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