A protostar is a young stellar object that is still gathering mass from its surrounding cloud of gas and dust. This is the earliest phase in the life cycle of a star, before the onset of sustained hydrogen fusion in its core. For a star similar in mass to our Sun, this process can last for about 500,000 years. The protostar stage begins when a dense cloud fragment collapses under gravity and forms an opaque, pressure-supported core.
Giant Molecular Clouds
The stellar birth process is confined to specific regions of space known as Giant Molecular Clouds (GMCs). These vast complexes are the coolest and densest portions of the Interstellar Medium (ISM). They are primarily composed of molecular hydrogen gas, which does not emit visible light, along with small amounts of cosmic dust.
GMCs can span hundreds of light-years and contain enough material to form thousands to millions of stars. The extremely low temperatures within these clouds, typically ranging from 10 to 30 Kelvin, allow molecules to form and gravity to overcome the outward thermal pressure. The densest regions within these clouds are where star-forming cores begin to take shape.
Initiating Gravitational Collapse
For a stable molecular cloud to form a star, its internal pressure must be overcome, triggering a gravitational collapse. This process often relies on external events to compress the gas past the threshold for gravitational instability. The critical condition for collapse, known as the Jeans criterion, dictates that a cloud fragment must have enough mass or be cold enough for gravity to overcome pressure.
Several dynamic events can provide the necessary compression. Shock waves originating from a supernova can sweep through a nearby cloud and squeeze the material. Collisions between two molecular clouds can also create powerful localized compression. Furthermore, density waves that propagate through the spiral arms of a galaxy can create regions of enhanced density, promoting star formation.
Core Formation and Accretion
Once the collapse is underway, the inwardly falling material quickly forms a dense, opaque central object. The protostar’s luminosity does not come from nuclear reactions but from the gravitational potential energy released as the gas contracts and heats up. This core continues to grow by pulling in the surrounding gas and dust, a process known as accretion.
The conservation of angular momentum is a significant factor, causing the slowly rotating cloud fragment to spin up considerably as it shrinks. This rapid rotation prevents material from falling directly onto the protostar, instead flattening the matter into a rotating accretion disk. Material spirals inward through this disk, feeding the growing central protostar with mass.
A defining feature of the protostar stage is the presence of powerful bipolar outflows, or jets, which are collimated streams of gas ejected from the poles of the system. These jets help the protostar shed excess angular momentum and clear away the surrounding cloud material, allowing accretion to continue. The protostar remains deeply embedded in its dusty envelope, making it observable primarily at infrared and radio wavelengths.
Transition to a Young Star
The protostar phase concludes when the central core has gathered nearly all of its final mass and the infalling gas is depleted. As the core continues to contract under its self-gravity, the internal temperature and pressure rise dramatically. When the core temperature reaches approximately 10 million Kelvin, sustained nuclear fusion of hydrogen into helium begins.
This ignition creates a powerful outward pressure that halts the gravitational contraction, bringing the star into a state of hydrostatic equilibrium. The object has now transitioned into a pre-main sequence star, such as a T Tauri star, which is still contracting but is no longer heavily accreting material. This newly formed star will continue to contract slowly until it settles onto the main sequence, where it will spend the majority of its life fusing hydrogen in its core. For a star the mass of the Sun, the entire process from initial collapse to main sequence takes about 50 million years.