The Formation and Structure of Stars

Chapter 9

The Interstellar Medium (ISM)

•Gas: ~75% H, 25% He, traces of “metals”

•1% “dust” (silicates, carbon, heavy elements coated with ice, About the size of the particles in smoke)

•150 m average distance between dust grains

•“Dense” => ~10 to 1000 atoms/cm3

•“Thin” ~ 0.1 atoms/cm3

Structure of the ISM

• HI clouds:

• Hot intercloud medium:

The ISM occurs mainly in two 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.

3 types of nebula

1. Emission

2. Reflection

3. Dark

Q: Why do emission nebula look red and reflection nebula blue?

We see absorption in elements where the background stars are too hot to form these lines

Narrow width (low temperature; low density)

Multiple components (several clouds of ISM with different radial velocities)

=> Comes from the ISM

Evidence for the ISM

Interstellar reddening

Q: Why do astronomers rely heavily on IR observations?

Q: How do we know the ISM exists?

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.

Stellar formation from the ISM:

Must be triggered by high mass stars –

• Give off intense radiation

• Explode as SNs

Collapsing cloud can form 10 to 1000 stars

• Association

• Cluster

SN_triggered_ssc2004-04v2.wmv

The Contraction of a Protostar

Q: Why do you think there’s a lower limit on the mass of a main-seq. star?

The Contraction of a Protostar

Sun: ~30 million years

15 M: 160,000 years

0.2 M: 1 billion years

From Protostars to Stars

Ignition of H He fusion processes

Star emerges from the

enshrouding dust cocoon

Protostellar Disks and Jets – Herbig-Haro Objects

Herbig-Haro Object HH34

Q: What are the bipolar flows evidence of?

Globules

Bok globules:

~ 10 – 1000 solar masses;

Contracting to form protostars

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

exposed by the ionizing radiation from nearby massive stars

Observations of star formation:

200 solar mass star

N 11B

Trifid

V838 Mon

Tarantula

N 49

The Source of Stellar EnergyStars produce energy by nuclear fusion of

hydrogen into helium.

In the sun, this happens primarily through the proton-proton (P-P) chain

Q: How does the sun fuse H to He?

The CNO Cycle

Happens in stars > 1.1 M

More efficient that the P-P chain.

Requires high T (>16 million K)

Q: Why does the CNO require a higher temp. than the P-P chain?

Fusion into Heavier Elements

Fusion into elements heavier than C, O:

requires high temperatures (>600 million K);

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

Stellar structure

Conservation of mass:

Weight of each shell = total weight

Conservation of energy:

E(out) = E(from within)

Hydrostatic equilibrium:

Pressure balances gravity

Energy transport:

Describes flow of energy

24dM

rdr

24dL

r edr

2

dP GM

dr r

3 2

3

16

dT L

dr ac T r

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 (II)

Outward pressure force must exactly balance the weight of all layers above, everywhere in the star.

This is why we find stable stars on such a narrow strip (main sequence) in the Hertzsprung-Russell diagram.

Pressure-temperature thermostat

Q: How does the P-T thermostat control the reactions in stars?

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

Basically the same structure for all stars close to 1 solar mass.

Q: Why is convection in stars important?

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.

…why stars initially all line up along the main sequence, and why there’s a mass-luminosity relation….

The Life of Main-Sequence StarsStars gradually exhaust their hydrogen fuel.

They gradually becoming brighter, evolving off the zero-age main sequence (ZAMS).

3.5 2.5

fuel 1

rate of consumption

M

M M

Lifetime of a main-sequence star (90% of total life is on main-seq.)

The Lifetimes of Stars on the Main Sequence

The Orion Nebula: An Active Star-Forming Region

The Trapezium

The Orion Nebula

Infrared image: ~ 50 very young, cool, low-

mass starsX-ray image: ~ 1000 very young, hot stars

less than 2 million years old

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 disksProtostars with protoplanetary disks

B3

B1

B1

O6

IR + visual

IR

Gas blown away from protostars