Download A-36_SF

The Birth of Stars
and Planets
Plan for the next ~45 min
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!)
The Problem With Star Formation
(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?
The Winter Sky
Some detective work…
Cloudy
Bluer Stars
Bunched
No Cloud
Redder Stars
Spread Out
Bluer Stars
Redder Stars
Younger
Older
On the Main Sequence, bluer stars are both
hotter and more massive. These stars “live fast
and die young.”
LM
3.5
and   M /L
Therefore,

1
M 2.5
Color of stars in a cluster is therefore an indicator
 (as we will measure 
of age
in EL#3)
Cloudy
Younger
No Cloud
Older
Cloud provides raw material for star formation
After largest stars “turn on,” they blow away their
birth material.
Bunched
Younger
Spread Out
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)
Which group of stars is older?
??
bluer
redder
??
bluer
redder
From Cloud to Star
I.
I.
II.
III. IV.
Molecular Cloud
II. Protostar
III. Pre-Main Sequence Star w/Disk
IV. Main-Sequence Star (w/Disk?)
I. Molecular Cloud
I.
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 (H2).
… but cold H2 is mostly invisible!
For areas not lit by stars, need to look at
IR/radio wavelengths
Carbon Monoxide in the
Orion Molecular Cloud
I. Molecular Cloud
I.
Equilibrium and Cloud Collapse:
What is equilibrium?
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, 105 Msun)
Clumps (0.1 pc, few Msun)
Cores (<0.1 pc, ~Msun)
When cores collapse, gravity wins! And we have a protostar…
II. Protostar
II.
Twinkle, twinkle, little protostar…
What makes a protostar shine?
Answer: Gravity!
Gravitational Binding Energy:
Energy loss by radiation:
M2
E  
R
E L

Kelvin-Helmholtz time:


2
E M

E  RL
BUT recall that luminosity is much greater for more massive stars!
So massive stars contract more quickly than low-mass stars.
III. Pre-MS Star w/Disk
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!
III. Pre-MS Star w/Disk
III.
From leftover star dust to solar systems
Part 1: dust grains stick together
Part 2: forming a planetary core
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
It’s easy to get dust grains to stick together;
less so for rocks.
Part 3: accreting gas
Timing is everything
A competing idea: gravitational instability leads to fragmentation
C. Dominik
III to IV: Transition Disks
III-IV
There’s a hole in the middle of the disk!
Evidence from spectra…
And you can actually see them!
IV. Main Sequence Star
IV.
Back to equilibrium
MS star is in hydrostatic and thermodynamic
equilibrium, burning hydrogen to helium.
Hydrostatic equilibrium: balance between
gravitational and thermal pressure
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. Main Sequence Star
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.
Summary I
• 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
Summary II
• Stars pass through several stages as
they form
– 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!