Not Quite a Star

The evolution of a protostar is characterized by a dramatic increase in temperature, especially at the core of the protostar. Still too cool to trigger nuclear fusion reactions, its core reaches a temperature of 1 million Kelvin. The protostar is also still very large, about 100 times larger than the sun. While its surface temperature at this stage is only half that of the sun, its area is so much larger that it is about 1,000 times more luminous than the sun. At this stage, it has the luminosity and radius of a red giant. A solar mass star will not look like this again until it is on its death bed, about 10 billion years later.

Despite the tremendous heat produced by its continuing collapse, which exerts an outward-directed force on the protostar, the inward-directed gravitational force is still greater.

Through the course of some 10 million years, the protostar's core temperature increases 5 fold, from 1 million Kelvin to 5 million, while its density greatly increases and its diameter shrinks, from 100 times to 10 times that of the sun.

Despite the increase in temperature, the protostar becomes less luminous, because its surface area is reduced. Contraction continues, but slows as the protostar approaches equilibrium between the inward-directed force of gravity and the outward-directed force of heat energy. It is approaching that part of its life that we called the main sequence.

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