1. Star Formation (Compare: Solar System Formation)

2. Where Stars come from: the Interstellar Medium Gas –Single atoms and molecules –Mostly hydrogen (90%), 9% helium; deficient in heavier elements Dust –Microscopic clumps of atoms/molecules –Size ~ 10  7 m, similar to the wavelength of visible light –Composition is not well known Temperature depends on the proximity of stars, typically ~100 K Density is very low! –Gas: about 1 atom/cm 3 D; Dust: even less dense

3. How do we know it’s there? Cold gas or dust doesn’t glow –they are dark –We might “see” them blocking light of other objects (Dark Nebulae) Gas & Dust clouds are very dilute –they might not be blocking other object’s light totally –Usually they will reduce (redden) the light of other objects

4. Reminder: Kirchhoff’s Laws Cool gas absorbs light at specific frequencies  Dark Lines: “fingerprints of the elements”

5. Looking Through Dust Clouds

6. Seeing Through Gas and Dust EM radiation is appreciably scattered or absorbed only by particles with size comparable to its wavelength (or larger) Gas –Emission and absorption lines –Doesn’t block EM radiation Dust –Grain size is comparable to the wavelength of visible light –Dims visible light and high frequency EM radiation –Transparent to longer wavelength radio and infrared radiation, though

7. Scattering in Earth’s Atmosphere

8. The Interstellar Medium Dust dims and reddens the light from distant stars

9. Dust Clouds What happens to the blue light scattered by the dust clouds? It’s still there, and sometimes can be seen M20 Pleiades

10. Nebulae Any irregularly shaped cloud of gas and dust May be bright or dark, depending on temperature Types: –Emission (bright) Nebulae –Dark Nebulae –Reflection Nebulae Historic Remark: Only some of the 109 “nebulae” catalogued by Charles Messier in 18th Century are actual nebulae; most are star clusters and galaxies

11. Dark Nebulae Classic Example: Horsehead Nebula in Orion Can’t see what’s behind a dark nebula, that’s why we see it!

12. Dark Nebulae Dark Nebulae do emit light of their own, though Temperatures ~ 10 to 100 K; black body radiation peaks in the radio to infrared frequencies f peak in infrared frequencies

13. Dark Nebulae Now you see it  Now you don’t (infrared frequencies) (visible frequencies) Rho Ophiuchi (visible light) Rho Ophiuchi (infrared)

14. Emission Nebulae Regions of hot glowing gas –Temperatures ~ 8000K Made to glow by ultraviolet radiation emitted by new O- or B-type (hot) stars located inside Emission lines from the nebula are easily distinguished from the continuous spectrum and absorption lines of stars within Color predominantly red, the color of a particular hydrogen emission line (the “H  line”)

15. Trifid Nebula (M20) Good example for dark dust lanes in front of an emission nebula

16. Emission Nebulae Example: Orion Nebula (M 42) hot glowing gas Temperatures ~ 8000K Made to glow by ultraviolet radiation emitted by young O- or B-type (hot) stars located inside Color predominantly red, the color of a particular hydrogen emission line (“H  ”)