1. Star Formation (Compare: Solar System Formation)

2. 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
Dust: some evidence for silicates (rock), carbon and iron, probably contains some dirty ice (methane, ammonia, water ice) – like comet material
Because of large volumes, the masses are enormous. Mass of interstellar matter within our galaxy is 20 billion solar masses !

3. 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

4. Reminder: Kirchhoff’s Laws
Cool gas absorbs light at specific frequencies
 Dark Lines: “fingerprints of the elements”

5. Looking Through Dust Clouds

6. 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”)

7. Dark Nebulae
Keyhole Nebula
Dustlanes Nebula

8. 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!

9. 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?
They come in all variety of shapes, a few are abnormal
They display variety of sizes (smallest ~30cm, largest 2.5m)
There are few varieties of skin color
There are VAST differences in behavior.
Can we find evidence of evolution from one state to another in individuals?
Infants—Small People—Large People
Where do people come from? Are they eternal? Dead people?
Create MODELS

10. 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

11. 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

12. Example: Orion Nebula
Orion Nebula is a place where stars are being born

13. Path in the Hertzsprung-Russell Diagram
Gas cloud becomes smaller, flatter, denser, hotter  Star

14. 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
First 6 stages take million years, less than 1% of the time it spends on the main sequence

15. Mass Matters Larger masses Smaller 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

16. 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
The position of a star in the main sequence is found to be determined by its mass, which varies from 0.2 solar masses (red dwarfs) to solar masses.
In other words, given the mass of a star in the main sequence, we can predict (approximately) its absolute luminosity and its spectral class (temperature)
Clear progression from low-mass red dwarfs to high-mass blue giants

17. Homework
Cosmic distance ladder III: Use formulae and descriptions given in question text of question