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? 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

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 Gravity Gravity

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 Star can be plotted on the H-R diagram for the first time

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 Star can be plotted on the H-R diagram for the first time

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 First 6 stages take 40-50 million years, less than 1% of the time it spends on the main sequence

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 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

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 10-20 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

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

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