Transcript Lecture11
Chapter 4: Formation of stars Insterstellar dust and gas • Viewing a galaxy edge-on, you see a dark lane where starlight is being absorbed by dust. An all-sky map of neutral hydrogen in the Milky Way. The plane of the galaxy is highly obscured by absorbing gas and dust. Looking toward the Galactic centre, in visible light. The interstellar medium • Stars are born from this gas and dust, collectively known as the interstellar medium. • During their lifetime, stars may return some material to the ISM through surface winds or explosive events • In supernova explosions, most of the star is dispersed throughout the ISM. Composition of the ISM • Hydrogen is by far the most common element in the ISM Molecular (H2) Neutral (HI) Ionized (HII) • Also contains helium and other elements. The solid component is in the form of dust. Properties of interstellar dust • Dust grains form by condensing out of a cooling cloud of interstellar gas. • Facilitate many chemical reactions They provide the only mechanism known for forming H2 • Radiate efficiently in the infrared, and therefore provide an effective means of cooling • Makes up ~10% of the ISM by mass • Composition: graphite, SiC, silicates, H2, H2O Types of molecular clouds Translucent clouds T=15-50 K n~5x108-5x109 m-3 M~3-100 MSun R~ 1-10 pc Giant molecular clouds T~20 K n~1x108-3x108 m-3 M~106 MSun R~50 pc Giant molecular cloud cores T~100-200 K n~1x1013-3x1015 m-3 M~10 – 1000 MSun R<1 pc The Jeans mass A simple energetic argument can give a rough approximation for the conditions required for a molecular cloud to collapse and form stars. 2K U 0 The virial theorem relates (time-averaged) kinetic to potential energy, for a stable, gravitationally bound system: This indicates a stability criterion: if the kinetic energy is too low, the cloud will collapse under the force of gravity It can be shown that a uniformdensity cloud will collapse if the mass exceeds the Jeans mass (or, equivalently, if the radius exceeds the Jeans length) 375k 3 M J 3 3 4 G mH 15k RJ 4GmH 1/ 2 1/ 2 1/ 2 T3 3 1/ 2 T Example: Diffuse HI clouds What is the Jeans mass for a typical diffuse cloud? M 100M Sun T 50 K n 5 108 m 3 Example: molecular cloud cores Typical conditions in molecular cloud cores: 10 M / M Sun 1000 T 150 K n 5 10 m 14 3 The sites of star formation • Could occur in giant molecular clouds with masses up to ~3x106Msun, in core regions where T≤30K Additional support provided by turbulence, magnetic fields, rotation need a trigger to start formation of small, dense cores where gravity can dominate possible triggers: supernova shock wave; stellar winds, spiral arm density waves Break Star formation A slowly-rotating, Jeans-unstable core of a molecular cloud can start to collapse Evolution of a solar mass protostar 1. Initially the clump is able to radiate all its gravitational energy efficiently, and collapses quickly. 2. As the core density increases the energy goes into heating the cloud. The core reaches approximate hydrostatic equilibrium, with a radius of ~5 AU. This is the protostar. Evolution of a solar mass protostar 3. 4. 5. Above the protostar, the rest of the cloud is still in free-fall. Rotation of the cloud means this collapsing material forms a disk. Eventually T becomes high enough that molecular hydrogen dissociates; this absorbs some of the energy supporting the protostar, so the core begins to collapse further, until it becomes ~30% larger than the present Solar radius (but still much less massive). The protostar continues to accrete material from the infalling cloud. Stellar disks Young main sequence stars often still have disks, even after the molecular cloud has been dispersed. Infrared-emitting dust disk around b-Pic. The central star has been subtracted. The dust disk around Vega. At least one large planet is known to exist within this disk. Disk shapes We can see that many disks are thinner in the centre than the edges. Why is this? Evolution of a solar mass protostar • When the star begins nuclear fusion it releases a large amount of energy in a bipolar jet, which: Prevents further collapse of material? Disperses gas disk? Gets rid of angular momentum? • As dust agglomerates into planetesimals, or is ejected by the jet, the central star becomes visible. Here we can actually see the stellar disk, illuminated by the central, obscured, star Herbig-Haro objects • Jets associated with star formation interact with the surrounding ISM, exciting the gas and forming bright, emission line objects. These are HH objects. Observations: T Tauri stars • T Tauri stars are a class of pre-main sequence stars with M<3 MSun with unusual spectra Strong Balmer emission lines Emission lines of Ca II and Fe Absorption lines of Li Forbidden lines [OI] and [SII]: indicate low gas densities • First view of the star as the cocoon is blown away. Pre-main sequence evolution Next lecture Planet formation • Condensation of solids from the solar nebula • Planet growth by: Collisional accretion Gravitational attraction • Final fate of planetesimals