esitähti

Esitähti is a Finnish term that translates to "pre-star" or "protostar" in English. It refers to a very early stage in the formation of a star, before nuclear fusion has begun in its core. During this phase, a massive cloud of gas and dust collapses under its own gravity. As the cloud contracts, its density and temperature increase, particularly at the center. This collapsing core radiates energy, primarily in the infrared spectrum, due to the release of gravitational potential energy.

The protostar is still enveloped in its parent molecular cloud, which can obscure it from visible light observation. Astronomers typically detect protostars through their infrared emissions. The process of accretion is also a defining characteristic of this stage, as the protostar continues to gather material from the surrounding disk of gas and dust. This infalling material can form powerful bipolar jets of plasma that are ejected from the protostar's poles.

The duration of the protostar phase varies depending on the mass of the star being formed, but it is a relatively brief period in the star's overall lifespan. Once the core temperature and pressure become sufficient to ignite nuclear fusion of hydrogen into helium, the object transitions from being a protostar to a main-sequence star. The study of protostars is crucial for understanding stellar evolution and the formation of planetary systems.