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0. Star Formation. 0. 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 . 0. Parameters of Giant Molecular Clouds. Size: r ~ 50 pc.

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Star Formation


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Giant Molecular Clouds

Barnard 68

Infrared

Visible

Star formation← collapse of the cores ofgiant molecular clouds:Dark, cold, dense cloudsobscuring the light of stars behind them.


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Parameters of Giant Molecular Clouds

Size: r ~ 50 pc

Mass: ~ 106 Msun

Temp.: ~ 20 K

Density: ~ 100 – 300 cm-3

Hot, dense cores:

r ~ 0.05 - 1 pc

M ~ 10 - 100 Msun

T ~ 100 - 200 K

n ~ 107 – 109 cm-3

Bok globules:

r ~ 1 pc

M ~ 1 - 1000 Msun

T ~ 10 K

n ≥ 104 cm-3


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Trifid Nebula

Bok Globules


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Globules

Bok Globules:

~ 10 – 1000 solar masses;

Contracting to form protostars


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Globules

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


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Fragmentation

  • MJ ~ T3/2r-1/2

  • ~ r-1/2

  • during isothermal collapse

  • Fragmentation

    => Stars do not form isolated, but in large groups, calledOpen Clusters of Stars.

Open Cluster M7



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Protostars

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.


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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


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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


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Young Star Clusters

→ O/B associations

Stars fully evolved towards ZAMS

8 million years

Containing very young, hot O/B stars

Luminosity

Stars still contracting and evolving towards ZAMS

ZAMS

30 million years

Temperature


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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 nH2a

a ≈ 3.1x10-13 (T/8000 K)-1/2 cm3 s-1

rs = [(3 N>13.6 eV) / (4 p nH2a)]1/3


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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


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Protostellar Disks and Jets – Herbig Haro Objects (II)

Herbig Haro Object HH34


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Protostellar Disks and Jets – Herbig Haro Objects (III)

Herbig Haro Object HH30


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The Orion Nebula: An Active Star-Forming Region


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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: ~ 1000 very young, hot stars

The Orion Nebula


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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


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