Protostar

A protostar is a very young star in its earliest phase of stellar evolution. It is an object that is still actively gathering mass from its parent molecular cloud. This stage is critical because it marks the very beginning of a star's life cycle. For a low-mass star, such as our Sun or one even smaller, this phase lasts about 500,000 years. Understanding protostars helps scientists understand how the massive structures in our universe are born.

The process begins within small, dense cores found inside larger molecular clouds. Initially, these dense cores exist in a state of balance. Self-gravity pulls the material inward to compress the object. At the same time, gas pressure and magnetic pressure push outward to inflate it. As the core collects more mass from the surrounding cloud, self-gravity eventually overwhelms these outward pressures. This causes the core to undergo a gravitational collapse. Theoretical models suggest this collapse may spread from the inside toward the outside.

As the gas collapses toward the center of the dense core, a low-mass protostar forms. Much of the infalling material does not hit the star directly. Instead, the gas forms a protoplanetary disk that orbits the growing object. This happens because of the conservation of angular momentum. As the collapse continues, more gas impacts the disk rather than the star itself. The outer surface of the protostar consists of shocked gas that has fallen from the inner edge of this disk. This makes the surface very different from the stable photosphere of a mature star.

Protostars have different internal mechanics than the stars we see in the night sky. The interior of a protostar is cooler than that of an ordinary star. In a mature star, hydrogen-1 atoms fuse together at the center. However, in a protostar, hydrogen-1 is not yet fusing with itself. Instead, a hydrogen isotope called deuterium, or hydrogen-2, fuses with hydrogen-1. This reaction creates helium-3 and releases heat. This heat creates an outward pressure that helps determine the size of the youngest pre-main-sequence stars.

Astronomers categorize these young objects into different classes based on their light emissions. These classes help researchers identify how far along the star is in its development. Class 0 sources are identified by submillimeter radiation and last about 10,000 years. Class I sources emit far-infrared radiation and last about 100,000 years. Class II sources emit near-infrared radiation and last about 100,000 years. Finally, Class III sources emit visible light and last about 10,000,000 years. While it is commonly believed that Class 0 and Class I sources are protostars, there is no definitive evidence for this exact identification yet.

The history of our understanding of these objects changed significantly in the 20th century. Chushiro Hayashi first suggested the modern theoretical picture of protostars in 1966. In the earliest versions of these models, scientists greatly overestimated the size of protostars. Later numerical calculations corrected this error. These calculations showed that protostars are actually only modestly larger than main-sequence stars of the same mass. Modern observations have confirmed this theoretical result. We now see that the largest pre-main-sequence stars are indeed of modest size.

Detecting a protostar is a unique challenge for astronomers because of interstellar dust. The thick dust in the surrounding dense core absorbs all visible light photons. Because the light is blocked, protostars cannot be detected at optical wavelengths. They also cannot be placed on a standard Hertzsprung–Russell diagram like more evolved stars. Instead, the dust absorbs the energy and reradiates it at much longer wavelengths. Consequently, astronomers must look for radiation in the infrared and millimeter regimes to find them.

The life of a protostar ends when the infalling gas is finally depleted. At this point, the object becomes a pre-main-sequence star. This star will continue to contract until it reaches the main-sequence stage. The main-sequence stage begins at the onset of hydrogen fusion, where hydrogen is turned into helium. This transition marks the shift from a gathering mass to a stable, shining star. This entire journey from a collapsing cloud to a stable star is one of the most important processes in astrophysics.