and

How do we learn about star formation? What can you see with your very own eyes or through our very own telescope?
Going through each stage of the star formation process, with emphasis on circumstellar disks (I hope you’ll like them as much as I do!)

(One of them, anyway…) It’s very slow (~10 Myr), so we can’t watch it happen from beginning to end What can we do instead?
Study groups of stars!
How long does a particular evolutionary stage last?
Do all objects go through that evolutionary stage?

Cloudy Bluer Stars Bunched No Cloud Redder Stars Spread Out

Bluer Stars Redder Stars

Younger Older
On the Main Sequence, bluer hotter and more massive and die young.” stars are both . These stars “live fast
L

M
3.5
and   M /L Therefore,   1
M
2.5
Color of stars in a cluster is therefore an indicator of age (as we will measure in EL#3)

Cloudy No Cloud Younger Older
Cloud provides raw material for star formation After largest stars “turn on,” they blow away their birth material.

Bunched Spread Out Younger Older
Formation of stars from a cloud tends to yield bunched/clustered stars Galactic rotation smears out a cluster, dispersing young stars (rotation period: ~200 Myr)

??
??
bluer redder bluer redder

I.
II.
III. IV.
I. Molecular Cloud II. Protostar III. Pre-Main Sequence Star w/Disk IV. Main-Sequence Star (w/Disk?)

Q: Why “molecular”?
Space between stars is filled with warm gas, mostly atomic H. The densest and coldest regions (where stars will form) have most of their mass in molecular form (H 2 ). … but cold H 2 is mostly invisible!
I.
For areas not lit by stars, need to look at IR/radio wavelengths
Carbon Monoxide in the Orion Molecular Cloud

Equilibrium and Cloud Collapse: What is equilibrium?
I.
Equilibrium is pressure balance: nothing is happening!
Types of pressure in clouds: •Gravitational (inward) •Thermal (outward) •Magnetic field (outward) What initiates cloud collapse?
•Cloud-cloud collisions •Shocks from supernovae •Spiral arm passage Cloud (10 pc, 10 5 M sun ) Clumps (0.1 pc, few M sun ) Cores (<0.1 pc, ~M sun )
When cores collapse, gravity wins! And we have a protostar…

Twinkle, twinkle, little protostar…
What makes a protostar shine?
Answer: Gravity!
II.
Gravitational Binding Energy:
E
 
M
2
R
Energy loss by radiation:
E
 
L
  
E

E
 
M
2 
RL
BUT recall that luminosity is much greater for more massive stars!
So massive stars contract more quickly than low-mass stars.


III.
Almost a star… now with planet-forming potential!
Where do disks come from?
How do we know that there are disks?
0. Orbits in our own solar system 1. The Peculiar Story of Vega 2. Of course… the Hubble Space Telescope 3. Radio astronomy!

From leftover star dust to solar systems III.
Part 1: dust grains stick together Part 2: forming a planetary core It’s easy to get dust grains to stick together; less so for rocks.
Part 3: accreting gas Timing is everything QuickTime™ and a YUV420 codec decompressor are needed to see this picture.
C. Dominik
A competing idea: gravitational instability leads to fragmentation

III-IV There’s a hole in the middle of the disk!
Evidence from spectra… And you can actually see them!

Back to equilibrium
MS star is in hydrostatic and thermodynamic equilibrium, burning hydrogen to helium.
Hydrostatic equilibrium: balance between gravitational and thermal pressure
IV.
Thermodynamic equilibrium: energy generated by fusion = luminosity
What happened to the disk?
It’s probably still there! e.g. Sun’s zodiacal light, or Vega’s debris disk

IV.
“Debris” disk?
All the original small dust grains should have been blown from the system by the star. Any remaining dust must be from collisions of planetesimals!
Evidence of planetary systems: clumpy disks
QuickTime™ and a Cinepak decompressor are needed to see this picture.

• We learn about star formation by studying groups of stars – Color indicates age: hot, massive, blue stars die quickly – …but not before they blow away the cloud they were born from – Galactic rotation disperses clustered stars

– Molecular clouds are in equilibrium until collapse – Protostars shine by gravity as they contract – Disks form through conservation of angular momentum • Their properties tell us about planet formation process • Inner holes and clumps provide evidence of young solar systems • Debris disks get their dust from grinding of planetesimals – Main Sequence stars are in equilibrium again!