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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!
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?
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
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 GravityGravity
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
Example: Orion Nebula
• Orion Nebula is a place where stars are being born
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
Path in the Hertzsprung-Russell Diagram
Gas cloud becomes smaller, flatter, denser, hotter Star
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
Path in the Hertzsprung-Russell Diagram
Gas cloud becomes smaller, flatter, denser, hotter Star
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
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
Mass Matters• Larger 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
Mass and the Main Sequence
• The position of a star in the main sequence is determined by its massAll we need to know
to predict luminosity and temperature!
• Both radius and luminosity increase with mass
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!
Main Sequence Lifetimes
Mass (in solar masses) Luminosity Lifetime
10 Suns 10,000 Suns 10 Million yrs
4 Suns 100 Suns 2 Billion yrs
1 Sun 1 Sun 10 Billion yrs
½ Sun 0.01 Sun 500 Billion yrs
Is the theory correct? Two Clues from two Types of Star Clusters
Open Cluster
Globular Cluster
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)
Deep Sky Objects: Open Clusters
•Classic example: Plejades (M45)
•Few hundred stars
•Young: “just born”
Still parts of matter around the stars
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