1. Stellar nurseries • Interstellar dust • Interstellar gas
• Birth of stars

2. Extinction and reddening
Interstellar dust
Extinction and reddening
Dust: grains of various sizes (~ 0.1 to 1 m)
non uniform distribution in galaxies – associated to interstellar gas
Scatters light
Cross section ~ 1/λ
→ absorption goes down from UV to IR
→ extinction + reddening
(similar to sunset)
Typical extinction curve

3. Dense clouds When optical depth  >> 1 in the optical → opaque
Interstellar dust - 2
Dense clouds
When optical depth  >> 1 in the optical → opaque
→ block light from background sources
→ appear like `holes´
To see through:
observe in IR (or X-rays)
(these clouds emit in far-IR)
Barnard 68 cloud (ESO-VLT) – size ~ 0.5 L.Y.

4. Interstellar gas HII regions Most spectacular interstellar gas clouds
Size up to 500 pc
Density:
ρ ~ 100 mp/m3
~ 10–25 × ρatm (typical atm. lensity at sea level)
→ deep vacuum
Orion nebula (HST mosaic)

5. Emission from HII regions
Interstellar gas - 2
Emission from HII regions
Gas (mostly hydrogen) ionised by stellar radiation
I = 13.6 eV = 2.2 ×10–18 J → λ < 90 nm (far-UV)
Only hot stars (O and B) emit enough energy at these wavelengths
[ spectroscopic notation: XI = neutral X, XII = X ionized once, XIII = X ionized twice... → HII = ionized H ]
Recombination e– with proton → H excited
→ emission due to de-excitation (in the visible: Balmer series down to level n = 2) E I hν
n=1
n=2
n=3

6. Interstellar gas - 3
HI regions
Contain most of the interstellar gas, size ~20 L. Y., M ~50 M
~ times denser than HII regions (ρ ~ 107 mp/m3)
T ~ 10 to 100 K → emit in IR
Easiest detection: 21 cm line of neutral hydrogen
Splitting of fundamental level due to hyperfine structure (interaction between nucleus spin and orbital angular momentum of the e–)
e– thermally excited if T > ~0.1 K
kB = 1.38 ×10–23 J/K (Boltzmann constant) E hν
n=1

7. Interstellar gas - 4
Molecular clouds
Higher density → gas atoms combine into molecules (or radicals)
H2: the most abundant, but very hard to detect
Detection from CO radio emission
T ~ 10 to 100 K
ρ ~ 1010 mp/m3
Giant clouds: ~ 40 L.Y.
M ~ 105 M
« Horsehead » nebula (CFHT)

8. Interstellar molecules
Interstellar gas - 5
Interstellar molecules
Many molecules discovered, including organic ones
Examples:
H2 CO
H2O CH4
NH3 CH3OH
HCOOH (formic acid)
CH3O (dimethyl ether)
HCN (hydrogen cyanide)
Carina nebula (HST)

9. Interstellar gas - 6
Colours of nebulae
When images are in true colours (combinations of filters)...
Emission from heated gas → red (mostly H)
Reflexion of stellar light by dust (less opaque in red) → blueish
Absorption of background light → dark
Upper left: reflexion of Antares light (red supergiant, hardly emits blue light → yellowish)
Antares and Rho Ophiuchi (AAO)

10. Triggering of star formation
Birth of stars
Triggering of star formation
If cloud density high enough → collapses under its own gravity
Collapse favored when an external source compresses the cloud:
• galactic spiral arm (higher density region rotating at a speed ≠ from stars and gas)
• supernova explosion → shock wave
• winds from neighbouring stars
M17, Omega nebula (HST)

11. Globules `Globules´ form
Birth of stars - 2
Globules
`Globules´ form
The most massive/denser collapse faster → hottest, most massive stars form first
For the others, competition between gravitational collapse and radiation from hot stars (ionize matter)
Rem: image colours R (red) = SII (673 nm) G (green) = H (656 nm) B (blue) = OIII (501 nm)
Gas pillars in M16 (HST)

12. Gravitational collapse
Birth of stars - 3
Gravitational collapse
Kelvin – von Helmholz contraction
→ release of energy
→ T and L increase
Ex: the protosun reached L
High energy release during a short time
Image: Thackeray globules in IC2944 cluster
Size ~ 1 L.Y., M ~ 15 M
Globules in IC2944 (HST)

13. Birth of stars - 4
Birth of the star
• M > 0.08 M → the stellar core reaches a T° high enough for hydrogen fusion
• M < 0.08 M → no 1H fusion
Brief episode of deuterium fusion
Stabilisation at R ~ RJupiter
Residual heat emission
→ L decreases from ~ 10–3 to ~ 10–6 L
→ brown dwarf (`failed star´)
• M < 1.3% M → no 2H fusion
→ planet
Molecular cloud BHR71 (VLT)

14. Protostars Conservation of angular momentum
Birth of stars - 4
Protostars
Conservation of angular momentum
• Collapse → R decreases → v increases
→ the cloud flattens (centrifugal force)
→ disk around the central star (→ possible planets)
• Start of nuclear reactions in stellar core
→ radiation pressure on circumstellar matter
→ jets of matter in direction perpendicular to the disk
Young stars (HST)

15. Herbig-Haro objects = nebulae associated to stellar formation
Birth of stars - 6
Herbig-Haro objects
= nebulae associated to stellar formation
• Protostar inside a dense cloud
• Surrounded by a protoplanetary disk
• Jets perpendicular to the disk (v ~ 250 km/s)
• Shock waves when jets collide with surrounding matter
→ compression, heating and emission
Image: HH-34, in the Orion nebula region One of the jets hidden by dust Lobes at ~ 1 L.Y. from the star H emission (here associated with green colour)
HH-34 (VLT)

16. Stellar nurseries End of chapter… • Interstellar dust
• Interstellar gas
• Birth of stars
End of chapter…