A protostar is a forming stellar object created when a dense region of a molecular cloud collapses under gravity and concentrates gas and dust into an increasingly compact, hot central body. It is a stage of stellar formation that occurs before sustained hydrogen fusion begins in the core. Star formation usually starts when a cold molecular cloud becomes gravitationally unstable or when external processes compress part of the cloud.
As the material collapses, conservation of angular momentum causes the rotating material to form a flattened accretion disc around the growing central object. Magnetic fields, turbulence, radiation and interactions with surrounding material influence the collapse. The central object gains mass as gas from the surrounding envelope and disc accretes onto it. A protostar therefore differs from an ordinary main sequence star because its luminosity is dominated initially by gravitational contraction and accretion rather than stable hydrogen fusion.
The material from which protostars form is primarily molecular gas containing hydrogen, helium and small amounts of heavier elements, together with dust grains. Molecular clouds are cold and dense compared with the wider interstellar medium, allowing molecules such as molecular hydrogen to exist. Regions within these clouds can become denser through turbulence, gravitational instability, cloud collisions, stellar feedback or other processes. Once gravity overwhelms opposing support, the region collapses.
As density increases, the collapse can become increasingly concentrated into dense cores. Conservation of angular momentum prevents all material from falling directly into the centre, so rotation promotes disc formation. The protostar, disc and surrounding envelope together constitute a complex evolving system. Observations at infrared, millimetre and radio wavelengths are particularly important because newly forming stars are often deeply embedded in dust that blocks visible light.
Protostars become hot primarily because gravitational potential energy is converted into thermal energy during contraction and accretion. As matter falls deeper into the gravitational potential well, its kinetic energy and gravitational energy are converted into heat and radiation. Accretion shocks can produce additional heating when material impacts the protostellar surface. Unlike a main sequence star, a protostar does not initially require hydrogen fusion to maintain its luminosity. Eventually, continued contraction raises the central temperature and density.
If the object has sufficient mass, the conditions become suitable for sustained hydrogen fusion and the object settles onto the main sequence. If its mass remains below the threshold required for stable hydrogen burning, it becomes a brown dwarf instead. The eventual mass of the forming object is therefore crucial to determining its later evolutionary path.
Protostars are frequently surrounded by accretion discs because conservation of angular momentum causes infalling material to orbit rather than fall directly inward. Viscous and magnetic processes can transfer angular momentum through the disc, allowing some material to spiral inward while other material moves outward. Protostellar discs are also important because they can become the sites where planets eventually form. The disc can contain gas, dust grains and larger solid particles that collide, stick and grow.
Young stellar systems can therefore link the processes of star formation and planet formation. Strong magnetic fields and rotation can also launch bipolar jets and outflows from the vicinity of the protostar and its disc. These outflows remove angular momentum and can inject energy and momentum into the surrounding molecular cloud, influencing both the protostar and its environment.
Protostars can produce powerful outflows and jets, often aligned approximately with the rotational axis of the accretion disc. These structures are thought to arise through interactions among magnetic fields, rotating accretion discs and infalling material. Jets can travel enormous distances through the surrounding cloud and produce bright shock fronts where fast material collides with slower gas. They can also help regulate the final mass of the forming star by removing material and energy from the system.
The feedback from young stars can influence nearby star formation, sometimes compressing neighbouring gas and sometimes dispersing it. Protostellar evolution is therefore not an isolated collapse but part of a larger ecosystem in which stars interact with their molecular cloud, neighbouring stars, magnetic fields and radiation.
The duration of the protostellar stage depends on the mass and accretion history of the object. Massive protostars can grow rapidly and reach high temperatures while still accreting substantial material, whereas lower mass protostars evolve more slowly. Once hydrogen burning becomes established, the object becomes a pre main sequence star and eventually reaches the main sequence.
For low and intermediate mass stars, the pre main sequence phase can involve contraction along evolutionary tracks such as the Hayashi track and, later, the Henyey track. The details depend on opacity, convection, composition and mass. The transition to the main sequence is therefore gradual in structural terms rather than a sudden switch that occurs at one universal moment. The main sequence is conventionally identified when sustained hydrogen fusion supplies the dominant internal energy source.
Protostars are difficult to observe in visible light because they are embedded in dense dust and gas. Infrared observations can penetrate some of this dust, while longer wavelength observations at millimetre and submillimetre wavelengths reveal cold dust and molecular gas. Radio observations can trace molecular outflows and jets, and high resolution interferometers can resolve discs and compact structures. Astronomers classify young stellar objects into observational categories based partly on the amount of surrounding envelope and disc material. These categories do not always correspond to perfectly distinct physical stages because star formation is continuous and objects can have overlapping properties. Nevertheless, observations of protostars provide direct evidence about the processes that transform diffuse interstellar gas into stars and planetary systems.
A common misconception is that a protostar is simply a smaller version of a normal star. A protostar is physically distinct because its structure and energy budget are strongly influenced by accretion and gravitational contraction, and sustained core hydrogen fusion has not yet become the dominant energy source. Another misconception is that all collapsing gas becomes a star.
Objects below the mass needed for stable hydrogen fusion become brown dwarfs, while fragmentation can produce multiple stars from a single molecular cloud core. Star formation also does not occur by simple spherical collapse because rotation, magnetic fields, turbulence, radiation and angular momentum create complex three dimensional structures. Protostars are therefore important transitional objects that connect molecular clouds with mature stars and provide the physical setting in which stellar masses, discs, jets and eventually planetary systems develop.
