1. The Fundamental Problem in studying the stellar lifecycle
We study the subjects of our research for a tiny fraction of its lifetime
Sun’s life expectancy ~ 10 billion (1010) years
Careful study of the Sun ~ 370 years
We have studied the Sun for only 1/27 millionth of its lifetime!

2. Suppose we study human beings…
Human life expectancy ~ 75 years
1/27 millionth of this is about 74 seconds
What can we learn about people when allowed to observe them for no more than 74 seconds?
They come in all variety of shapes, a few are abnormal
They display variety of sizes (smallest ~30cm, largest 2.5m)
There are few varieties of skin color
There are VAST differences in behavior.
Can we find evidence of evolution from one state to another in individuals?
Infants—Small People—Large People
Where do people come from? Are they eternal? Dead people?
Create MODELS

3. Theory and Experiment Theory:
Need a theory for star formation
Need a theory to understand the energy production in stars  make prediction how bight stars are when and for how long in their lifetimes
Experiment: observe how many stars are where when and for how long in the Hertzsprung-Russell diagram
 Compare prediction and observation

4. Hydrostatic Equilibrium
Two forces compete: gravity (inward) and energy pressure due to heat generated (outward)
Stars neither shrink nor expand, they are in hydrostatic equilibrium, i.e. the forces are equally strong
Heat
Gravity
Gravity

5. Star Formation & Lifecycle
Contraction of a cold interstellar cloud
Cloud contracts/warms, begins radiating; almost all radiated energy escapes
Cloud becomes dense  opaque to radiation  radiated energy trapped  core heats up

6. Example: Orion Nebula
Orion Nebula is a place where stars are being born

7. Protostellar Evolution
increasing temperature at core slows contraction
Luminosity about 1000 times that of the sun
Duration ~ 1 million years
Temperature ~ 1 million K at core, 3,000 K at surface
Still too cool for nuclear fusion!
Size ~ orbit of Mercury
Star can be plotted on the H-R diagram for the first time

8. Path in the Hertzsprung-Russell Diagram
Gas cloud becomes smaller, flatter, denser, hotter  Star

9. Protostellar Evolution
increasing temperature at core slows contraction
Luminosity about 1000 times that of the sun
Duration ~ 1 million years
Temperature ~ 1 million K at core, 3,000 K at surface
Still too cool for nuclear fusion!
Size ~ orbit of Mercury
Star can be plotted on the H-R diagram for the first time

10. Path in the Hertzsprung-Russell Diagram
Gas cloud becomes smaller, flatter, denser, hotter  Star

11. A Newborn Star
Main-sequence star; pressure from nuclear fusion and gravity are in balance
Duration ~ 10 billion years (much longer than all other stages combined)
Temperature ~ 15 million K at core, 6000 K at surface
Size ~ Sun
First 6 stages take million years, less than 1% of the time it spends on the main sequence

12. Failed Stars: Brown Dwarfs
Too small for nuclear fusion to ever begin
Less than about 0.08 solar masses or 13 Jupiters
Give off heat from gravitational collapse
Luminosity ~ a few millionths that of the Sun

13. Mass Matters Larger masses Smaller masses higher surface temperatures
higher luminosities
take less time to form
have shorter main sequence lifetimes
Smaller masses
lower surface temperatures
lower luminosities
take longer to form
have longer main sequence lifetimes

14. Mass and the Main Sequence
The position of a star in the main sequence is determined by its mass
All we need to know to predict luminosity and temperature!
Both radius and luminosity increase with mass
The position of a star in the main sequence is found to be determined by its mass, which varies from 0.2 solar masses (red dwarfs) to solar masses.
In other words, given the mass of a star in the main sequence, we can predict (approximately) its absolute luminosity and its spectral class (temperature)
Clear progression from low-mass red dwarfs to high-mass blue giants

15. Stellar Lifetimes
From the luminosity, we can determine the rate of energy release, and thus rate of fuel consumption
Given the mass (amount of fuel to burn) we can obtain the lifetime
Large hot blue stars: ~ 20 million years
The Sun: 10 billion years
Small cool red dwarfs: trillions of years
The hotter, the shorter the life!
Hot blues have more fuel but they go through it faster
Small stars have less fuel but they burn slowly

16. Main Sequence Lifetimes
Mass (in solar masses) Luminosity Lifetime
10 Suns ,000 Suns Million yrs
4 Suns Suns Billion yrs
1 Sun Sun Billion yrs
½ Sun Sun Billion yrs

17. Is the theory correct? Two Clues from two Types of Star Clusters
 Open Cluster
Globular Cluster 

18. Star Clusters
Group of stars formed from fragments of the same collapsing cloud
Same age and composition; only mass distinguishes them
Two Types:
Open clusters (young  birth of stars)
Globular clusters (old  death of stars)

19. Deep Sky Objects: Open Clusters
Classic example: Plejades (M45)
Few hundred stars
Young: “just born”
Still parts of matter
around the stars

20. What do Open Clusters tell us?
Hypothesis: Many stars are being born from a interstellar gas cloud at the same time
Evidence: We see
“associations” of stars
of same age
 Open Clusters