Molecular Clouds and Gravitational Collapse
Stars form inside vast, cold clouds of molecular gas where the densest pockets, when their gravity overwhelms the forces holding them up, collapse to seed new stars.
Definition
Molecular clouds are cold, dense regions of the interstellar medium composed mainly of molecular hydrogen, and gravitational collapse is the runaway contraction of a region within them once self-gravity exceeds the opposing pressure and support.
Scope
The topic covers the physical properties of giant molecular clouds and their dense cores, the Jeans criterion for gravitational instability, the supporting roles of thermal pressure, supersonic turbulence, and magnetic fields, and the onset and early stages of collapse that precede protostar formation.
Core questions
- What are the properties of molecular clouds?
- Under what conditions does a cloud become gravitationally unstable?
- What supports clouds against collapse?
- How does collapse begin within a dense core?
Key concepts
- giant molecular cloud
- Jeans mass
- dense core
- supersonic turbulence
- magnetic support
- free-fall time
- interstellar dust
Key theories
- Jeans instability
- A self-gravitating gas cloud collapses when its mass exceeds the Jeans mass, the threshold at which gravity overcomes thermal pressure; this criterion sets the scale at which cold, dense regions of molecular clouds become unstable and begin to contract.
- Turbulent and magnetic support
- Molecular clouds are supported not only by thermal pressure but by supersonic turbulence and magnetic fields; this support keeps most cloud material from collapsing and means only special, dense cores reach the conditions for star formation.
Mechanisms
Cooling by molecules and dust keeps molecular clouds cold, lowering the Jeans mass so that gravity can act; within a cloud, turbulence creates transient dense regions, and where a core's mass exceeds the local Jeans mass and magnetic support is overcome, the core begins to contract on roughly its free-fall time toward forming a protostar.
Clinical relevance
The properties and stability of molecular clouds set where, when, and how efficiently stars form, controlling the star-formation rate of galaxies and the supply of the dense gas observed in tracers such as carbon monoxide and dust continuum.
History
Jeans derived the instability criterion bearing his name in 1902; later work established the molecular nature and turbulent, magnetized structure of star-forming clouds, with the balance between gravity, turbulence, and magnetic fields synthesized in the modern theory reviewed by McKee and Ostriker.
Key figures
- James Jeans
- Christopher McKee
- Eve Ostriker
- Leon Mestel
Related topics
Seminal works
- jeans1902
- mckee2007
Frequently asked questions
- Why are star-forming clouds so cold?
- Molecular hydrogen and trace molecules, together with dust grains, radiate away heat efficiently and shield the interior from starlight, keeping these clouds at only about ten to twenty degrees above absolute zero, which lowers the pressure and allows gravity to win.
- What is the Jeans mass?
- It is the critical mass above which a region of gas at a given temperature and density can no longer support itself against gravity and begins to collapse; colder, denser gas has a smaller Jeans mass and so collapses more readily.