M3 Interstellar Medium and Star Formation

Topic

The central figure in this model is the giant molecular cloud that collapses to give rise to a star.

Star formation takes place within these molecular clouds—cold, dense regions of the interstellar medium—when the cloud's mass exceeds a critical value known as the Jeans mass; this value decreases as the cloud becomes colder and denser. Because the Jeans mass decreases in this way, colder, more compact clouds collapse much more readily than warmer, diffuse ones.

The cloud's collapse begins as an isothermal process, as the gas manages to cool efficiently by emitting molecular radiation while being compressed; however, it becomes adiabatic once the density crosses a specific threshold. Crossing this threshold causes the gas temperature to rise and, at that very moment, gives birth to the actual protostar at the center of the collapsing cloud.

Accretion luminosity—derived from the gravitational potential energy of gas falling onto the protostar and converting into radiation—dominates its brightness throughout this phase; this luminosity increases with the gas infall rate and decreases as the protostar's radius grows.

Meanwhile, the Salpeter initial mass function describes how the masses of newly born stars are distributed within the cloud: low-mass stars are by far the most numerous, yet the few massive stars that form dominate both the group's total luminosity and the chemical enrichment the cloud eventually contributes to the galaxy as a whole. Star formation, however, generates its own feedback: the bipolar jets, winds, and ionizing photons emitted by newborn stars act as a brake on the very cloud that spawned them, limiting the efficiency of the entire process to just around one percent of that cloud's original mass. Thus, from the giant molecular cloud with which this document begins to the self-braking feedback with which it concludes—and taking in the Jeans mass, the protostar, and accretion luminosity along the way—star formation results in a single cloud ultimately limiting, through the products of the few stars it manages to form, its own capacity to continue forming more.

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