Stellar Evolution: The Life Cycle of Stars

Dense, dark clouds, possibly forming stars in the future

Young stars, still in their birth

nebulae

Aging supergiant

Stars are not eternal.They are being born, live a finite life time, and die.

Parameters of Giant Molecular Clouds

Size: 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.

Stars are formed during the collapse of the cores of Giant Molecular Clouds.

Contraction of Giant Molecular Cloud Cores

• Thermal Energy (pressure)

• Magnetic Fields

• Rotation (angular momentum)

External trigger required to initiate the collapse of clouds

to form stars. Horse Head Nebula

• Turbulence

Factors resisting the collapse of a gas cloud:

Shocks Triggering Star Formation

Globules = sites where stars are being born right now!

Trifid Nebula

Sources of Shock Waves Triggering Star Formation (1)Previous star formation can trigger further star formation through:

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

Massive stars die young => Supernovae tend to

happen near sites of recent star formation

Sources of Shock Waves Triggering Star Formation (2)

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 (3)

Giant molecular clouds are very large and may occasionally

collide with each other

c) Collisions of giant

molecular clouds

Sources of Shock Waves Triggering Star Formation (4)

d) Spiral arms in galaxies like our Milky Way:

Spiral arms are probably

rotating shock- wave patterns.

Protostars

Protostars = 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

Heating By ContractionAs a protostar contracts, it heats up:

Free-fall contraction→ Heating

From cloud to protostar: gravity is the key for the collapse

Initial cloud with some rotation (even very, very small)

Cloud spins up as it collapse

A protostar

From a protostar to a true star

• Gas is heated when it is compressed• The central part of a protostar is compressed the

most, and when the temperature there reaches 10 million K, hot enough to ignite hydrogen fusion, the collapse is halted by the heated generated by the nuclear reaction

• A new star is born, and its internal structure is stabilized, because the energy produced in the center matches the amount of radiation from the surface

From Protostars to Stars

Higher-mass stars evolve more rapidly

from protostars to stars than less massive

stars

From Protostars to Stars

The Birth Line:

Star emerges from the enshrouding dust cocoon

The Orion Nebula: Evidence of Star Formation

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

Visual image of the Orion Nebula

Protostars with protoplanetary disks

B3

B1B1

O6

Open Clusters of StarsLarge masses of Giant Molecular Clouds => Stars do not form isolated, but in large groups, called Open Clusters of Stars.

Young Star ClustersUltraviolet radiation and strong stellar winds from young, hot, massive stars in open star clusters are compressing the surrounding gas.

30 Doradus

NGC 602

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

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 (2)

Herbig-Haro Object HH34

Protostellar Disks and Jets – Herbig-Haro Objects (3)

Herbig-Haro Object HH30

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

The Source of Stellar Energy

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

Recall from our discussion of the sun:

Sun must produce energy, or else it would cool off quickly (≈104 yr)

Stars produce energy by nuclear fusion of hydrogen into helium.

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.

41H --> 4He + energy ( E = mc2

)Two ways to do this fusion reaction:

In the Sun, about 500 million tons/sec are needed!

If M<1.1Mo: p-p chain

If M>1.1 Mo: CNO cycle

Energy output of p-p cycle depends mildly on T: 10% ΔT 46% ΔE, with 50% of energy being generated in 11% of mass

Energy output of CNO has steep dependence on T: 10% ΔT 340% ΔE, with 50% of energy being generated in 2% of mass

p-p cycle is a “direct way to fuse 4 H into 1 He

CNO cycle needs the help of C, N and O (catalysts)C, N and O simply assist the reaction, but do not partecipate

Final output is the same: 4 H fuse into 1 He

Energy TransportEnergy generated in the star’s center must be

transported to the surface.

Physicists know of three ways in which energy can be transported:

Energy Transport (2)However, in stars, only two energy transport

mechanisms play a role:

Inner layers:

Radiative energy transport

Outer layers (including photosphere):

Convection

Bubbles of hot gas rising up

Cool gas sinking downGas particles

of solar interior-rays

Energy Transport Structure

Inner radiative, outer convective

zone

Inner convective, outer radiative

zone

CNO cycle dominant PP chain dominant

Balance happens thanks toflow (transport) of radiation

from center (hotter) to surface (colder)

• Conduction, radiation, convection• Opacity is key to efficiency of radiation

transport• p-p stars: radiative core, convective

envelope• CNO stars: convective core, radiative

envelope• Small stars (M<~0.4 Mo) all convective

Hydrostatic Equilibrium

Imagine 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 (2)

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.

Pressure and Temperature of a Gas

Outward thermal pressure of coreis larger than inward gravitational pressure

Core expands

Expanding core cools

Nuclear fusion ratedrops dramatically

Outward thermal pressureof core drops (and becomessmaller than inward grav. pressure)

Core contracts

Contracting core heats up

Nuclear fusion raterises dramatically

The Stellar Thermostat

Summary: Stellar Structure

MassSun

Radiative Core, convective envelope;

Energy generation through PP Cycle

Convective Core, radiative envelope;

Energy generation through CNO Cycle

A main-sequence star can hold its structure for a very long time (depending on its mass),

until it has H to burn into HeAnd then????

ThermalPressure

GravitationalContraction

Stellar Evolution in a Nutshell

Mass controls the evolution of a star!

M < 8 MSun M > 8 MSun

Mcore < 3MSunMcore > 3MSun

Review Questions

1. Where are the birth places of stars?

2. What are the main components of a protostar?

3. When and how is a new star born?

4. What prevents a star from collapsing?