prōtos + astēr

Greek prōtos (πρῶτος) means first, and astēr (ἀστήρ) means star. A protostar is a star in its earliest phase — a dense core of molecular gas that has begun to collapse under its own gravity but has not yet reached the temperature and pressure needed for hydrogen fusion. The term entered astrophysical vocabulary in the mid-twentieth century as infrared astronomy revealed objects that were too hot to be ordinary gas clouds but too cool to be stars.

The collapse begins when a region of a molecular cloud — itself a cold, dark accumulation of hydrogen and dust — reaches a critical density. A shockwave from a nearby supernova or a passing density wave in a spiral arm can trigger the collapse. The Jeans mass, named after physicist James Jeans who calculated it in 1902, defines the threshold: above this mass, gravity overwhelms thermal pressure, and the cloud begins to fall inward.

A protostar in the Milky Way's Orion Nebula takes about 100,000 years to form — fast by astronomical standards. The collapsing gas heats as it compresses, eventually reaching thousands of degrees at the core. A disc of leftover material forms around the protostar, and from this disc, planets may eventually coalesce. Our solar system began this way, approximately 4.6 billion years ago, from a cloud astronomers call the solar nebula.

Protostars are invisible to optical telescopes — the surrounding dust blocks visible light. They were first detected in the 1960s using infrared instruments. The Spitzer Space Telescope and the James Webb Space Telescope have since revealed thousands, glowing in wavelengths the eye cannot see. Every star in the sky was once a protostar. Every protostar is a star waiting to happen.