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 (H 2 )  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  eV, corresponding to a wavelength =21 cm. - corresponds to radio frequencies, 1420 MHz

A map of neutral H in the Milky Way The Milky Way in optical light

Neutral Hydrogen in the Milky Way HI gas in the Milky Way clearly reveals spiral structure

Properties of interstellar dust Grain sizes: 1nm-10  m (i.e. similar to visible light) Composition: graphite, SiC, silicates, H 2, H 2 O

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: where d is measured in parsecs, and a is the number of magnitudes of extinction along the line of sight. How is this related to the optical depth?

Molecular clouds When hydrogen becomes dense enough, molecules of H 2 form: H 2 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 C 3 H 2.

Types of molecular clouds Translucent clouds T=15-50 K n~5x x10 9 m -3 M~3-100 M Sun R~ 1-10 pc a V ~1-5 Giant molecular clouds T~20 K n~1x x10 8 m -3 M~10 6 M Sun R~50 pc Giant molecular cloud cores T~ K n~1x x10 15 m -3 M~10 – 1000 M Sun R<1 pc a V ~

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. 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: The two are related by:

Example: Diffuse HI clouds Calculate the Jeans mass for diffuse clouds This is much greater than the typical cloud mass. Thus, diffuse clouds are stable and do not collapse.

Example: molecular cloud cores What is the Jeans mass for a molecular cloud core? 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. The time it takes for the shell containing mass M r to collapse to r=0 is the free- fall time scale:

Example: cloud collapse Notice the collapse starts off slowly, but the density increases sharply during the final stages.