Star formation begins with the most abundant material in the universe, where vast, diffuse clouds of gas and dust condense under their own weight to create luminous stars. A protostar represents the earliest stage of this stellar evolution, defined as a dense, hot core that is still gathering mass. It has not yet achieved the extreme temperatures required for sustained nuclear fusion in its center. This article explains the physical forces and external triggers necessary to initiate the collapse of a stable cloud and begin the birth of a star.
The Starting Material: Molecular Clouds
Stars are born within massive concentrations of interstellar matter known as molecular clouds, often referred to as stellar nurseries. These clouds are overwhelmingly composed of molecular hydrogen and atomic helium, along with trace amounts of heavier elements and fine silicate dust grains. They are extremely cold environments, maintaining temperatures typically ranging between 10 and 30 Kelvin (about negative 440 degrees Fahrenheit).
Molecular clouds are significantly denser than the average interstellar medium, though still millions of times less dense than Earth’s atmosphere. This high density allows atoms to combine into molecules and provides the mass needed for gravity to dominate. These clouds can span hundreds of light-years and contain the mass equivalent of up to ten million Suns, making them the necessary raw material for large-scale star formation.
Overcoming Stability: The Initial Trigger for Collapse
Star formation is fundamentally a competition between two opposing forces: the outward push of thermal pressure and the inward pull of gravity. Thermal pressure, generated by the random motion of gas particles, works to keep the cloud diffuse and stable. Gravity attempts to pull every particle toward the cloud’s center.
For a cloud to collapse, gravity must decisively overcome the resistance of thermal pressure. This critical condition is quantified by the concept of Jeans Instability, named after physicist Sir James Jeans. This instability defines a minimum mass, the Jeans Mass, that a region of the cloud must possess to spontaneously collapse. If a cloud fragment’s mass exceeds the Jeans Mass for its temperature and density, its self-gravity is sufficient to initiate a runaway collapse.
Because most molecular clouds are in a state of quasi-equilibrium, they require an external event to compress them past this critical Jeans threshold. A common trigger is the powerful shockwave generated by a nearby supernova explosion. This rapidly expanding wave sweeps up and compresses the surrounding interstellar gas, effectively increasing the density of a molecular cloud region.
Other external forces include the gravitational influence of density waves moving through the spiral arms of galaxies. The collision of two separate molecular clouds can also provide the necessary compression to trigger instability and collapse. Once the cloud is compressed and the Jeans criterion is met, the collapse proceeds rapidly, with the densest regions falling inward.
The Mechanics of Gravitational Contraction and Fragmentation
Following the initial trigger, the large molecular cloud begins a highly non-uniform gravitational contraction. Instead of collapsing into a single object, the cloud fragments into multiple, smaller, gravitationally bound clumps called pre-stellar cores. This hierarchical fragmentation occurs because the Jeans Mass decreases as the density increases during the initial collapse.
As material falls inward, gravitational potential energy converts into kinetic energy, which is thermalized, causing the core’s temperature to rise. Initially, the core remains relatively cool because it is “optically thin,” allowing the heat generated by the collapse to be radiated away into space. This radiative cooling keeps the collapse nearly isothermal, meaning the temperature does not increase as quickly as the density.
As the core becomes denser and more opaque, the radiation becomes trapped within the gas. This trapping of thermal energy causes the core to heat up dramatically, forming a small, pressure-supported object known as the “first core.” This sudden increase in internal pressure temporarily slows the infall of material, marking the transition to the distinct protostar phase. The gravitational contraction will continue to dominate this core’s energy output for hundreds of thousands of years.
Defining the Protostar Phase
The true protostar phase begins when the central core becomes opaque and starts accumulating mass from the surrounding envelope of gas and dust. This earliest stage of stellar life lasts for approximately 500,000 years for a Sun-like star. A defining characteristic of this stage is the formation of a flattened, rotating structure called an accretion disk around the central object.
The disk forms because the original cloud core possessed rotation. As the material contracts, the conservation of angular momentum causes the spin rate to increase significantly. Material in the disk slowly spirals inward, feeding mass onto the growing protostar. The energy released by this accretion process is the primary source of the protostar’s luminosity, which can be far greater than the future main-sequence star.
Another sign of a protostar is the presence of powerful, collimated jets, known as bipolar outflows, expelled from the poles of the system. These jets are launched by magnetic fields interacting with the accretion disk and serve the function of carrying away excess angular momentum. By expelling this angular momentum, the protostar continues gathering mass from the disk, allowing the central core to contract and heat up enough to achieve the final condition for stardom: the onset of sustained hydrogen fusion.