Transcript
Page 1: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

The Formation and Structure of Stars

Chapter 9

Page 2: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

The Big Picture

• Stars exist because of gravity.

• Gravity causes interstellar material to collapse to form stars.

• Gravity determines how much energy stars generate.

• Gravity dictates how stars evolve and die.

• Mass determines gravity;Mass is the #1 property of a star.

Page 3: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

The space between the stars is not empty, but filled with very dilute gas and

dust, producing some of the most beautiful objects in the sky.

We are interested in the ISM because:

a) dense interstellar clouds are the birth place of stars

b) dark clouds alter and absorb the light from stars behind them

The Interstellar Medium (ISM)

Page 4: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Various Appearances of the ISM

Page 5: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Three Kinds of Nebulae

• Emission Nebula• Reflection Nebula• Dark Nebula

Page 6: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

The Fox Fur Nebula

NGC 2246

1) Emission Nebulae (HII Regions)

A very hot star illuminates a cloud of hydrogen gas;

Its ultraviolet light ionizes hydrogen;

Electrons recombine with protons, cascade down to the ground state, and produce emission lines, dominated byred Hα photons.

The Keyhole Nebula

Page 7: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

2) Reflection Nebulae

Star illuminates a nearby cloud of gas and dust;

Blue light is much more likely to be scattered by dust than red light;

Reflection nebula appears blue.

*The same physics for the blue sky and the red sunset!

Page 8: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Emission and Reflection Nebulae

Page 9: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

3) Dark Nebulae

Barnard 86

Dense clouds of gas and dust absorb the light from the stars behind;

Appear dark in front of the brighter background, which is often an emission nebula.

Horsehead Nebula

Page 10: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Interstellar Reddening

Visible Infrared

Barnard 68

Blue light is strongly scattered and absorbed by interstellar (dust) clouds.

Red light can more easily penetrate the cloud, but is still absorbed to an extent (“interstellar extinction”).

(Infrared radiation is hardly absorbed.)

Interstellar clouds make background stars appear

redder.

The physics of reflection nebula revisited!

Page 11: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Interstellar Absorption LinesThe interstellar medium produces

absorption lines in the spectra of stars.

Distinguished from stellar absorption lines via:

a) Absorption from wrong ionization states

Narrow absorption lines from Ca II: Too low ionization state and too narrow for the O star in

the background; multiple lines of same transitionb) Narrow (sharp) lines (temperature & density too low)

c) Multiple components (several clouds of ISM

with different radial velocities)

Page 12: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Four Components of the ISM

ComponentTemperature

(K)Density

(atoms/cm3)Gas

Percent of total mass

Molecular clouds 20 – 50 103 – 105

Molecules(H2 and others) ~ 25%

HI clouds 50 – 150 1 – 1000 Neutral hydrogen Other atoms ionized

~ 25%

Intercloud medium 103 – 104 0.01 Partially ionized ~ 50%

Coronal gas 105 – 106 10–4 – 10–3

Highly ionized,from hot stars and supernovae

~ 5%

Note: Emission nebulae (HII regions) occur only near very hot stars, so they comprise very small fraction of the ISM.

EXTRA

Page 13: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Various Views of the Interstellar Medium

Infrared observations reveal the presence of cool, dusty gas.

X-ray observations reveal the presence of hot gas.

Page 14: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Shocks triggering star formation

Henize 206 (infrared)

Page 15: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

The Contraction of a Protostar

Page 16: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

From Protostars to Stars

Ignition of41H → 4He

fusion processes

Star emerges from the enshrouding dust cocoon(T Tauri stage)

Page 17: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Evidence of Star Formation

Nebula around S Monocerotis:

Contains many massive and very young stars,

including T Tauri stars: strongly variable and bright in the infrared.

Page 18: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

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 or bipolar outflows) seen as Herbig-Haro objects

Page 19: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Example:

Herbig-Haro Object HH34

Page 20: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Globules

Bok globules:

~ 10 – 1000 solar masses

Contracting to form protostars

Page 21: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

EGGsEvaporating gaseous globules (“EGGs”): Newly forming stars

exposed by the ionizing radiation from nearby massive stars

Page 22: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Energy Generation (§7.2)Energy generation in the sun (and all other stars):

nuclear fusion= fusing together 2 or more lighter nuclei to produce heavier ones.

Nuclear fusion can generate energy up to the production of iron.

For elements heavier than iron, energy is produced by nuclear fission.

Binding energy due to the strong force

Page 23: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Energy generation in the Sun:The Proton-Proton Chain (§7.2)

Basic reaction:

41H → 4He + energy

4 protons have 0.048x10-27 kg (= 0.7 %) more mass than 4He.

⇒ Energy gain = mc2

= 0.43x10-11 joules per reaction

Need large proton speed (high temperature) to overcome

Coulomb barrier (electrical repulsion between protons).

Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second!.

T ≥ 107 K = 10 million K

Page 24: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

The Solar Neutrino Problem• The solar interior cannot be observed directly because it is highly opaque to radiation.• But neutrinos can penetrate huge amounts of material without being absorbed.

Davis solar neutrino experiment

• Early solar neutrino experiments detected a much lower flux of neutrinos than expected (→ the “solar neutrino problem”).

• Recent results have proven that neutrinos change (“oscillate”) between different types (“flavors”), thus solving the solar neutrino problem.

Page 25: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

The Source of Stellar EnergyRecall from our discussion of the Sun:

Stars produce energy by nuclear fusion of hydrogen into helium.

In the Sun, this happens primarily

through the proton-proton

(PP) chain

Basic reaction:

41H → 4He + energy

Page 26: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

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Page 27: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Fusion into Heavier Elements than C & O:

Occurs only in very massive stars (more than 8 solar masses)

—why?.

Requires very high temperatures (why?).

Page 28: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Stellar Models—the theory of stars

Divide a star’s interior into concentric shells — “Onion skin layer model”

Within each shell and between neighboring shells, require that the laws of physics are obeyed:

• Conservation of Mass

• Conservation of Energy

• Hydrostatic Equilibrium

• Energy Transport

Four laws of stellar structure:

Page 29: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Hydrostatic EquilibriumIn each layer:

This condition uniquely determines the interior structure of the star.

Stable stars on a narrow strip (main sequence) in the H-R

diagram.

Outward force of thermal

pressure

Inward force of gravity

(weight of all layers above)

=

Page 30: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Energy TransportEnergy generated in the star’s center must be transported to the surface.

Inner layers of the Sun:

Radiation

Outer layers of the Sun:

ConvectionEnergy carried by photons Energy carried by convective

motion of large masses

Page 31: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

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

Star’s total mass determines which part of

the star has convection or radiation (cf. Ch. 10)

Sun

Page 32: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Calculating the stellar structure:Take the four equations representing the four laws of stellar structure and solve them simultaneously!

Hydrostatic equilibrium

Energy transport

Conservation of mass

Conservation of energy

Star’s mass (and chemical composition) completely

determines the properties of the star.

Page 33: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Interactions of Stars and their EnvironmentSupernova explosions of

the most massive stars inflate and blow away remaining gas of star

forming regions.

Young, massive stars excite the remaining gas of their star forming regions, forming HII regions.

Page 34: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

The Life of Main-Sequence Stars

As stars gradually exhaust their hydrogen fuel,

they become brighter, and evolve off the zero-age main sequence.

Page 35: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

The Lifetimes of Stars on the Main Sequence

Page 36: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

TheOrion Nebula:

a region of active star formation

Page 37: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

The Trapezium

The Orion Nebula

The 4 trapezium stars: Brightest, young stars

(< 2 million years old) in the central region of the

Orion nebula

X-ray image: ~ 1000 very young, hot starsInfrared image: ~ 50 very young, cool, low-

mass stars

Only one of the trapezium stars is hot

enough to ionize hydrogen in the Orion

nebula

Page 38: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

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

Protostars with protoplanetary disks

B3

B1

B1

O6

Page 39: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

Stellar Structure— Cause and Effect

Mass (M)

→Gravity (weight)

→Pressure

→Density

Radius

Temperature→Fusion Rates

→→Luminosity

Available Fuel

Time of StabilityMain Sequence Lifetime (“Life Expectancy”)

Hydrostatic equilibrium

Pressure-Temperature Thermostat

~ M

L ~ M 3.5

Lifetime = M/L ~ M –2.5

Mass-Luminosity Relation

Page 40: The Formation and Structure of Stars Chapter 9. The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form

“Red” in Astronomy

• red emission nebulae• red supergiants/giants/dwarfs• red shift (in the Doppler effect)• Interstellar reddening

• blue reflection nebulae• red sunset• blue sky


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