The Formation of StarsChapter 11

The last chapter introduced you to the gas and dust between the stars. Here you will begin putting together observations and theories to understand how nature makes stars. That will answer four essential questions:

• How are stars born?

• How do stars make energy?

• How do stars maintain their stability?

• What evidence do astronomers have that theories of star formation are correct?

Guidepost

Astronomers have developed a number of theories that explain the birth of stars. Are they true? That raises one of the most important questions you will meet concerning science:

• How certain can a theory be?

As you learn how nature makes new stars you will see science in action as evidence and theory combine to produce real understanding.

Guidepost

I. Making Stars from the Interstellar MediumA. Star Birth in Giant Molecular CloudsB. Heating By ContractionC. Protostars (原恆星 )D. Evidence of Star Formation

II. The Source of Stellar EnergyA. A Review of the Proton-Proton ChainB. The CNO Cycle

III. Stellar StructureA. Energy TransportB. What Supports the Sun?C. Inside StarsD. The Pressure-Temperature Thermostat

Outline

IV. The Orion NebulaA. Evidence of Young Stars

Outline (continued)

The Life Cycle of StarsDense, dark clouds, possibly forming stars in the future

Young stars, still in their birth

nebulae

Aging supergiant

Credit: You-Hua Chu (UIUC)

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分子雲- 孕育恆星的搖籃

Giant Molecular Clouds

• Size ~ 50-200 pc• M~104-106M⊙

• H2 ~ 300 #/cm3

– Most of the molecular gas are in modest extinction regions

• T ~ 8-50K（ peak at mm, submm, infrared）

Molecular Cores

VisibleInfrared

Barnard 68

Star formation → collapse of the cores of giant molecular clouds: Dark, cold, dense clouds obscuring the light of stars behind them

(More transparent in infrared light)

Parameters of Molecular Cores

Temp.: a few K

R ~ 0.1 pc

M ~ 1 Msun

Much too cold and too low density to ignite thermonuclear processes

Clouds need to contract and heat up in order to form stars.

Contraction of Molecular Cores

• Thermal Energy (pressure)

• Magnetic Fields

• Rotation (angular momentum)

External trigger can help to initiate the collapse of clouds

to form stars. Horse Head Nebula

• Turbulence

Factors resisting the collapse of a gas cloud:

Shocks Triggering Star Formation

Globules = sites where stars will form or are being born right now!

Trifid Nebula

Sources of Shock Waves Triggering Star Formation (1)

Previous star formation can trigger further star formation through:

a) Shocks from supernovae

(explosions of massive stars):

Massive stars die young =>

Supernovae tend to happen near sites of recent star formation

Sources of Shock Waves Triggering Star Formation (2)

Previous star formation can trigger further star formation through: b) Ionization

fronts of hot, massive O or B

stars which produce a lot of

UV radiation:

Massive stars die young => O and B stars only exist

near sites of recent star formation

Sources of Shock Waves Triggering Star Formation (3)

Giant molecular clouds are very large and may occasionally

collide with each other

c) Collisions of giant

molecular clouds

Sources of Shock Waves Triggering Star Formation (4)

d) Spiral arms in galaxies like our Milky Way:

Spirals’ arms are probably

rotating shock wave patterns.

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

Heating By ContractionAs a protostar contracts, it heats up:

Free-fall contraction→ Heating

From Protostars to StarsHigher-mass stars evolve more rapidly from protostars to stars

Protostellar Disks

Conservation of angular momentum leads to the formation of protostellar disks birth place of planets and moons

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 (2)

Herbig-Haro Object HH34

Protostellar Disks and Jets – Herbig-Haro Objects (3)

Herbig-Haro Object HH30

From Protostars to Stars

The Birth Line:

Star emerges from the enshrouding dust cocoon

Evidence of Star Formation

Nebula around S Monocerotis:

Contains many massive, very young stars,

including T Tauri Stars: strongly variable; bright

in the infrared.

Evidence of Star Formation (2)

The Cone Nebula

Optical Infrared

Young, very massive star

Smaller, sun-like stars,

probably formed under

the influence

of the massive

star.

Evidence of Star Formation (3)

Star Forming Region RCW 38

Globules

~ 10 to 1000 solar

masses;

Contracting to form

protostars

Bok Globules:

Globules (2)Evaporating Gaseous Globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars

Open Clusters of StarsLarge masses of Giant Molecular Clouds => Stars do not form isolated, but in large groups, called Open Clusters of Stars.

Open Cluster M7

The Source of Stellar Energy

In the sun, this happens primarily through the proton-proton (PP) chain

Recall from our discussion of the sun:

Stars produce energy by nuclear fusion of hydrogen into helium.

The CNO Cycle

In stars slightly more massive than the sun, a more powerful

energy generation mechanism than

the PP chain takes over:

The CNO Cycle.

Energy TransportEnergy generated in the star’s center must be

transported to the surface.

Inner layers:

Radiative energy transport

Outer layers (including photosphere):

Convection

Bubbles of hot gas rising up

Cool gas sinking downGas particles

of solar interior-rays

Stellar Structure

Temperature, density and pressure decreasing

Energy generation via nuclear fusion

Energy transport via radiation

Energy transport via convection

Flo

w o

f en

erg

y

Basically the same structure for all stars with approx. 1 solar

mass or less

Sun

Hydrostatic Equilibrium

Imagine a star’s interior composed of individual

shells

Within each shell, two forces have to be in equilibrium with

each other:

Outward pressure from the interior

Gravity, i.e. the weight from all layers above

Hydrostatic Equilibrium (2)

Outward pressure force must exactly balance the

weight of all layers above everywhere in

the star.

This condition uniquely determines the interior structure of the star.

This is why we find stable stars on such a narrow strip

(Main Sequence) in the Hertzsprung-Russell diagram.

H-R Diagram (showing Main Sequence)

Energy Transport Structure

Inner radiative, outer convective

zone

Inner convective, outer radiative

zone

CNO cycle dominant PP chain dominant

Summary: Stellar Structure

MassSun

Radiative Core, convective envelope;

Energy generation through PP Cycle

Convective Core, radiative envelope;

Energy generation through CNO Cycle

The Orion Nebula: An Active Star-Forming Region

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

Visual image of the Orion Nebula

Protostars with protoplanetary disks

B3

B1B1

O6

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Star formation standard model (star formation paradigm)

Shu et al. 1987Shu et al. 1987

Standard Evolutionary Scenario

Cloud collapse Protostar with disk

infall

outflow

Formation planets Solar system

Factor 1000 smaller

t=0 t=105 yr

t=106-107 yr t>108 yr

Single isolated low-mass star

n~104-105 cm-3

T~10 K

n~105-108 cm-3

T~10-300 K

Stages

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The Standard model (current version)