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(dark matter) (dark matter) (dark matter) Stellar halo Bulge (= bar) (dark matter) (dark matter) Disk • thick • thin • gas (dark matter) (dark matter) The Interstellar Medium • = the gas disk • Includes ionized, neutral, molecular gas (H+, H0, H2) = (H I, H II, H2) Molecular clouds [12.1,12.2] • Massive interstellar gas clouds • Up to ~105 M • 100’s of LY in diameter. Fraction by mass H 73% He 25% Metals 2% • High density by interstellar medium standards • Up to 105 atoms 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. 1 CO map of Milky Way Molecular clouds are in the disk and are associated with dust. Optical (lII = +/- 90o) Galactic Latitude Galactic Latitude 20° 20 (a) 10° 10 0° 0 -10° -10 -20° -20 90° 90 70° 70 60° 60 50° 50 40° 40 30° 30 20° 20 10° 10 0° 0 350° -10 340° -20 330° -30 320° -40 310° -50 300° -60 290° -70 280° -80 270° -90 -30 330° -40 320° -50 310° -60 300° -70 290° -80 280° -90 270° (b) 10° 10 Galactic Latitude Galactic Latitude 80° 80 Mm-wave map of CO within 2.5 kpc Galactic Longitude 20° 20 50 100 150 opt_inner 200 250 0°0 -10° -10 -20° -20 90 90° 80 80° 70 70° 60 60° 50 50° 40 40° 30 30° 20 20° 10 10° 00° -10 350° Galactic Longitude Galactic Longitude -20 340° Dame, Hartmann & Thaddeus (2000, ApJ) Figure 6 M101 H I (21 cm) traces spiral structure M51 Molecular clouds occur in spiral arms CO contours over red image CO contours over 21 cm map 2 Computer simulation of Star Formation in a Molecular Cloud By Matthew Bate, University of Exeter UK • Collapse and fragmentation of 50 solar-mass cloud. • Initially 1 light-year in diameter. Star 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). 3 Orion 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 Clouds Total complex 4 x 105 M View on sky Pla ne 1.0 x 105 M 0.8 0.9 Ga lac tic Orion A Orion B Mon R2 Edge-on view Sun 4 recombination The 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. photoionization Ne III O II S II Ar III O II Ionizing O star He I pc Flux Æ 6 0. OI Hα O III Hγ Hα Hβ S III Wavelength Æ HII region is small cavity at edge of much bigger molecular cloud Molecular cloud Earth Ionized gas Ionizing stars • Ionized region has “blown out” of near side of dense cloud. • “Blister” HII Region • Many more similar star-formation regions buried deep inside cloud. 5 M16 “Pillars of Creation” EGGs: Evaporating Gaseous Globules of denser gas. Contain stars in process of formation. NGC 3603 • Dense molecular clouds are being eroded away by ionizing radiation from newly-formed stars. Nearby star-forming regions: SOAR mosaic 100 O stars M 17 SOAR mosaic SOAR mosaic 200 O stars M 17 NGC 3603 12 O stars One O star 30 Doradus, in the Large Magellanic Cloud Orion 6 1 kpc NGC 300 1 kpc Most distant known galaxy I Zw 18 30 Dor LMC 30 Dor 30 Doradus is prototype of “starbursts” seen in more distant galaxies. 6 deg 7