1. The Nebular Hypothesis
How do stars form?
The Nebular Hypothesis

2. Observations Stars can be seen in various stages of formation.
Stars seem to have been forming continuously since the formation of the Universe.
Star formation continues today.
Observations synthesized into the Nebular Hypothesis.

3. Our Sun: an example of stellar evolution
Our Sun began as a nebula, approximately 5 billion years ago.
A nebula is an enormous cloud of gasses (mainly Hydrogen) and dust
Nebula may become disturbed by shock waves, for example from a nearby supernova.

4. Example of a nebula

5. Nebula begins to contract
As the molecules of gas and dust move closer together, they experience stronger gravitational attraction.
Newton’s Law of Universal Gravitation
Fg = g(m1m2)/d2
Most mass concentrates at the center as the nebula contracts.

6. Nebula begins to take a more definite shape.

7. Shape becomes spherical with equatorial disk

8. Will a star form?
If mass is sufficient, gravity at the center of the sphere may be great enough to “squash” atoms together.
4 H atoms are fused to form 1 He atom in a nuclear reaction (not chemical).
Nuclear reaction is nuclear fusion, which releases tremendous energy.
A star is born!

9. Stable stars
Radiant energy produced by fusion causes the star to expand.
Gravity holds the star together.
Gas pressure/radiant energy is balanced by gravity, so the star is stable.
The Hertzsprung-Russell (H-R) Diagram shows these stars as the main sequence.

10. The H-R Diagram

11. Another version of the H-R Diagram

12. H-R Diagram
Shows relationships among size, temperature and brightness (luminosity or magnitude).
Larger, stable stars are hotter and brighter.
Large, hot stars burn out faster than smaller, cooler stars.

13. Stellar Evolution and the H-R Diagram
What happens when a star exhausts its nuclear fuel?
Depends on size
Star core collapses on itself, but heats the outer envelope.
Result may be: White dwarf, white dwarf with planetary nebula, red giant, neutron star or black hole.

14. Stellar collapse
May result in gravitational heating and eventual burnout.
May result in renewed fusion (He is fused this time).
Collapse may produce explosion (supernova).
Remnant of supernova may be a neutron star or a black hole.

15. Products of fusion H is fused to form He
He is fused to form C and other, heavier chemical elements.
Heavier elements are recycled into new nebulae, and/or new stars and planets.
Implication?

16. Heavy Elements
Since all elements heavier than H are produced by fusion in stars,
We are made of Stardust !!!

17. Summary: Nebular Hypothesis

18. So what happened to the disk?
The disk that surrounds the central star may
1) be swallowed as the star initially expands.
2) remain as a disk or a series of rings
3) may form planets that orbit the central star.

19. Our Solar System
Sun began to radiate energy about 5 billion years ago.
Surrounding disk condensed into 9 (possibly 8) planets and an asteroid belt.
Earth is one of those nine planets.
Earth condensed approximately 4.6 billion years ago.

20. How do we know the timing?
Age of Sun via chemical composition and known rate of fusion: about 5 Ga
Oldest Earth rock: 3.98 Ga
Age of oldest Moon Rocks: 4.2 Ga
Age of Meteorites: 4.5 Ga
Ga = Giga-annum = billion years