1. We are “star stuff” because the elements necessary for life were made in stars

2. How do stars form?

3. Stars are born in molecular clouds consisting mostly of hydrogen molecules

4. Stars form in places where gravity can overcome thermal pressure in a cloud

5. HST Photo: Trifid Nebula
Cloud heats up as gravity causes it to contract
Conservation of energy
Contraction can continue if thermal energy is radiated away

6. Download a movie of optical-to-IR transformation from the Spitzer Space Teelscope site
Star-forming clouds emit infrared light because of the heat generated as stars form

7. Orion Nebula is one of the closest star-forming clouds
Infrared light from Orion

8. Solar-system formation is a good example of star birth

9. As gravity forces a cloud to become smaller, it begins to spin faster and faster

10. As gravity forces a cloud to become smaller, it begins to spin faster and faster
Conservation of angular momentum

11. As gravity forces a cloud to become smaller, it begins to spin faster and faster
Conservation of angular momentum
Gas settles into a spinning disk because spin makes it hard to for gas cloud to collapse perpendicular to spin axis

12. Angular momentum leads to:
Rotation of protostar
Disk formation
… and sometimes …
Jets from protostar
Fragmentation into binary

13. Disks and jets seen around young stars

14. Protostar to Main Sequence
Protostar contracts and heats until core temperature is sufficient for hydrogen fusion.
Contraction ends when energy released by hydrogen fusion balances energy radiated from surface.
Takes 50 million years for star like Sun (less time for more massive stars)

15. Summary of Star Birth
Gravity causes gas cloud to shrink and fragment
Core of shrinking cloud heats up
When core gets hot enough, fusion begins and stops the shrinking
New star is now on the (long-lasting) main sequence

16. How massive are newborn stars?

17. A cluster of many stars can form out of a single cloud.

18. Very massive stars are rare
Luminosity
Low-mass stars are common
Temperature

19. Stars more massive than 150 MSun would blow themselves apart
Luminosity
Stars less massive than
0.08 MSun can’t sustain fusion
Temperature

20. Pressure Gravity
If M > 0.08 MSun, then gravitational contraction heats core until fusion begins
If M < 0.08 MSun, degeneracy pressure stops gravitational contraction before fusion can begin

21. Degeneracy Pressure:
Laws of quantum mechanics prohibit more than one electron (or neutron) from occupying same state in same place at same time

22. Thermal Pressure:
Depends on heat content
The main form of pressure in most stars
Degeneracy Pressure:
Particles can’t be in same state in same place
Doesn’t depend on heat content

23. Brown Dwarf An object less massive than 0.08 MSun
Gains thermal energy from gravitational contraction
Radiates infrared light
Cools off after degeneracy pressure stops contraction … cools off forever!
VERY dim, VERY red, VERY hard to spot …‘Nemesis’ in our own solar system?

24. What have we learned? • How do stars form?
Stars are born in cold, relatively dense molecular clouds.
As a cloud fragment collapses under gravity, it becomes a protostar surrounded by a spinning disk of gas.
The protostar may also fire jets of matter outward along its poles. Protostars rotate rapidly, and some may spin so fast that they split to form close binary star systems.

25. What have we learned? • How massive are newborn stars?
Newborn stars come in a range of masses, but cannot be less massive than 0.08 solar masses.
Below this mass, degeneracy pressure prevents gravity from making the core hot enough for efficient hydrogen fusion, and the object becomes a “failed star” known as a brown dwarf.

26. Clicker questions on activity #32, pages 109-112

27. 1A: Does the force of gravity increase or decrease as you get closer to the star’s center – say halfway to the center?
Increases
Decreases
Stays the same
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28. What are the gravity and pressure at the exact center of the Sun
What are the gravity and pressure at the exact center of the Sun? (Assume the Sun is perfectly symmetrical.)
Zero gravity, high pressure
High gravity, high pressure
High gravity, zero pressure
Zero gravity, zero pressure
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29. 2: In Figure 3, which plot shows the dependence of average particle speed on temperature?B
Neither
Both are possible
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30. 3: What would the pressure of a gas cloud in space do if the temperature of the gas was increased?
Decrease
Stay the same
Increase
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31. 4: What should start to happen to the temperature and pressure in a star’s core when fusion stops in the core?
Temp. decreases, pressure increases
Temp. increases, pressure goes down
Both T & P increase
Both T & P decrease
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32. 5B: The force of gravity on the mass m is…
Four times stronger at the surface of star A than at the surface of star B
Four times stronger at the surface of star B than at the surface of star A
Two times stronger at the surface of star A than at the surface of star B
Two times stronger at the surface of star B than at the surface of star A
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33. 5D: As a star contracts, the force of gravity at its surface…
Decreases
Stays the same
Increases
It depends on other properties of the star
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34. 7: Some star cores stop collapsing by forming a gas where one or the other of two particle types are degenerate. What are those particle types?
Electrons or Neutrinos
Neutrons or Neutrinos
Electrons or Neutrons
Hydrogen or Neutrons
Hydrogen or Helium
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35. 10. If the core of a star is sufficiently massive, neither kind of degeneracy pressure will stop its collapse, and the result is a…
Black Dwarf
Black Hole
Brown Dwarf
White Dwarf
White Hole
Yellow Submarine
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