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SH2 136: A Spooky Nebula Ghoulish dust clouds: a region of star formation

Halloween corresponds roughly to the cross-quarter day: half-way between equinox and solstice

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Your next test will be November 14

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The Formation of StarsChapter 11

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Giant Molecular Clouds: stellar nurseries

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.)

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Giant molecular clouds – stellar nurseries

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Orion Deep FieldH-alpha line 656 nm (red light)

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Great Orion Nebula

1500 light-years away

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The Horsehead Nebula

Slide 9Star Forming Region RCW 38

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Parameters of Giant Molecular Clouds

Size: r ~ 50 pcMass: > 100,000 Msun

Dense cores:

Temp.: a few 0K

R ~ 0.1 pcM ~ 1 Msun

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

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Coldest spots in the galaxy:T ~ 1-10 K

Composition:• Mainly molecular hydrogen• 1% dust

EGGs = Evaporating Gaseous Globulesftp://ftp.hq.nasa.gov/pub/pao/pressrel/1995/95-190.txt

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Jeans instability:

Thermal pressure cannot support the gas cloud against its self-gravity. The cloud collapses and fragments.

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Contraction of Giant Molecular Cloud Cores

• Thermal Energy (pressure)

• Magnetic Fields

• Rotation (angular momentum)

External trigger required to initiate the collapse of clouds

to form stars.

Horse Head Nebula

• Turbulence

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Shocks Triggering Star Formation

Globules = sites where stars are being born right now!

Trifid Nebula

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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

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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

Slide 19The Bubble Nebula (Cassiopeia)

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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.

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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.

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Jeans instability:

Thermal pressure cannot support the gas cloud against its self-gravity. The cloud collapses and fragments.

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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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Heating By Contraction

As a protostar contracts, it heats up:

Free-fall contraction→ Heating

Heating does not stop contraction because the core cools down due to radiation

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Protostars: warm clumps of gas surrounded by infalling matter

Disks: planet formation?!

Why disks?

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Role of angular momentum

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Protostellar Disks

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

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2211 RMVRMV

12

1

1

2 R

R

V

V

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Protostellar Disks and Jets – Herbig Haro ObjectsDisks 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 (2)

Herbig Haro Object HH34

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Protostellar Disks and Jets – Herbig Haro Objects (3)

Herbig Haro Object HH30

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From a protostar to a young star: very hot; still accreting matter

Observed in the infrared, becauseinfalling gas and dust obscure light

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• The matter stops falling on the star• Nuclear fusion starts in the core• Planets can be formed from the remaining disk

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From Protostars to Stars

Ignition of H He fusion processes

Star emerges from the enshrouding dust cocoon

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Evidence of Star Formation

Nebula around S Monocerotis:

Contains many massive, very young stars,

including T Tauri Stars: strongly variable; bright

in the infrared.

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Evidence of Star Formation (2)

The Cone Nebula

Optical Infrared

Young, very massive star

Smaller, sunlike stars,

probably formed under

the influence

of the massive

star

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Evidence of Star Formation (3)

Star Forming Region RCW 38

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Globules

~ 10 to 1000 solar masses;

Contracting to form protostars

Bok Globules:

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

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Open Clusters of Stars

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

Open Cluster M7

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Open Clusters of Stars (2)

Large, dense cluster of (yellow and red) stars in the foreground; ~ 50 million years old

Scattered individual (bright, white) stars in the background; only ~ 4 million years old

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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.

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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.

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Net result is the same: four hydrogen nuclei fuse to form one helium nucleus

Why p-p and CNO cycles? Why so complicated?

Because simultaneous collision of 4 protons is too improbable

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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

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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.

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H-R Diagram (showing Main Sequence)

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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

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Conduction, Convection, and Radiation

(SLIDESHOW MODE ONLY)

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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

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Energy Transport Structure

Inner radiative, outer convective

zone

Inner convective, outer radiative

zone

CNO cycle dominant PP chain dominant

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Summary: Stellar Structure

MassSun

Radiative Core, convective envelope;

Energy generation through PP Cycle

Convective Core, radiative envelope;

Energy generation through CNO Cycle