Star Formation Compare Solar System Formation Where Stars

Star Formation (Compare: Solar System Formation)

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

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

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

Looking Through Dust Clouds

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

Scattering in Earth’s Atmosphere

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

Dust Clouds • What happens to the blue light scattered by the dust clouds? • It’s still there, and sometimes can be seen M 20 Pleiades

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 18 th Century are actual nebulae; most are star clusters and galaxies

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

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 fpeak in infrared frequencies

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

Emission Nebulae • Regions of hot glowing gas – Temperatures ~ 8000 K • 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”)

Trifid Nebula (M 20) Good example for dark dust lanes in front of an emission nebula

Emission Nebulae Example: Orion Nebula (M 42) • hot glowing gas Temperatures ~ 8000 K • 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 ”)