The Formation of Stars

Fox Fur Nebula

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The Life Cycle of StarsDense, dark

clouds, possibly forming stars in

the future

Young stars, still in their birth

nebulae

Ageing supergiant

Movie: Lifes of Stars

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.

(More transparent in infrared light.)

How hot does the core of a cloud/star need to be to ignite nuclear fusion?

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1. 1,000 K

2. 10,000 K.

3. 100,000 K.

4. 1 million K.

5. 10 million K.

Parameters of Giant Molecular CloudsSize: r ~ 50 pcMass: > 100,000 Msun

Dense cores:

Temp.: a few 0K

R ~ 0.1 pcM ~ 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.

External trigger required to initiate the collapse of clouds to form stars.

→ Compression waves (shocks)

Which of the following is an example of a shock wave?

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1. A large ocean wave.

2. The tides.

3. The loud, sudden bang of the sound of a supersonic air plane.

4. A Tsunami.

5. An Earthquake.

Shocks Triggering Star Formation

Globules = sites where stars are being born right now!

Trifid Nebula

Sources of Shock Waves Triggering Star Formation

Previous star formation can trigger further star formation through:

a) Shocks from supernovae (explosions of massive stars):

Which stars live the shortest lives?

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1. Very massive O and B stars.

2. Very massive K and M stars.

3. Intermediate-mass A and F stars.

4. Low-mass O and B stars.

5. Low-mass K and M stars.

Sources of Shock Waves Triggering Star Formation

Previous star formation can trigger further star formation through:

a) Shocks from supernovae

(explosions of massive stars):

Massive (O, B) stars die young =>

Supernovae tend to happen near sites of recent star formation

What is “Ionization”?

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1. Electrons being ejected out of an atom.

2. Electrons being lifted into an excited state in an atom.

3. Electrons annihilating on their anti-particles.

4. Two atomic nuclei combining to form a heavier nucleus.

5. A heavy atomic nucleus being split up into two lighter nuclei.

Sources of Shock Waves Triggering Star Formation

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

c) Spiral arms in galaxies like our Milky Way:

Spirals arms are probably

rotating shock wave patterns.

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

O/B Associations

Special type of Open Cluster:

Orion Nebula; Trapezium Stars

Very young clusters still contain very massive, very hot O and B

stars, dominating the luminosity of the association and ionizing large

regions of gas around them.

(Bok) Globules

~ 10 – 1000 solar masses;

Contracting to form protostars

Compact, dense pockets of gas which may contract to form stars.

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

exposed by the ionizing radiation from nearby massive stars

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.

What happens to a rotating gas cloud when it contracts?

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1. Its rate of rotation will slow down, and it takes on a disk-like shape.

2. Its rotation will slow down, but its shape remains approximately spherical.

3. Its rate of rotation will remain constant, and its shape remains approximately spherical.

4. Its rate of rotation will speed up, and it takes on a disk-like shape.

5. Its rotation will speed up, but its shape remains approximately spherical.

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

Herbig Haro Object HH30

Disks of matter accreted onto the protostar (“accretion

disks”) often lead to the formation of jets (directed

outflows): Herbig Haro Objects

Once thermonuclear fusion in the center of a star is ignited, the star will heat up and become

more luminous. Thus, which way will it “move” in the Hertzsprung-Russell (HR) diagram?

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1. Towards the lower right.

2. Towards the lower left.

3. Towards the upper right.

4. Towards the upper left.

5. It will basically stay at the same point in the HR diagram.

From Protostars to Stars

Ignition of H → He fusion

Birth Line: Star emerges

from the enshrouding dust cocoon

Star Forming Region RCW 38