1. Life of a Star Formation to Red Giant
(Power Point 13)

2. Student Learning Objectives
Describe stellar formation
Describe the interstellar medium and its effects on light
Compare and contrast types of nebulae
Describe and or diagram the life of a star
List the stages of the Red Giant Phase

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

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
Image Credit: APOD 2007 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. Image Credit: APOD 2009 June 15
Dark Nebula
Dark nebulae
many dust grains
Dust blocks light
May contain embedded objects.
Image Credit: 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.
“A Two Micron All Sky Survey Analysis of the Stability of Southern Bok Globules”
The Astrophyiscal Journal
Published 9 September 2009

10. What is the process of star formation?
ISM clouds are essentially stable.
The inward pull of gravity is balanced by outward gas pressure.
Balanced
Inward Pull
Gravity pulls toward centers of mass
Gravity collapses cloud material
Outward Push
Magnetic fields maintain shape
Rotation and turbulence resist collapse

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

12. Cloud Collapse
The cloud begins to collapse as gravity wins against the low (10 K) gas pressure.
Image Credit: phys.org

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

14. Image Credit: Gemini Observatory/Artwork by Jon Lomberg
Bok Globules form in the dark nebulae.
Small, warm, dense regions in cloud
Heated by contraction
Bok globules become either a star or Brown Dwarf.
Brown Dwarf < 0.08 Msun
Image Credit: Gemini Observatory/Artwork by Jon Lomberg

15. Proto-star
A proto-star is an object that has enough mass to evolve into a star.
Bok globule accretes mass
Object is heating and rotating as it accretes mass
The proto-star is a warm dense core with an associated envelope of material.
Contraction + Accretion  Heat + Rotation

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

17. HR Diagram Birth Tracks
Star
The proto-star becomes a star when it “turns on”, begins nuclear fusion.
The star remains on the main-sequence for 90% of its lifetime.
HR Diagram Birth Tracks
Image Credit: pics about space

18. 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?

19. Young Stars with High Velocity Jets
Image Credit: Hubble Archive, NASA, & ESA

20. Low mass stars last longer at each stage.
How long do stars live?
Low mass stars last longer at each stage.
Mass
(Solar Units)
Formation
(years)
Main-Sequence 1
1 Billion
90 Billion 5
5 Million
60 Million 10
0.6 Million
10 Million

21. 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

22. Practice
Which type of nuclear fusion (PPC or CNO) occurs in the following types of stars?
Sun
Blue Star
M-type Star
Which nuclear fusion process do you expect to produce more energy per second? Why?

23. 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

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

25. 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? Why?

26. What happens when the star runs out of fuel?
The Red Giant phase begins when main-sequence nuclear fusion ends.
All hydrogen in core has been used
Core is not hot enough to fuse helium
Core and surrounding layers collapse
Gravity “wins”

27. The Lowest Mass Stars
Lowest mass stars will have accessed all hydrogen.
Star accesses hydrogen from all layers (convection)
Fusion ends when all hydrogen is gone
Remnant collapses, heats, then slowly fades
Stars less than 0.4 Msun become a Red Dwarf remnant.

28. Red Dwarf vs Sun Image Credit: NASA
Image Credit: NASA

29. A Red Dwarf with an Earth
Image Credit: NASA

30. Practice
Why does fusion only occur in the core of a main-sequence star; what is it about the core of a main-sequence star that makes nuclear fusion possible?
Which would last longer? Why?
A star with a total mass of 0.3 Msun
A star like our Sun with a core that is 0.5 Msun

31. 2nd Stage Red Giant Pressure “wins”
Hydrogen begins burning in a heated shell surrounding the inert helium core.
Outer layers expand and cool.
Pressure “wins” He H

32. Practice Explain why collapsing layers heat.
Explain why the outer layers cool, as the red giant expands.
What makes the outer layers expand?

33. 3rd Stage Red Giant Pressure “wins”
Helium fusion begins in core of red giant when T=100 Million Kelvin
Pressure “wins”
Helium Fusion in Core + Hydrogen Fusion in Shell = Outer Layers Expand More

34. Low Mass Red Giant Final Stage
Image Credit: The Astronomy Cafe

35. Low Mass Layers near end
Image Credit: Openstax

36. High Mass Layers near end
Image Credit: Openstax

37. Practice Stars pulsate during the Red Giant phase. Why?
Name the heaviest element(s) formed in the core at the end of the Red Giant stage.
Lowest Mass Star
Low Mass Star
High Mass Star

38. Practice What will happen to Earth when the Sun becomes a red Giant?
Where will the surface of the Sun be?

39. Diameter 100+ larger Mercury & Venus in Sun Earth at surface of Sun
Our Sun as a Red Giant
Diameter 100+ larger Mercury & Venus in Sun Earth at surface of Sun

40.
Video by Mana Brau