1. Star Formation

2. Giant Molecular Clouds
Barnard 68
Infrared
Visible
Star formation ← collapse of the cores of giant molecular clouds: Dark, cold, dense clouds obscuring the light of stars behind them.

3. Parameters of Giant Molecular Clouds
Size: r ~ 50 pc
Mass: ~ 106 Msun
Temp.: ~ 20 K
Density: ~ 100 – 300 cm-3
Hot, dense cores:
r ~ pc
M ~ Msun
T ~ K
n ~ 107 – 109 cm-3
Bok globules:
r ~ 1 pc
M ~ Msun
T ~ 10 K
n ≥ 104 cm-3

4. Trifid Nebula
Bok Globules

5. Contracting to form protostars
Globules
Bok Globules:
~ 10 – 1000 solar masses;
Contracting to form protostars

6. Globules
Evaporating Gaseous Globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars

7. during isothermal collapse
Fragmentation
MJ ~ T3/2 r-1/2
~ r-1/2
during isothermal collapse
Fragmentation
=> Stars do not form isolated, but in large groups, called Open Clusters of Stars.
Open Cluster M7

8. Free Fall of a GMC Core

9. Protostars Protostars = pre-birth state of stars:
Protostars = pre-birth state of stars:
Hydrogen to Helium fusion not yet ignited
Still enshrouded in opaque “cocoons” of dust => barely visible in the optical, but bright in the infrared.

10. Pre-Main-Sequence Evolution
Hydrostatic equilibrium reached
→ Adiabatic collapse
→ Heating
Isothermal collapse of the protostar: Gravitational energy release in equilibrium with radiative cooling:
Hayashi Track
Isothermal collapse
→ Formation of a protostellar core

11. Pre-Main-Sequence Evolution
Star emerges from the enshrouding dust cocoon
Hayashi Track
First thermonuclear reactions:
p + 21H → 32He + g
Radiative core develops
p + p → 21H + e+ + ne
and 126C-burning steps of CNO cycle set in
Core expands; T drops
126C supply exhausted; final adjustment to ZAMS

12. Containing very young, hot O/B stars
Young Star Clusters
Containing very young, hot O/B stars
→ O/B associations
Stars fully evolved towards ZAMS
8 million years
Luminosity
Stars still contracting and evolving towards ZAMS
ZAMS
30 million years
Temperature

13. rs ~ 0.3 pc for a typical O6 star
HII Regions
The Strömgren Sphere
rs ~ 0.3 pc for a typical O6 star
H ionization rate = recombination rate
N>13.6 eV = (4/3) p rs3 nH2 a
a ≈ 3.1x10-13 (T/8000 K)-1/2 cm3 s-1
rs = [(3 N>13.6 eV) / (4 p nH2 a)]1/3

14. Protostellar Disks and Jets – Herbig Haro Objects
Disks of matter accreted onto the protostar (“accretion disks”) often lead to the formation of jets (directed outflows; bipolar outflows): Herbig Haro Objects

15. Protostellar Disks and Jets – Herbig Haro Objects (II)
Herbig Haro Object HH34

16. Protostellar Disks and Jets – Herbig Haro Objects (III)
Herbig Haro Object HH30

17. The Orion Nebula: An Active Star-Forming Region

18. The Trapezium
The 4 trapezium stars: Brightest, very young (less than 2 million years old) stars in the central region of the Orion nebula
Only one of the trapezium stars is hot enough to ionize hydrogen in the Orion nebula
Infrared image: ~ 50 very young, cool, low-mass stars
X-ray image: ~ very young, hot stars
The Orion Nebula

19. Spectral types of the trapezium stars
Kleinmann-Low nebula (KL): Cluster of cool, young protostars detectable only in the infrared
The Becklin-Neugebauer Object (BN): Hot star, just reaching the main sequence
Spectral types of the trapezium stars B3 B1 B1 O6
Visual image of the Orion Nebula
Protostars with protoplanetary disks