A protostar represents the earliest stage in a star’s life cycle, preceding the ignition of nuclear fusion in its core. It is a massive concentration of gas and dust that has begun to collapse under its own weight, but is not yet hot enough to sustain fusion. Because it is deeply embedded within the thick cloud of material from which it is forming, the protostar is not visible at optical wavelengths. This stage is relatively brief, lasting only about 500,000 years for a sun-like star, as it rapidly gathers its future mass.
The Cosmic Cloud of Origin
The journey of a star begins within immense, cold regions of space known as giant molecular clouds or nebulae. These clouds are composed primarily of molecular hydrogen and helium, along with trace amounts of dust. Within these clouds, pockets of slightly denser material exist, which begin to pull in surrounding gas and dust due to mutual gravitational attraction.
The initial collapse of this dense pocket into a pre-stellar core is often slow, but external forces can accelerate it. Shockwaves from a nearby supernova or the gravitational influence of passing spiral arms can compress a section of the cloud. This compression increases the local density past a threshold, triggering the runaway gravitational collapse that begins the star formation sequence.
The Physics of Growth and Heating
Once the dense core forms, the protostar enters a prolonged phase defined by gravitational contraction and accretion. As the core shrinks, the conversion of gravitational potential energy into kinetic energy heats the material intensely. This heat is the primary source of the protostar’s luminosity, since it has not yet begun to fuse hydrogen.
As cloud material falls inward, conservation of angular momentum causes the gas to flatten into a rapidly spinning accretion disk, or protoplanetary disk. Material from this disk slowly spirals inward, feeding mass onto the forming star. This disk is where planets will eventually form, marking the simultaneous beginning for both the star and its planetary system.
A result of this rapid accretion and rotation is the creation of spectacular bipolar outflows, or jets, that shoot out from the protostar’s poles at high speeds. These jets, often observed as Herbig-Haro objects, are driven by the interaction between the star’s magnetic field and the inner edge of the accretion disk. The outflows are a mechanism to shed excess angular momentum, which prevents the protostar from spinning itself apart.
Ignition: Transition to a Star
The protostar phase concludes when the continuous infall of gas and dust ceases, and the object transitions into a pre-main-sequence star. For low-mass stars like the sun, this intermediate stage is known as the T Tauri phase. T Tauri stars are still powered by gravitational contraction, but they no longer actively gather mass and are characterized by strong stellar winds and brightness variations.
For a star to be considered fully born, its core must achieve at least 10 million Kelvin, the threshold for sustained hydrogen fusion to begin. This process provides an internal pressure that precisely balances the crushing inward force of gravity, a state known as hydrostatic equilibrium. Once this balance is reached, the star settles onto the main sequence, where it will spend the majority of its life.
The time required to reach this fusion-ignition temperature varies greatly by mass. Objects with insufficient mass, specifically less than about 8% of the sun’s mass, never reach the required core temperature for hydrogen fusion. Instead, they cool and contract until electron degeneracy pressure halts the collapse, resulting in a failed star known as a brown dwarf.
How Scientists Detect Protostars
Observing protostars directly is challenging because they are completely enshrouded by the dense, opaque cloud of gas and dust from which they formed. The surrounding material blocks nearly all visible light, making it impossible to study them with traditional optical telescopes. Astronomers must use specialized instruments that can penetrate this cosmic veil.
Protostars emit an enormous amount of heat as they contract and accrete matter. The surrounding dust absorbs this energy, warming up significantly, and then re-radiates the energy at much longer wavelengths. This re-radiation occurs primarily in the infrared and millimeter-wave parts of the electromagnetic spectrum, requiring specialized telescopes to detect this thermal glow.
Modern instruments like the James Webb Space Telescope (JWST) use powerful infrared capabilities to peer through these dust clouds and study the faint heat signatures of forming stars. Detecting the bipolar outflows, which are often visible in the infrared as molecular hydrogen jets, is another method for confirming the presence of an embedded protostar.