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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 Gravity Heat 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 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 mass All 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) Lifetime 10 Suns 10 Million yrs 4 Suns 2 Billion yrs 1 Sun 10 Billion yrs ½ Sun 500 Billion yrs Luminosity 10,000 Suns 100 Suns 1 Sun 0.01 Sun 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