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• I nterstellar dust • Interstellar gas • Birth of stars

Stellar nurseries. • I nterstellar dust • Interstellar gas • Birth of stars. Interstellar dust. Extinction and reddening Dust: grains of various sizes (~ 0.1 to 1  m). non uniform distribution in galaxies – associated to interstellar gas Scatters light Cross section ~ 1/ λ

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• I nterstellar dust • Interstellar gas • Birth of stars

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  1. Stellar nurseries • Interstellar dust • Interstellar gas • Birth of stars

  2. Interstellar dust Extinction and reddening Dust: grains of various sizes (~ 0.1 to 1 m) non uniform distribution in galaxies – associated to interstellar gas Scatters light Cross section ~ 1/λ → absorption goes down from UV to IR → extinction + reddening (similar to sunset) Typical extinction curve

  3. Interstellar dust - 2 Dense clouds When optical depth >> 1 in the optical → opaque → block light from background sources → appear like `holes´ To see through: observe in IR (or X-rays) (these clouds emit in far-IR) Barnard 68 cloud (ESO-VLT) – size ~ 0.5 L.Y.

  4. Interstellar gas HIIregions Most spectacularinterstellargasclouds Size up to 500 pc Density: ρ ~ 100 mp/m3 ~ 10–25× ρatm (typical atm. lensity at sea level) → deep vacuum Orion nebula (HST mosaic)

  5. E I n=3 hν n=2 n=1 Interstellar gas - 2 Emission from HIIregions Gas (mostlyhydrogen) ionised by stellar radiation I = 13.6 eV = 2.2 ×10–18 J → λ < 90 nm (far-UV) Only hot stars (O and B) emit enough energy at these wavelengths [ spectroscopic notation: XI = neutral X, XII = X ionized once, XIII = X ionized twice... → HII = ionized H ] Recombination e– with proton → H excited → emission due to de-excitation (in the visible: Balmer series down to level n = 2)

  6. E n=1 hν Interstellar gas - 3 HI regions Contain most of the interstellar gas, size ~20 L. Y., M ~50 M ~ 100 000 times denser than HII regions (ρ ~ 107mp/m3) T ~ 10 to 100 K → emit in IR Easiest detection: 21 cm line of neutral hydrogen Splitting of fundamental level due to hyperfine structure (interaction between nucleus spin and orbital angular momentum of the e–) e– thermally excited if T > ~0.1 K kB = 1.38 ×10–23 J/K (Boltzmann constant)

  7. Interstellar gas - 4 Molecular clouds Higher density → gas atoms combine into molecules (or radicals) H2: the most abundant, but very hard to detect Detection from CO radio emission T ~ 10 to 100 K ρ ~ 1010mp/m3 Giant clouds: ~ 40 L.Y. M ~ 105M « Horsehead » nebula (CFHT)

  8. Interstellar gas - 5 Interstellar molecules Many molecules discovered, including organic ones Examples: H2 CO H2O CH4 NH3 CH3OH HCOOH (formic acid) CH3O2 (dimethyl ether) HCN (hydrogen cyanide) Carina nebula (HST)

  9. Interstellar gas - 6 Colours of nebulae When images are in true colours (combinations of filters)... Emission from heated gas → red (mostly H) Reflexion of stellar light by dust (less opaque in red) → blueish Absorption of background light → dark Upper left: reflexion of Antares light (red supergiant, hardly emits blue light → yellowish) Antares and Rho Ophiuchi (AAO)

  10. Birth of stars Triggering of star formation If cloud density high enough → collapses under its own gravity Collapse favored when an external source compresses the cloud: • galactic spiral arm (higher density region rotating at a speed ≠ from stars and gas) • supernova explosion → shock wave • winds from neighbouring stars M17, Omega nebula (HST)

  11. Birth of stars - 2 Globules `Globules´ form The most massive/denser collapse faster → hottest, most massive stars form first For the others, competition between gravitational collapse and radiation from hot stars (ionize matter) Rem: image colours R (red) = SII (673 nm) G (green) = H (656 nm) B (blue) = OIII (501 nm) Gas pillars in M16 (HST)

  12. Birth of stars - 3 Gravitational collapse Kelvin – von Helmholz contraction → release of energy → T and L increase Ex: the protosun reached 500 L High energy release during a short time Image: Thackeray globules in IC2944 cluster Size ~ 1 L.Y., M ~ 15 M Globules in IC2944 (HST)

  13. Birth of stars - 4 Birth of the star • M > 0.08 M → the stellar core reaches a T° high enough for hydrogen fusion • M < 0.08 M → no 1H fusion Brief episode of deuterium fusion Stabilisation at R ~ RJupiter Residual heat emission → L decreases from ~ 10–3 to ~ 10–6 L → brown dwarf (`failed star´) • M < 1.3% M → no 2H fusion → planet Molecular cloud BHR71 (VLT)

  14. Birth of stars - 4 Protostars Conservation of angular momentum • Collapse → R decreases → v increases → the cloud flattens (centrifugal force) → disk around the central star (→ possible planets) • Start of nuclear reactions in stellar core → radiation pressure on circumstellar matter → jets of matter in direction perpendicular to the disk Young stars (HST)

  15. Birth of stars - 6 Herbig-Haro objects = nebulae associated to stellar formation • Protostar inside a dense cloud • Surrounded by a protoplanetary disk • Jets perpendicular to the disk (v ~ 250 km/s) • Shock waves when jets collide with surrounding matter → compression, heating and emission Image: HH-34, in the Orion nebula region One of the jets hidden by dust Lobes at ~ 1 L.Y. from the star H emission (here associated with green colour) HH-34 (VLT)

  16. Stellar nurseries • Interstellar dust • Interstellar gas • Birth of stars End of chapter…

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