Sky science

Protostar

Direct answer

Protostar: A Protostar is a collapsing young stellar object still gaining mass before sustained core hydrogen fusion defines the main sequence.

It is an early gravitational stage of star formation.

Astronomy & SkywatchingThe Great Almanac DictionaryUpdated 2026-09-26

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At a glance

Found in: Astronomy & Skywatching
Related: Hertzsprung-Russell Diagram, Main Sequence, Red Giant, Red Supergiant

A refracting telescope on its mount, pointed at a field of stars — Observable sky · visual guide **Protostar**
It is an early gravitational stage of star formation.
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The story behind Protostar

Twentieth-century infrared astronomy began penetrating stellar nurseries inaccessible to optical surveys.

A one-minute story

The Wandering Stars

How the moving planets were watched, named and found, and what a transit is.

Why Protostar matters

Protostar links cold molecular clouds with luminous newborn stars and disks.

Protostar in practice: an example

A protostar can drive bipolar outflows while material continues falling through a surrounding disk.

Sources and editorial method

Research references used for this entry include:

NASA Science: Stars - NASA overview of stellar birth, main-sequence evolution, giant phases, remnants, and supernovae.
NASA Science: Universe Glossary - NASA definitions for common stellar, galactic, orbital, and high-energy astronomy terms.

Source: The Great Almanac Dictionary
Category: Astronomy & Skywatching
Last updated: 2026-09-26

Enter a stellar nursery

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Hertzsprung-Russell DiagramIt maps stars by brightness and temperature. Main SequenceIt is the longest stable phase of most stars. Red GiantIt is a swollen, luminous late stellar phase. Red SupergiantIt is a late expansive phase of a high-mass star. AstronomyAstronomy is the science of celestial objects, space, and the physical universe. SkywatchingSkywatching is direct observation of the changing day and night sky.

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Common questions

What is a Protostar?

A Protostar is a collapsing young stellar object still gaining mass before sustained core hydrogen fusion defines the main sequence.

What do people get wrong about a Protostar?

Not every dense cloud core completes formation as a star.