The Formation and

Structure of Stars

Chapter 9

Star Formation

The following slides are from a naturalistic viewpoint.

Blue stars are so hot they would burn up before they

could become a million years

old

Star formation, which has never been observed, is a

justification for the observably

young universe.

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)

The Various Appearances of the ISM

Three kinds of nebulae

1) Emission Nebulae (HII Regions)

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

its 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 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 statesNarrow 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.

The Various Components of

the Interstellar Medium

Infrared observations reveal the

presence of cool, dusty gas.X-ray observations reveal the

presence of hot gas.

Shocks Triggering

Star Formation

Henize 206

(infrared)

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.

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 (II)

Herbig-Haro Object HH34

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

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 Equilibrium

Imagine a stars 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 (II)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 stars 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

of

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 stars mass (and chemical composition) completely

determines its properties.

Thats 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

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