St*r F*rmation!
NGC 602 and Beyond Credit: NASA, ESA, and the Hubble Heritage Team (STScI / AURA) - ESA/Hubble Collab.
Basics:
-Stars are born in giant molecular clouds, which have cold, dense cores.
- Stars are not born alone! They are born in structures ranging from multiple
systems to clusters.
- Star formation in a self-limiting process. Feedback from the young stars and
supernova explosions stops it. (The global environment of the clouds is also selfregulated.)
X-rays and the Eagle Nebula Credit: X-ray: NASA/CXC/J.Linsky (U.Colorado) et al.; Optical: NASA/ESA/STScI/
J.Hester, P.Scowen (ASU)
Molecular Cloud Basics
Mass ≈ 104 - 106 Msun,
Size ≈ 3 pc.,
<ρ> ≈ 104 cm-3 (higher in core)
Compare to Sun: <ρ> ≈ 1024 cm-3 (must collapse by 1020 in density!)
Back to the clouds….
<T> ≤ 100 K,
<T>core ≤ 10 K
About 1/2 the interstellar mass in the galaxy is in molecular form. ( ≈ 104
GMCs).
Interstellar molecules in general
- Absorb and emit over broad wavelength bands.
- Their wavelengths are typically in the IR-radio regions.
- Molecules are present only where the ISM is cold.
- They are dissociated by UV, and must be shielded.
- They often form on grain surfaces.
-they are abundant in large, dense “molecular clouds,” near star formation
complexes. (Their formation rates depend on density.)
- They include: H2, OH, H2O, CO, CN, CH4, CO2, C2H5OH, CH3NH2, NH3,
SiO,…, and amino acid building blocks.
- Molecular line observations provide info on:
- cloud temperatures
- cloud internal densities
- turbulent velocity dispersions.
Orion: A Nearby Molecular Cloud Complex
Canaries Sky Credit & Copyright: A. Vannini, G. Li Causi, A. Ricciardi, A. Garatti This gorgeous view of stars, nebulae,
and the Milky Way comes from the dark night sky above the lovely island of La Palma in the Canaries archipelago. The
picture was made by a group of experienced astrophotographers who traveled there to take advantage of the ideal observing
conditions near La Palma's Observatorio del Roque de los Muchachos. Skygazers can easily pick out several of their favorite
astronomical objects in this wide angle time exposure which covers about 40 degrees on winter the sky.
Orion constituents:
Dust: “Horsehead nebula,” T ≈ 30K, emits copious IR.
HII Region: “Orion nebula” around the Trapezium stars, is expanding outwards.
HI Emission: red nebulosity throughout the region.
An Area of Active Star Formation
-Lots of young massive OB stars.
- Dense molecular clouds.
- Expanding HII regions (and jets and winds) compressing the nearby ISM.
Stimulating further SF?
A living example of the continuing process of star formation in the ISM.
Physical Processes in Star Formation
• Stars form as a result of local gravitational collapse, but gravity is resisted by gas
and turbulent pressure.
• Early theory developed by James Jeans.
- At a given density & temperature there is a critical size (Jean’s length) or
mass (Jeans mass). Only clouds larger or more massive than these values
can collapse. (Gravity beats pressure.)
Jeans mass ∝ ρ−1/2 Τ3/2.
- Only cold, dense clouds can collapse.
• In a molecular cloud (e.g., M ≈ 105 Msun), MJeans ≈ 300 Msun.
How to get from clumps with M ≈ 300Msun, R ≈ 1 pc to protostars?
1.
Increase density
E.g., kick the cloud:
cloud collisions, stellar winds, supernovae.
2. Fragmentation in collapse (a la Jeans)
Initial collapse of cloud ––>
Fragmentation into Jeams clumps ––>
ρ increases, MJ decreases ––>
More collapse, further fragmentation
––> Star Cluster?
Timescale = free-fall time ~ (Gρ)-1/2.
But turbulence greatly complicates the picture!
Observations of Pre-Main Sequence Stars
Mostly IR observations of sites within molecular clouds.
- Lots of dust surrounding cloud.
- Dust heated up to 1000K ––> thermal IR.
Bok globules: dark knots against bright background. Protostar nurseries.
IR stars & Cocoon nebulae: compact nebulae, bright in IR.
T-Tauri stars (“naked” or “dressed”):
- cool stars with irregular brightness variations, several subclasses.
- blowing away the cocoon?
- evidence for protoplanetary disks.
Bipolar flows and YSOs:
- collimated flows of gas (with molecules) away from IR star.
- More disks!
Quick Summary of Pre-Main Sequence Evolution
At least the quasi-static stage, which has two parts…
100 Msun
-10
3 Msun
1 Msun
Mbol
0
0.1 Msun
10
M.S.
4.4
(25,000)
4.0
Log(T)
3.6
(4000)
1.
Hyashi phase:
The star is almost completely convective, with low T, low ionization.
Convection ––> T ≈ const.
Contraction ––> L ~ R2T4 decreases.
Timescale ≤ 106 yr.
2. Radiative core develops and grows.
T increases ––> so ionization increases, and opacity decreases (near
surface).
R decreases, but Teff increases. Competition between R, T
––> L ≈ const.
This phase is ended with the onset of H burning.