Star Formation

Giant Molecular Clouds

VisibleInfrared

Barnard 68

Star formation ← collapse of the cores of giant molecular clouds: Dark, cold, dense clouds

obscuring the light of stars behind them.

Parameters of Giant Molecular Clouds

Size: r ~ 50 pcMass: ~ 106 Msun

Hot, dense cores:

Temp.: ~ 20 K

r ~ 0.05 - 1 pcM ~ 10 - 100 Msun

Density: ~ 100 – 300 cm-3

T ~ 100 - 200 Kn ~ 107 – 109 cm-3

Bok globules:

r ~ 1 pcM ~ 1 - 1000 Msun

T ~ 10 Kn ≥ 104 cm-3

Bok Globules

Trifid Nebula

Globules

Bok Globules:

~ 10 – 1000 solar masses;

Contracting to form protostars

GlobulesEvaporating Gaseous Globules (“EGGs”): Newly forming stars

exposed by the ionizing radiation from nearby massive stars

Free Fall of a GMC Core

Fragmentation

MJ ~ T3/2 -1/2

~ -1/2

during isothermal collapse

Fragmentation

=> Stars do not form isolated, but in large groups,

called Open Clusters of Stars.Open Cluster M7

Pre-Main-Sequence Evolution

→ Adiabatic collapse

Isothermal collapse

→ Formation of a protostellar core

→ Heating

Isothermal collapse of the protostar: Gravitational energy release in

equilibrium with radiative cooling:

Hayashi Track

Hydrostatic equilibrium reached

ProtostarsProtostars =

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.

Pre-Main-Sequence Evolution

Hayashi Track

Star emerges from the enshrouding dust cocoon

First thermonuclear

reactions:

p + 21H → 3

2He +

p + p → 21H + e+ + e

and 126C-burning

steps of CNO cycle set in

Radiative core develops

Core expands; T drops

126C supply exhausted; final

adjustment to ZAMS

Young Star ClustersContaining very young,

hot O/B stars

→ O/B associations

Lum

ino

sity

Temperature

ZAMS

Stars fully evolved

towards ZAMS

Stars still contracting

and evolving towards ZAMS

8 million years

30 million years

HII Regions

The Strömgren Sphere

H ionization rate = recombination rate

N>13.6 eV = (4/3) rs3 nH

2

rs = [(3 N>13.6 eV) / (4 nH2

≈ 3.1x10-13 (T/8000 K)-1/2 cm3 s-1

rs ~ 0.3 pc for a

typical O6 star

T Tauri Stars• Intermediate stage between dust-enshrouded

(IR sources) and ZAMS

• Large-amplitude variability on time scales of days• 0.5 – 3 M0

• Strong emission lines (Balmer, Ca II, Fe)• Forbidden lines [O], [SII]• Often associated with protostellar/protoplanetary disks

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

Protostellar Disks and Jets – Herbig Haro Objects (II)

Herbig Haro Object HH34

Protostellar Disks and Jets – Herbig Haro Objects (III)

Herbig Haro Object HH30

The Orion Nebula: An Active Star-Forming Region

The Trapezium

The Orion Nebula

The 4 trapezium stars: Brightest, very young

(less than 2 million years old) stars in the central region of the

Orion nebula

Infrared image: ~ 50 very young, cool, low-

mass starsX-ray image: ~ 1000 very young, hot stars

Only one of the trapezium stars is hot

enough to ionize hydrogen in the Orion

nebula

The Becklin-Neugebauer Object (BN): Hot star, just reaching the main

sequence

Kleinmann-Low nebula (KL): Cluster

of cool, young protostars

detectable only in the infrared

Spectral types of the trapezium

stars

Visual image of the Orion NebulaProtostars with protoplanetary disks

B3

B1

B1

O6