Fate of comets

1. Fate of comets Sun • This “Sun-grazing” comet was observed by the SOHO spacecraft a few hours before it passed just 50,000 km above the Sun's surface. • The comet did not survive its passage, due to the intense solar heating and tidal forces. • Shoemaker-Levy collided with Jupiter in 1994 • Was previously tidally disrupted into a string of fragments • Each fragment hit Jupiter with the energy of a 10 megaton nuclear bomb explosion

2. Astronomy picture of the day: http://antwrp.gsfc.nasa.gov/apod/astropix.html

3. Chapter 4: Formation of stars

4. 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.

5. 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.

6. 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.

7. 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

8. Types of molecular clouds • Giant molecular clouds • T~20 K • n~1x108-3x108 m-3 • M~106 MSun • R~50 pc • Translucent clouds • T=15-50 K • n~5x108-5x109 m-3 • M~3-100 MSun • R~ 1-10 pc • Giant molecular cloud cores • T~100-200 K • n~1x1013-3x1015 m-3 • M~10 – 1000 MSun • R<1 pc

9. 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. • 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 uniform-density cloud will collapse if the mass exceeds the Jeans mass (or, equivalently, if the radius exceeds the Jeans length)

10. Example: Diffuse HI clouds • What is the Jeans mass for a typical diffuse cloud?

11. Example: molecular cloud cores • Typical conditions in molecular cloud cores:

12. 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

13. Break

14. Star formation • A slowly-rotating, Jeans-unstable core of a molecular cloud can start to collapse. It will form a disk – why?

15. Evolution of a solar mass protostar • Initially the clump is able to radiate all its gravitational energy efficiently, and collapses quickly. • 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.

16. Evolution of a solar mass protostar • 3. Above the protostar, the rest of the cloud is still in free-fall. Rotation of the cloud means this collapsing material forms a disk. • 4. 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). • 5. The protostar continues to accrete material from the infalling cloud.

17. 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

18. 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.

19. Stellar disks • Young main sequence stars often still have disks, even after the molecular cloud has been dispersed. • The dust disk around Vega. At least one large planet is known to exist within this disk. • Infrared-emitting dust disk around b-Pic. The central star has been subtracted.