A Brown Dwarf
See related question
A protostar must reach about 10 million degrees Celsius for nuclear fusion to start in its core, triggering the transition into a true star. This marks the point where hydrogen atoms begin fusing into helium, releasing energy in the process. So, a protostar will become a full-fledged star after nuclear fusion begins at this temperature.
The nuclear fusion of hydrogen into helium
Hydrogen is the most likely substance to undergo nuclear fusion. In the core of stars, hydrogen nuclei combine to form helium through the fusion process, releasing vast amounts of energy in the form of heat and light.
The core of the protostar reached an extremely high temperature
The core of a protostar must reach temperatures of at least 10 million degrees Celsius for nuclear fusion to begin. At this temperature, hydrogen atoms can overcome their mutual repulsion and fuse to form helium, releasing energy in the process.
The basic idea is that the protostar contracts, under the influence of gravity, until it gets dense and hot enough to undergo nuclear fusion. You can find more details at the Wikipedia article "Protostar".
A protostar must reach about 10 million degrees Celsius for nuclear fusion to start in its core, triggering the transition into a true star. This marks the point where hydrogen atoms begin fusing into helium, releasing energy in the process. So, a protostar will become a full-fledged star after nuclear fusion begins at this temperature.
A protostar; also a brown dwarf.
the nuclear fusion of hydrogen into helium
The nuclear fusion of hydrogen into helium
A protostar is heated up by gravitational forces causing it to contract and increase in temperature. Once the core reaches a high enough temperature and pressure, nuclear fusion reactions begin, releasing energy and making the protostar shine as a star.
That sounds like the description of a protostar.
A protostar is a star the is developing as is on the way to starting nuclear fusion. A brown dwarf is a failed star, one that failed to gather enough mass to start fusion.
Hydrogen is the most likely substance to undergo nuclear fusion. In the core of stars, hydrogen nuclei combine to form helium through the fusion process, releasing vast amounts of energy in the form of heat and light.
The core of the protostar reached an extremely high temperature
Nuclear fusion. In the case of stars, it is often called nucleosynthesis.
A protostar. See related question.