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Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode). The Formation of Stars. Chapter 11. Guidepost.

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Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode).


Previous chapters have used the basic principles of physics as a way to deduce things about stars and the interstellar medium. All of the data we have amassed will now help us understand the life stories of the stars in this chapter and those that follow.

In this chapter, we use the laws of physics in a new way. We develop theories and models based on physics that help us understand how stars work. For instance, what stops a contracting star and gives it stability? We can understand this phenomenon because we understand some of the basic laws of physics.

Throughout this chapter and the chapters that follow, we search for evidence. What observational facts confirm or contradict our theories? That is the basis of all science, and it must be part of any critical analysis of what we know and how we know it.


I. Making Stars from the Interstellar Medium

A. Star Birth in Giant Molecular Clouds

B. Heating By Contraction

C. Protostars

D. Evidence of Star Formation

II. The Source of Stellar Energy

A. A Review of the Proton-Proton Chain

B. The CNO Cycle

III. Stellar Structure

A. Energy Transport

B. What Supports the Sun?

C. Inside Stars

D. The Pressure-Temperature Thermostat

outline continued
Outline (continued)

IV. The Orion Nebula

A. Evidence of Young Stars

the life cycle of stars
The Life Cycle of Stars

Dense, dark clouds, possibly forming stars in the future

Aging supergiant

Young stars, still in their birth nebulae

giant molecular clouds
Giant Molecular Clouds

Barnard 68



Star formation collapse of the cores of giant molecular clouds: Dark, cold, dense clouds obscuring the light of stars behind them.

(More transparent in infrared light.)

parameters of giant molecular clouds
Parameters of Giant Molecular Clouds

Size: r ~ 50 pc

Mass: > 100,000 Msun

Temp.: a few 0K

Dense cores:

R ~ 0.1 pc

M ~ 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.

contraction of giant molecular cloud cores
Contraction of Giant Molecular Cloud Cores

Horse Head Nebula

  • Thermal Energy (pressure)
  • Magnetic Fields
  • Rotation (angular momentum)
  • Turbulence

External trigger required to initiate the collapse of clouds to form stars.

shocks triggering star formation
Shocks Triggering Star Formation

Trifid Nebula

Globules = sites where stars are being born right now!

sources of shock waves triggering star formation 1
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
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
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
Sources of Shock Waves Triggering Star Formation (4)

d) Spiral arms in galaxies like our Milky Way:

Spirals’ arms are probably rotating shock wave patterns.


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 contraction
Heating By Contraction

As a protostar contracts, it heats up:

Free-fall contraction

→ Heating

protostellar disks
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
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

from protostars to stars
From Protostars to Stars

Star emerges from the enshrouding dust cocoon

Ignition of H He fusion processes

evidence of star formation
Evidence of Star Formation

Nebula around S Monocerotis:

Contains many massive, very young stars,

including T Tauri Stars: strongly variable; bright in the infrared.

evidence of star formation 2
Evidence of Star Formation (2)

Smaller, sunlike stars, probably formed under the influence of the massive star

Young, very massive star



The Cone Nebula

evidence of star formation 3
Evidence of Star Formation (3)

Star Forming Region RCW 38


Bok Globules:

~ 10 to 1000 solar masses;

Contracting to form protostars

globules 2
Globules (2)

Evaporating Gaseous Globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars

open clusters of stars
Open Clusters of Stars

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

Open Cluster M7

open clusters of stars 2
Open Clusters of Stars (2)

Large, dense cluster of (yellow and red) stars in the foreground; ~ 50 million years old

Scattered individual (bright, white) stars in the background; only ~ 4 million years old

the source of stellar energy
The Source of Stellar Energy

Recall 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
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.

energy transport
Energy Transport

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

Outer layers (including photosphere):


Inner layers:

Radiative energy transport

Bubbles of hot gas rising up

Cool gas sinking down

Gas particles of solar interior


stellar structure
Stellar Structure

Energy transport via convection


Energy transport via radiation

Flow of energy

Energy generation via nuclear fusion

Basically the same structure for all stars with approx. 1 solar mass or less.

Temperature, density and pressure decreasing

the sun
The Sun


hydrostatic equilibrium
Hydrostatic Equilibrium

Imagine a star’s interior composed of individual shells.

Within each shell, two forces have to be in equilibrium with each other:

Gravity, i.e. the weight from all layers above

Outward pressure from the interior

hydrostatic equilibrium 2
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.

energy transport structure
Energy Transport Structure

Inner convective, outer radiative zone

Inner radiative, outer convective zone

CNO cycle dominant

PP chain dominant

summary stellar structure
Summary: Stellar Structure

Convective Core, radiative envelope;

Energy generation through CNO Cycle



Radiative Core, convective envelope;

Energy generation through PP Cycle

in the orion nebula
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





Protostars with protoplanetary disks

Visual image of the Orion Nebula

new terms
New Terms

shock wave

free-fall contraction



protostellar disk

birth line

T Tauri star

Bok globule

Herbig–Haro object

bipolar flow


T association

O association

CNO (carbon–nitrogen–oxygen) cycle


hydrostatic equilibrium

discussion questions
Discussion Questions

1. Ancient astronomers, philosophers, and poets assumed that the stars were eternal and unchanging. Is there any observation they could have made or any line of reasoning that could have led them to conclude that stars don’t live forever?

2. How does hydrostatic equilibrium relate to hot-air ballooning?

quiz questions
Quiz Questions

1. In which component of the interstellar medium do new stars form?

a. In the HI clouds.

b. In the HII intercloud medium.

c. In the hot coronal gas.

d. In molecular clouds.

e. Both a and d above.

quiz questions45
Quiz Questions

2. What force causes the contraction of a cloud of interstellar matter to form a star?

a. The electrostatic force.

b. The strong nuclear force.

c. The weak nuclear force.

d. The gravitational force.

e. All of the above.

quiz questions46
Quiz Questions

3. Which factor resists the contraction of a cloud of interstellar matter?

a. Thermal energy.

b. The interstellar magnetic field.

c. Rotation.

d. Turbulence.

e. All of the above.

quiz questions47
Quiz Questions

4. What triggers the gravitational collapse of material inside a molecular cloud?

a. Collisional cooling.

b. Shielding of the interstellar magnetic field.

c. Tidal forces slow the rate of rotation.

d. A subsidence in turbulence due to internal friction.

e. A passing shock wave.

quiz questions48
Quiz Questions

5. What is the source of a shock wave that passes through a molecular cloud and triggers star formation?

a. A supernova explosion.

b. The ignition of hot stars within the cloud.

c. A collision of molecular clouds.

d. A spiral wave pattern within a galaxy.

e. All of the above.

quiz questions49
Quiz Questions

6. What happens to the temperature and density inside a collapsing protostar?

a. Temperature and density both increase.

b. Temperature and density both decrease.

c. Temperature increases and density decreases.

d. Temperature decreases and density increases.

e. The product of temperature and density remains constant.

quiz questions50
Quiz Questions

7. What is a protostar's energy source?

a. Nuclear fusion.

b. Gravitational energy.

c. Chemical energy.

d. Both a and b above.

e. All of the above.

quiz questions51
Quiz Questions

8. What characteristic of the collapsing cloud that forms a protostar allows it to also form a protostellar disk?

a. Thermal energy.

b. The interstellar magnetic field.

c. Rotation.

d. Turbulence.

e. All of the above.

quiz questions52
Quiz Questions

9. At what wavelengths can we observe the early stages of protostar formation?

a. Infrared.

b. Visible.

c. Ultraviolet.

d. Both a and b above.

e. Both a and c above.

quiz questions53
Quiz Questions

10. What eventually halts the slow contraction of a newly forming star?

a. A second shock wave.

b. Electrostatic repulsion.

c. The Coulomb barrier.

d. Nuclear fusion.

e. Gravity.

quiz questions54
Quiz Questions

11. The gestation period for humans is 40 weeks. What was the gestation period for our Sun; that is, how much time passed between the onset of gravitational collapse and the Sun's arrival on the main sequence?

a. About 40 weeks.

b. About 30,000 years.

c. About 30 million years.

d. About 1 billion years.

e. About 5 billion years.

quiz questions55
Quiz Questions

12. According to Figure 11-5, the Protosun was cooler yet much more luminous than the Sun is now. How can this be true?

a. The Protosun had more mass.

b. The Protosun was much larger.

c. The rate of nuclear fusion was higher inside the Protosun.

d. Both a and c above.

e. Both b and c above.

quiz questions56
Quiz Questions

13. What evidence do we have that the Orion region is actively forming stars?

a. Protostars are seen here at infrared wavelengths inside their cocoons.

b. Some stars here are between the birth line and the main sequence.

c. Some visible stars in the Orion region have disks.

d. Some short-lived stars are located in this region.

e. All of the above.

quiz questions57
Quiz Questions

14. How does the CNO cycle differ from the proton-proton chain?

a. The CNO cycle requires a higher temperature than the proton-proton chain.

b. The rate of the CNO cycle is more temperature sensitive than the proton-proton chain.

c. The energy produced by one sequence through the CNO cycle is greater than for one sequence through the proton-proton chain.

d. Both a and b above.

e. All of the above.

quiz questions58
Quiz Questions

15. Which stars produce most of their energy by the CNO cycle?

a. Protostars.

b. Upper main sequence stars.

c. Lower main sequence stars.

d. Both a and b above.

e. Both a and c above.

quiz questions59
Quiz Questions

16. Which method of energy transport is NOT important inside most stars?

a. Conduction.

b. Convection.

c. Radiation.

d. Both a and b above.

e. Both a and c above.

quiz questions60
Quiz Questions

17. How does the extreme temperature sensitivity of the CNO cycle affect a star's interior?

a. The CNO cycle generation zone occupies a very small region.

b. CNO cycle stars have radiative cores and convective envelopes.

c. CNO cycle stars have convective cores and radiative envelopes.

d. Both a and b above.

e. Both a and c above.

quiz questions61
Quiz Questions

18. What prevents the enormous amount of energy released from the fusion reactions at a star's core from blowing the star apart?

a. Gas pressure.

b. Density.

c. Opacity.

d. Gravity.

e. All of the above.

quiz questions62
Quiz Questions

19. What would happen in the interior of a normal star if gravity were to shrink the star's size a small amount?

a. The interior temperature would increase.

b. The rate of fusion would increase.

c. The gas pressure would increase.

d. Both a and b above.

e. All of the above.

quiz questions63
Quiz Questions

20. Where in the Sun is the law of hydrostatic equilibrium at work?

a. At the visible surface.

b. At the outer boundary of the energy-generating core.

c. At the convective zone/radiative zone boundary.

d. About halfway between the center and visible surface.

e. At every point inside the Sun.


1. d

2. d

3. e

4. e

5. e

6. a

7. b

8. c

9. a

10. d

11. c

12. b

13. e

14. d

15. b

16. a

17. e

18. d

19. e

20. e