Download Integrative Studies 410 Our Place in the Universe

Star Formation
(Compare: Solar System Formation)
Where Stars come from: the
Interstellar Medium
• Gas
– Single atoms and molecules
– Mostly hydrogen (90%), 9% helium; deficient in heavier
elements
• Dust
– Microscopic clumps of atoms/molecules
– Size ~ 107 m, similar to the wavelength of visible light
– Composition is not well known
• Temperature depends on the proximity of stars, typically
~100 K
• Density is very low!
– Gas: about 1 atom/cm3 D;
Dust: even less dense
How do we know it’s there?
• Cold gas or dust doesn’t glow
– they are dark
– We might “see” them blocking
light of other objects (Dark
Nebulae)
• Gas & Dust clouds are very
dilute
– they might not be blocking other
object’s light totally
– Usually they will reduce (redden)
the light of other objects
Reminder: Kirchhoff’s Laws
Cool gas absorbs light at specific frequencies
 Dark Lines: “fingerprints of the elements”
Looking Through Dust Clouds
Emission Nebulae
Example: Orion Nebula (M 42)
• hot glowing gas
Temperatures ~ 8000K
• Made to glow by
ultraviolet radiation
emitted by young
O- or B-type (hot)
stars located inside
• Color predominantly
red, the color of a
particular hydrogen
emission line (“H”)
Dark Nebulae
Keyhole Nebula
Dustlanes Nebula
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
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
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
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
Homework
• Cosmic distance ladder III: Use formulae
and descriptions given in question text of
question