The Formation and Structure of Stars

Chapter 9

The space between the stars is not completely empty, but filled with very

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

We are interested in the interstellar medium 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)

Three kinds of nebulae1) Emission Nebulae (HII Regions)

A hot star illuminates a gas cloud;

excites and/or ionizes the gas

(electrons kicked into

higher energy states);

electrons recombining, falling back to ground state

produce emission lines The Fox Fur Nebula NGC 2246The Trifid Nebula

2) Reflection Nebulae

Star illuminates gas and dust cloud

star light is reflected by the dust

reflection nebula appears blue because blue light is scattered by larger angles than

red light

same phenomenon makes the day sky

appear blue (if it’s not cloudy)

Emission and Reflection Nebulae

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

Horsehead Nebula

Interstellar Reddening

Visible Infrared

Barnard 68

Blue light is strongly scattered and absorbed

by interstellar clouds.

Red light can more easily penetrate the cloud, but

it is still absorbed to some extent.

Infrared radiation is

hardly absorbed at

all.

Interstellar clouds make background stars appear

redder.

Interstellar Absorption LinesThe interstellar medium produces

absorption lines in the spectra of stars. These can be

distinguished from stellar absorption

lines through:

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 componentsb) Small line width

(too low temperature; too low density)

c) Multiple components

(several clouds of ISM with different radial velocities)

Structure of the ISM

• HI clouds:

• Hot intercloud medium:

The ISM occurs in two main types of clouds:

Cold (T ~ 100 K) clouds of neutral hydrogen (HI);

moderate density (n ~ 10 – a few hundred atoms/cm3);

size: ~ 100 pc

Hot (T ~ a few 1000 K), ionized hydrogen (HII);

low density (n ~ 0.1 atom/cm3);

gas can remain ionized because of very low density

Shocks Triggering Star Formation

The gas in the ISM needs to be compressed in order to collapse and form stars: Shocks traveling

through interstellar space can do this.

Shocks Triggering Star Formation

Compression of the ISM by Winds from Hot Stars

The Contraction of a Protostar

From Protostars to Stars

Ignition of H → He fusion processes

Star emerges from the

enshrouding dust cocoon

Evidence of Star FormationNebula around S Monocerotis:

Contains many massive, very young stars,

including T Tauri Stars: strongly variable; bright

in the infrared.

T Tauri Stars

Very young stars, still in the forming stage

Typically 100,000 – 10 million years old

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

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.

Winds from Hot StarsVery young, hot stars produce massive stellar winds,

blowing parts of it away into interstellar space.

Eta Carinae

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

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.

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

Fusion into Heavier Elements

Fusion into heavier elements than C, O:

requires very high temperatures; occurs only in very massive stars (more than 8

solar masses)

Hydrostatic EquilibriumImagine 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

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.

Energy TransportEnergy generated in the star’s center must be

transported to the surface.

Inner layers of the sun:

Radiative energy transport

Outer layers of the sun

(including photosphere):

Convection

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

Stellar ModelsThe structure and evolution of a star is

determined by the laws of:• Hydrostatic equilibrium

• Energy transport

• Conservation of mass

• Conservation of energy

A star’s mass (and chemical composition) completely determines

its properties.

That’s why stars initially all line up along the main sequence.

Interactions of Stars and their Environment

Young, massive stars excite the remaining gas of their

star forming regions, forming HII regions.

Supernova explosions of the most massive stars inflate and blow

away remaining gas of star forming regions.

The Life of Main Sequence Stars

Stars gradually exhaust their

hydrogen fuel.

In this process of aging, they are

gradually becoming brighter,

evolving off the zero-age main

sequence.

The Lifetimes of Stars on the Main Sequence