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… Bluer Stars Redder Stars Cloudy No Cloud Bunched Spread Out

Bluer StarsRedder Stars On the Main Sequence, bluer stars are both hotter and more massive. These stars “live fast and die young.” Color of stars in a cluster is therefore an indicator of age (as we will measure in EL#3) YoungerOlder and Therefore,

CloudyNo Cloud Cloud provides raw material for star formation After largest stars “turn on,” they blow away their birth material. YoungerOlder

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

Which group of stars is older? ?? bluerredderbluerredder

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

I. Molecular Cloud 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! For areas not lit by stars, need to look at IR/radio wavelengths I. Carbon Monoxide in the Orion Molecular Cloud

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

II. Protostar II. Twinkle, twinkle, little protostar… What makes a protostar shine? Answer: Gravity! Gravitational Binding Energy:Energy loss by radiation:Kelvin-Helmholtz time: 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? 1. The Peculiar Story of Vega 2. Of course… the Hubble Space Telescope 3. Radio astronomy! 0. Orbits in our own solar system

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 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

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!