How do stars form? From cold gas to a glowing fusion reactor

It begins in an uneven cloud

Interstellar molecular clouds consist mainly of gas and a small proportion of dust. They are neither completely still nor equally dense throughout. Flows and turbulence create concentrations of material, while thermal pressure and magnetic fields can resist contraction. Where gravity becomes strong enough, material begins to collapse. A cloud core shrinks and heats up as gravitational energy is released. This process may initially remain hidden because the surrounding envelope strongly attenuates visible light. Infrared and millimetre observations are therefore particularly important. A luminous young source need not yet be a fully fledged star: even a protostar can radiate considerable energy as material falls onto it. Observations therefore distinguish between different stages of development, rather than treating star formation as a single, sudden act of ignition.

Angular momentum turns collapse into a disc

Even a slowly rotating cloud has angular momentum. As it contracts, this becomes more important, rather as a figure skater spins faster with her arms drawn in. The material therefore does not all simply fall in a straight line towards the centre. A disc can form around the protostar, allowing more gas to move inwards. At the same time, outflows and narrow jets are observed in many young systems. Magnetic fields and the dynamics of the disc play an important role in this interplay. Researchers are still investigating how efficiently mass and angular momentum are transported during the different stages of development. The disc is also the place where planets may later form. Star and planet formation are therefore closely linked, although they do not follow the same course or timescale in every system.

Mass determines the subsequent life story

If pressure and temperature at the centre rise sufficiently, hydrogen fusion can take over as the long-term energy source. The forming star reaches a phase in which energy production and energy loss, together with its internal structure, establish an approximately stable equilibrium. How long this phase lasts depends strongly on its mass. Massive stars have more fuel, but consume it so much faster that their lives are substantially shorter than those of low-mass stars. Not every collapsing concentration of material reaches the conditions needed for sustained hydrogen fusion; below a certain mass, brown dwarfs form. Nor does all the original material become stars. Radiation, winds and later explosions alter the surrounding cloud. Star formation is thus both a local process of collapse and part of a larger cycle that influences subsequent generations.

Keep exploring

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• Dark matter: why astronomers infer invisible mass