Why do stars form in molecular clouds and not elsewhere?

Stars need very cold, dense gas to collapse under gravity. Molecular clouds are the coldest, densest parts of the interstellar medium, shielded from disruptive starlight, so they are the only places where gas can gather and collapse into stars.

If molecular clouds are mostly hydrogen molecules, why are they studied using carbon monoxide?

Molecular hydrogen is hard to detect directly because it emits little at the cold temperatures of clouds. Carbon monoxide, a trace molecule, radiates readily and reliably traces the hydrogen, so it is used as a proxy to map molecular gas.

Molecular Clouds and Star-Forming Regions

Cold, dense molecular clouds are the nurseries of stars, where gravity overcomes support and gas collapses into new stars and planetary systems.

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Definition

Molecular clouds are cold, dense regions of the interstellar medium where hydrogen is mostly molecular and shielded from starlight; star-forming regions are the parts of these clouds where self-gravity overcomes internal support and gas collapses to form stars.

Scope

This topic covers the structure and properties of giant molecular clouds, the scaling relations among their size, line width, and density, the roles of gravity, turbulence, and magnetic fields in regulating collapse, the Jeans criterion for instability, and the sequence by which dense cores form protostars.

Core questions

What are the physical properties and structure of giant molecular clouds?
What sets the conditions under which a cloud becomes gravitationally unstable?
How do turbulence and magnetic fields regulate star formation?
How does a dense core collapse to form a protostar?

Key theories

Larson's scaling relations: Larson found that molecular clouds obey relations linking their size, internal velocity dispersion, and density, evidence that supersonic turbulence and near-equilibrium structure govern the clouds.
Gravitational instability and collapse: When a cloud's self-gravity exceeds the support from thermal pressure, turbulence, and magnetic fields, set by criteria such as the Jeans mass, it collapses to form stars.
Inside-out protostellar collapse: Shu and collaborators described how a dense core collapses from the inside out, building a central protostar surrounded by an accretion disk and infalling envelope.

Clinical relevance

Molecular clouds are the immediate sites of all star and planet formation, so understanding them connects the interstellar medium to the origin of stars, planetary systems, and the chemical enrichment of galaxies.

History

The detection of interstellar molecules, especially carbon monoxide in the early 1970s, revealed giant molecular clouds as the dominant reservoir of dense gas. Larson's 1981 scaling relations and Shu's 1987 collapse theory framed how these clouds form stars, a picture later enriched by studies of turbulence and magnetic support.

Key figures

Frank Shu
Richard Larson
Christopher McKee
Eve Ostriker

Seminal works

larson1981
shu1987
mckee2007

Frequently asked questions

Why do stars form in molecular clouds and not elsewhere?: Stars need very cold, dense gas to collapse under gravity. Molecular clouds are the coldest, densest parts of the interstellar medium, shielded from disruptive starlight, so they are the only places where gas can gather and collapse into stars.
If molecular clouds are mostly hydrogen molecules, why are they studied using carbon monoxide?: Molecular hydrogen is hard to detect directly because it emits little at the cold temperatures of clouds. Carbon monoxide, a trace molecule, radiates readily and reliably traces the hydrogen, so it is used as a proxy to map molecular gas.