1. Star Formation & Main-sequence Stars
(Power Point 15)

2. Student Learning Objectives
Describe stellar formation
Describe the interstellar medium and its effects on light
Compare and contrast types of nebulae
Describe the process of star formation
Recall the balance that maintains stellar structure

3. What is the interstellar medium and how is it detected?
The interstellar medium (ISM) is gas and dust between the stars.
Giant clouds
Stellar nurseries
The ISM can be detected because it alters light from stars.
Interstellar extinction
Interstellar reddening
Absorption lines
Strong emission line
(21 cm)

4. Practice
The ISM produces a strong emission line at 21 cm (Radio). What does this tell us about the energy in an ISM cloud?
How many stars can a single ISM cloud produce?

5. Emission Nebula Emission nebulae
(HII regions)
Virtually no dust grains
Associated with young stars
The new star excites surrounding gasses.
Emission Nebula IC 1396
APOD December 24

6. Reflection Nebula Reflection nebulae The dust scatters starlight.
Very few dust grains
Associated with young stars
The dust scatters starlight.
Image Credit: NASA

7. Dark Nebula Dark nebulae May contain embedded objects.
many dust grains
Dust blocks light
May contain embedded objects.
APOD 2009 June 15

8. Practice
What is the key aspect that determines the appearance (type) of nebula?
In which nebula would you expect to find a star beginning to form?
Why does the excited gas of an emission nebula glow red?

9. Why are interstellar dust grains important?
Solid dust grains account for about 1% of ISM.
Grains align with magnetic field.
Magnetic maps indicate where stars are beginning to form.

10. The Astrophyiscal Journal, Published 9 September 2009
“A TWO MICRON ALL SKY SURVEY ANALYSIS OF THE STABILITY OF SOUTHERN BOK GLOBULES”
The Astrophyiscal Journal, Published 9 September 2009

11. What is the process of star formation?
ISM clouds are essentially stable.
Inward Pull
Gravity pulls toward centers of mass
Gravity may cause collapse
Balanced
Outward Push
Magnetic fields maintain shape
Rotation and turbulence resist collapse

12. Cloud Collapse
Cloud collapse begins when gravity wins against pressure.
Cloud is cold (10K)
Outward pressure against gravity is low
An event disrupts the gravity-pressure balance

13. Video by Our Universe Visualized
Video by Our Universe Visualized

14. Practice
Explain why rotation helps to hold up a cloud against gravity.
What are the possible events that could disrupt the gravity-pressure balance?

15. Image Credit: Gemini Observatory/Artwork by Jon Lomberg
Warm Dense Object
Bok Globules form first.
Embedded in dark nebulae
Dense gas regions heated by contraction
Pre-fusion masses
Bok globules become either a star or a Brown Dwarf.
Brown Dwarf < 0.08 Msun
Image Credit: Gemini Observatory/Artwork by Jon Lomberg

16. Proto-star
A proto-star is an object that has enough mass to evolve into a star.
Bok globule accretes “enough” mass
Warm dense core
Associated envelope of material
Object is heating and rotating
Contraction + Accretion  Heat + Rotation

17. Formation of a solar system
Video by Our Universe Visualized

18. HR Diagram Birth Tracks
Star
HR Diagram Birth Tracks
The star “turns on”.
Proto-star enters main-sequence, and is now classified as a star.
Image Credit: pics about space

19. Practice
Why does the proto-star get fainter and then grow in brightness?
When does a proto-star become a star? What is it that classifies an object as a star?

20. Image Credit: Hubble Archive, NASA, & ESA
Image Credit: Hubble Archive, NASA, & ESA

21. A star spends 90% of lifetime in the main-sequence stage.
How long do stars live?
A star spends 90% of lifetime in the main-sequence stage.
Mass
(Solar Units)
Formation
(years)
Main-Sequence 1
1 Billion
90 Billion 5
5 Million
60 Million 10
0.6 Million
10 Million

22. Temperature determines the type of nuclear fusion.
Low Mass
High Mass
T = 7 Million K
T = 16 Million K
PPC
CNO
4H  He
C + N + O + H  He

23. Practice Which type of nuclear fusion occurs in our Sun?
Which type of nuclear fusion occurs in a Blue Star?
What type of nuclear fusion occurs in an M-type Star?
Which nuclear fusion process do you expect to produce more energy per second?

24. What maintains stellar structure?
The outward radiation pressure balances the inward force of gravity, at every layer of the star. (maintains shape of star)
Hydrostatic Equilibrium
Inward Gravity
Outward Pressure

25. Hydrostatic Equilibrium at each layer
Image Credit: Astronomynotes.com

26. Practice
At which layer of the star, innermost or outermost, is gravity greatest? Why?
At which layer of the star, innermost or outermost, is the outward pressure greatest? Explain.
Compare hydrostatic equilibrium in a star to what you have experienced in a swimming pool.