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Lecture 14 Star formation Insterstellar dust and gas • Dust and gas is mostly found in galaxy disks, and blocks optical 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 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. Neutral hydrogen • HI can emit radiation if the electron flips its spin angular momentum vector. • This is a very small energy difference of only 5.9 meV, corresponding to a wavelength l=21 cm. - corresponds to radio frequencies, 1420 MHz The Milky Way in optical light A map of neutral H in the Milky Way Neutral Hydrogen in the Milky Way HI gas in the Milky Way clearly reveals spiral structure Properties of interstellar dust • Grain sizes: 1nm-10 mm (i.e. similar to visible light) • Composition: graphite, SiC, silicates, H2, H2O Interstellar dust • Interstellar dust is likely produced in the envelopes around red supergiant stars. • Radiate in the infrared (cooling mechanism) • Are easily destroyed by collisions Interstellar extinction Dust scatters starlight. Thus a star behind a dust cloud will appear fainter. The apparent magnitude of a star is therefore: ml M l 5 log 10 d 5 al where d is measured in parsecs, and al is the number of magnitudes of extinction along the line of sight. How is this related to the optical depth? al 1.086 l Molecular clouds When hydrogen becomes dense enough, molecules of H2 form: • H2 is nearly impossible to observe: there are no emission or absorption lines at visible or radio wavlengths Thus we rely on tracer molecules, most commonly CO but also CH, OH, CS and C3H2. Types of molecular clouds Translucent clouds T=15-50 K n~5x108-5x109 m-3 M~3-100 MSun R~ 1-10 pc aV~1-5 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 aV~50-1000 The sites of star formation The cores of molecular clouds are likely sites of new star formation The formation of protostars There are many unanswered questions about the formation of protostars Since they form in very dense, opaque clouds of dust and gas they are very difficult to observe in detail Break 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 This defines a critical mass, known as the Jeans mass: It can also be expressed as a radius, in terms of the Jeans length: 375k 3 M J 3 3 4 G mH 15k RJ 4GmH 1/ 2 1/ 2 1/ 2 T3 3 m 1/ 2 T m 5kT M J The two are related by: GmmH RJ Example: molecular cloud cores What is the Jeans mass for a molecular cloud core? 10 M / M Sun 1000 T 150 K n 5 1014 m 3 M J 4.5M Sun Thus these cores should be collapsing under the weight of their own gravity, consistent with their association with the sites of star formation. Cloud collapse A collapsing molecular cloud starts off simply: In free-fall, assuming the pressure gradients are too small to have much effect The gas is approximately isothermal, if gas is optically thin so energy can be efficiently radiated away. 8G0 r02 r0 dr 1 dt 3 r The time it takes for the shell containing mass Mr to collapse to r=0 is the freefall time scale: 1/ 2 3 1 t ff 32 G 0 Example: cloud collapse Notice the collapse starts off slowly, but the density increases sharply during the final stages.