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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).


The formation of stars

The Formation of Stars PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

Chapter 11


Guidepost
Guidepost PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show 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.


Outline
Outline PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

IV. The Orion Nebula

A. Evidence of Young Stars


The life cycle of stars
The Life Cycle of Stars PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

Dense, dark clouds, possibly forming stars in the future

Aging supergiant

Young stars, still in their birth nebulae


Giant molecular clouds
Giant Molecular Clouds PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

Barnard 68

Infrared

Visible

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 PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

d) Spiral arms in galaxies like our Milky Way:

Spirals’ arms are probably rotating shock wave patterns.


Protostars
Protostars PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

As a protostar contracts, it heats up:

Free-fall contraction

→ Heating


Protostellar disks
Protostellar Disks PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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
Protostellar Disks and Jets – Herbig Haro Objects (2) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

Herbig Haro Object HH34


Protostellar disks and jets herbig haro objects 3
Protostellar Disks and Jets – Herbig Haro Objects (3) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

Herbig Haro Object HH30


From protostars to stars
From Protostars to Stars PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

Star emerges from the enshrouding dust cocoon

Ignition of H  He fusion processes


Evidence of star formation
Evidence of Star Formation PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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

Young, very massive star

Infrared

Optical

The Cone Nebula


Evidence of star formation 3
Evidence of Star Formation (3) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

Star Forming Region RCW 38


Globules
Globules PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

Bok Globules:

~ 10 to 1000 solar masses;

Contracting to form protostars


Globules 2
Globules (2) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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


Open clusters of stars
Open Clusters of Stars PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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

Outer layers (including photosphere):

Convection

Inner layers:

Radiative energy transport

Bubbles of hot gas rising up

Cool gas sinking down

Gas particles of solar interior

g-rays


Conduction convection and radiation
Conduction, Convection, and Radiation PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

(SLIDESHOW MODE ONLY)


Stellar structure
Stellar Structure PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

Energy transport via convection

Sun

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 PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

(SLIDESHOW MODE ONLY)


Hydrostatic equilibrium
Hydrostatic Equilibrium PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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.


H r diagram showing main sequence
H-R Diagram (showing Main Sequence) PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode


Energy transport structure
Energy Transport Structure PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

Inner convective, outer radiative zone

Inner radiative, outer convective zone

CNO cycle dominant

PP chain dominant


Summary stellar structure
Summary: Stellar Structure PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

Convective Core, radiative envelope;

Energy generation through CNO Cycle

Sun

Mass

Radiative Core, convective envelope;

Energy generation through PP Cycle


The orion nebula an active star forming region
The Orion Nebula: An Active Star-Forming Region PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode


In the orion nebula
In the Orion Nebula PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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

B3

B1

B1

O6

Protostars with protoplanetary disks

Visual image of the Orion Nebula


New terms
New Terms PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

shock wave

free-fall contraction

protostar

cocoon

protostellar disk

birth line

T Tauri star

Bok globule

Herbig–Haro object

bipolar flow

association

T association

O association

CNO (carbon–nitrogen–oxygen) cycle

opacity

hydrostatic equilibrium


Discussion questions
Discussion Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions1
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions2
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions3
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions4
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions5
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions6
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions7
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions8
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions9
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions10
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions11
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions12
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions13
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions14
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions15
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions16
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions17
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions18
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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 questions19
Quiz Questions PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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.


Answers
Answers PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode

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


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