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 ~ 107 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/cm3 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

M20 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 18th 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 Now you don’t

• (infrared frequencies) (visible frequencies)

Rho Ophiuchi (visible light) Rho Ophiuchi (infrared)

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”)

Trifid Nebula (M20)

Good example for dark dust lanes in front of an emission nebula

Emission Nebulae Example: Orion Nebula (M 42)

• hot glowing gasTemperatures ~ 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”)