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

2. 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

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

4. 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

5. H II Regions

6. The Orion Region

7. 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,
Msun
Sites of star formation

8. 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

9. M16 the Eagle Nebula – “Pillars of Creation”

10. 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