1Bulge (= bar)Stellar haloDisk•thick•thin• gas(dark matter)(dark matter)(dark matter)(dark matter)(dark matter)(dark matter)(dark matter)Molecular clouds [12.1,12.2]• Massive interstellar gas clouds• Up to ~105M• 100’s of LY in diameter.• High density by interstellar medium standards• Up to 105atoms per cm3• Shielded from UV radiation by dust, so atoms combined into molecules.•Mostly H2, but not easily detectable•Also H2O, NH3, CO etc.• form emission lines in observable passbands • CO is usual tracer.– mm wave observations (Î low angular resolution)• All stars form in molecular clouds.The Interstellar Medium• = the gas disk• Includes ionized, neutral, molecular gas (H+, H0, H2) = (H I, H II, H2)Fraction by massH 73%He 25%Metals 2%290 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -9020100-10-20Galactic LongitudeGalactic Latitude50 100 150 200 250opt_inner90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -9020100-10-20Galactic LongitudeGalactic LatitudeGalactic Longitude0°10°20°-10°-20°Galactic Latitude0°10°20°-10°-20°Galactic Latitude(b)(a)0°10°20°30°40°50°60°70°80°90° 350° 340° 330° 320° 310° 300° 290° 280° 270°0°10°20°30°40°50°60°70°80°90° 350° 340° 330° 320° 310° 300° 290° 280° 270°Figure 6CO map of Milky WayOptical (lII= +/- 90o)Mm-wave map of CO within 2.5 kpcMolecular clouds are in the disk and are associated with dust.Dame, Hartmann & Thaddeus (2000, ApJ) M101H I (21 cm)traces spiral structureCO contours over red imageCO contours over 21 cm mapM51Molecular clouds occur in spiral arms3Computer simulation of Star Formation in a Molecular Cloud• Collapse and fragmentation of 50 solar-mass cloud.• Initially 1 light-year in diameter.ByMatthew Bate,University of ExeterUKStar formation thought to propagate in wave through dense molecular clouds• Photons from very luminous O stars heat and blow away surrounding gas.• So slightly older clusters no longer shrouded by dusty gas• Compression of gas further inside cloud causes inward wave of star formation (“triggered” star formation).4Orion Molecular Cloud• 100 pc across• 200,000 M• Only a few of its stars close to the near edge can be seen in visible light.• But infrared images penetrate the dust and show many more stars.The Orion-Monoceros Molecular CloudsOrion A 1.0 x 105MOrion B 0.8Mon R2 0.9Total complex 4 x 105MSunGalactic PlaneView on skyEdge-on view5The Orion Nebula• Ionized by UV phots from hot star.• Recombination lines from H, He.• + lines from heavier elements due to collisional excitation.• Î measure chemical abundances.Ionizing O starHαHβHγO IIIHαS IIO IIAr IIIHe IS IIINe IIIO IIO IFlux ÆWavelength Æphotoionizationrecombination0.6 pcHII region is small cavity at edge of much bigger molecular cloud• Ionized region has “blown out” of near side of dense cloud.• “Blister” HII Region• Many more similar star-formation regions buried deep inside cloud.MolecularcloudIonized gasIonizing starsEarth6M16 “Pillars of Creation”• Dense molecular clouds are being eroded away by ionizing radiation from newly-formed stars.EGGs: EvaporatingGaseous Globules of denser gas. Contain stars in process of formation.NGC 360330 Doradus, in theLarge Magellanic CloudNearby star-forming regions:NGC 3603M 17OrionSOAR mosaicSOAR mosaicM 17SOAR mosaicOne O star12 O stars100 O stars200 O stars76 deg30 DorI Zw 18NGC 3001 kpc30 Doradus isprototype of “starbursts” seen in more distant galaxies.30 DorLMC1 kpcMost distant known
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