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The protostar stage is one of the earliest steps in the life-cycle of a star. It lasts from the gravitational collapse of a giant molecular cloud and ends with the start of the hydrogen fusion. This paper will cover this stage from start to finish, covering both standard process of formation and a case when a molecular cloud does not have any angular momentum.
Life-cycle of a Protostar
Stars start their formation in molecular clouds. These clouds consist of primordial hydrogen, helium, and stardust. A molecular cloud can vary in density. Usually, the density ranges from a few hundred to several million atoms per cubic centimeter. This gaseous material is often in a state of motion, swirling inside a galaxy due to blast waves of exploding stars and gravitational forces. Star formation occurs in these clouds because of their low temperatures and high densities. Specifically, it takes place in smaller molecular clouds called dense cores. They start with a balance of self-gravity, magnetic pressure, and gas pressure. This reaction causes the cloud to inflate. When the core gathers enough mass from the surrounding cloud, the collapse begins. The gas starts to collapse toward the center of the core, creating the first stage of a protostar. At this point, it has a relatively low mass and a circumstellar disk orbiting around it. The disk is made up of dense gas and dust. During this stage, the collapse mostly affects the disk and not the protostar itself due to the angular momentum previously present in the cloud. The surface of the protostar consists of shocked gas from the inner edge of the disk. On the inside, a protostar is colder than a normal star, and hydrogen at its center is not undergoing nuclear fusion. When this fusion finally takes place, the heat causes the protostar to inflate leading to the next step of stellar evolution. While protostars generate energy, it does not come from the nuclear fusion, but rather the radiation released when the gas from the inner circle hits the surface of the protostar (Maldonado, J. et al. 3).
Normally these kinds of molecular clouds have two cores. The first core is called adiabatic core, and it appears before the protostar formation. When a cloud has angular momentum, the adiabatic core becomes the circumstellar disk as it was previously described. In contrast, molecular clouds without angular momentum have the adiabatic core fall onto the protostar. There it disperses over a period of several years. This reaction still leads to the creation of low-mass protostars by fragmentation of the filaments, but these stars rarely get above the hydrogen-burning limit. Because of this they often become brown-dwarf protostars and never reach the status of a proper star. There are not a lot of these types of stars, so the information about them is scarce. Most of the research data come from simulations of such formations (Whitworth. and Lomax 4).
The process described in this paper could last over a million years, and it happens on such a large scale that it would be inconceivable to people who first looked at these stars centuries ago. Be it a relatively rare and small brown dwarf star or a full main sequence star; we still have a lot to research about their formation. With these questions, unanswered astronomers have to strive for knowledge about this subject.
Maldonado, J. et al. “Searching For Signatures of Planet Formation in Stars with Circumstellar Debris Discs.” Astronomy & Astrophysics, vol 579, no. 20, 2015, pp. 1-41. EDP Sciences, Web.
Whitworth, A. and O. Lomax. “A Theoretical Perspective on the Formation and Fragmentation of Protostellar Discs.” Publications of the Astronomical Society of Australia, vol 33, no. 3, 2016, pp. 1-11. Cambridge University Press (CUP), Web.