Transcript Integrative Studies 410 Our Place in the Universe

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