The Fundamental Problem in studying the stellar lifecycle

1
The Fundamental Problem in studying the stellar
lifecycle

• We study the subjects of our research for a tiny
  fraction of its lifetime
• Suns 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?

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

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

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

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
• 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

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!

16
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

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