1. Star Chapter 19: A Traumatic Birth
Star Formation
Star Chapter 19:
A Traumatic Birth

2. Standards Understand the scale and contents of the universe, including
objects such as stars
Understand the concept of equilibrium

3. Stars evolve as they consume their fuel supply

4. Formation of stars like the sun
Gravitational competition
Star formation begins when a nebula in interstellar space starts to collapse under its own weight
The nebula shrinks and heats up
The center becomes hot enough to start fusion
Contraction stops and a star is born

5. Formation of stars like the sun
A star is in equilibrium: gravity pulling in exactly balances fusion pressure pushing out

6. Stages of Star Formation
Stage 1: An Interstellar Cloud
Cloud of cold atomic & molecular gas
Thousands of times mass of sun
Starts to collapse
Begins to fragment

7. Stages of Star Formation
Stage 2: A Collapsing Cloud Fragment
Sun-like star: fragment is 1 – 2 solar masses
~100 x size of solar system
Temperature & pressure increase and fragmentation stops

8. Stages of Star Formation
Stage 3: Fragmentation Ceases
Several tens of thousands of years after initial contraction
Size of solar system
Heats up & begins to resemble a star – called a protostar – has surface

9. Stages of Star Formation
Stage 4: Protostar
Shrinks, density & temperature increase
Size of Mercury’s orbit
Can be plotted on H-R diagram
Violent surface activity
Strong winds
Called T Tauri star

10. Stages of Star Formation
Stage 5: Protostellar Evolution
~10 x size of sun
Central temperature = 5,000,000 K
Gas is ionized
Evolution slows

11. Stages of Star Formation
Stage 6: Newborn Star
10 million years = true star
Central temperature = 10,000,000 K = fusion starts: H2  He

12. Stages of Star Formation
Stage 7: Main Sequence at Last
Endpoint of prestellar evolution, no matter what mass star has
When fusion starts & star becomes stable

13. Stars of Other Masses
The time required for an interstellar cloud to become a main sequence star depends strongly on its mass
The most massive O stars reach the 10 million Kelvin needed to start fusion in a million years (1/50 time taken by sun)
An M-type star less massive than our sun takes one billion years to form

14. Stars of Other Masses
Whatever the mass, the endpoint of the prestellar evolutionary track is the main sequence
A star is considered to have reached the main sequence when hydrogen burning (fusion) begins in the core and its properties settle down to stable values (i.e., it’s in equilibrium)

15. Stars of Other Masses
Zero-age main sequence (ZAMS) – main sequence band predicted by theory. Agrees well with observed main sequence stars, which provides strong support for the modern theory of star formation and stellar structure

16. Stars of Other Masses
The composition of a star affects its internal structure, which affects both its temperature and luminosity, and thus its place on the main sequence
Stars with more heavy elements are cooler and slightly less luminous than stars of the same mass with fewer heavy elements

17. Stars of Other Masses
The main sequence is not an evolutionary track – stars do not evolve along it
It is a way station where stars stop and spend most of their lives
Once on the main sequence, a star stays in the same location on the H-R diagram until it runs out of fuel and begins to die

18. Failed Stars Some cloud fragments are too small to become stars
Jupiter is a good example
There is not enough mass to start fusion. They continue to cool, eventually becoming compact, dark, cold fragments of unburned matter
They are known as brown dwarfs
Note: Jupiter would need to be 80 times more massive to start fusion

19. Failed Stars
The minimum mass needed to start fusion is about 0.08 solar masses
There may be vast numbers of these objects throughout the universe, but they are difficult to detect

20. Observations of Cloud Fragments and Protostars
The age of our entire civilization is much shorter than the time needed for a cloud to contract and form a star.
We can never observe individual objects proceed through a full panorama of star birth
We can, however, observe many different objects at different stages of stellar evolution