why are there many more low mass stars (M) than high mass stars (O)? A: Fragmentation of Cloud - much more likely to have many small “lumps” or fragments - much less likely to have few massive “lumps” or fragments Q. Why is there a maximum mass for a star and a Main Sequence Lifetimes: minimum mass? - determined by amount of fuel - rate of fuel consumption A: Heat from Gravitational Collapse - Too much mass: so much heat that the gas TM.S. = Mass / Luminosity pressure overwhelms gravity (> 150 M ) - Too little mass: not enough heat to start H fusion. (< 0.08 M ) - Object becomes Brown Dwarf .