The Interstellar Medium and Star Formation Material between the stars – gas and dust

Interstellar Dust -- the general obscuration and discrete dark clouds Dark nebulae Dust grains cause dimming and reddening of starlight due to absorption and scattering of light Scattering is wavelength dependent – blue light scattered more

Reflection nebulae -- dust grains reflect the light of embedded stars

Interstellar gas -- how do we “see” the gas? Exists in both cold (neutral hydrogen H I) and hot ( ionized hydrogen H II) “hot” gas – diffuse or emission nebulae – H II regions Photoionization – hydrogen ionized by UV radiation from embedded hot, young stars Free electrons captured and “recombine” with atom – producing emission as electron cascades down through energy levels Cold or H I (neutral) gas – produces an emission line at 21 cm – detected in radio

H II Regions

The Orion Region

Interstellar Molecules and Giant Molecular Clouds (GMCs) Numerous simple and complex molecules found in ISM - in cold, dark clouds, concentrated in the dark nebulae especially in the GMC’s GMCs -- Very large & massive - > 100 pc in diameter, 105 -- 106 Msun Sites of star formation

Star Formation Sites – cold, dark clouds of dust & gas Gravitational collapse Small dense core forms first Heat of collapse trapped, Escaping radiation seen in infrared “Protostar” -- star in formation, no nuclear reactions - a strong IR source, embedded in dark nebulae Examples – dark “thumbs” in H II regions, small dense globules

M16 the Eagle Nebula – “Pillars of Creation”

Stars in formation surrounded by material from the nebula, eventually collapses into a disk – recall Solar nebula Images of young stars surrounded by circumstellar disks and jets of material -- both accretion and bi-polar outflows -- Herbig-Haro objects Eventually photosphere heats up, Becomes visible – a pre-main sequence object When central core reaches 107o K nuclear reactions – p-p chain begin For a star like the Sun – 30 x 106 yrs