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A protostar is a large object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years. It starts with a core of increased density in a molecular cloud and ends with the formation of a T Tauri star, which then develops into a main sequence star. This is heralded by the T Tauri wind, a type of super solar wind that marks the change from the star accreting mass into radiating energy.

Observations have revealed that giant molecular clouds are approximately in a state of virial equilibrium—on the whole, the gravitational binding energy of the cloud is balanced by the thermal pressure of the cloud's constituent molecules and dust particles. Although thermal pressure is likely the dominant effect in counteracting gravitational collapse of protostellar cores, magnetic pressure, turbulence and rotation can also play a role (Larson, 2003). Any disturbance to the cloud may upset its state of equilibrium. Examples of disturbances are shock waves from supernovae; spiral density waves within galaxies and the close approach or collision of another cloud. If the disturbance is sufficiently large, it may lead to gravitational instability and subsequent collapse of a particular region of the cloud.

The British physicist Sir James Jeans considered the above phenomenon in detail. He was able to show that, under appropriate conditions, a cloud, or part of one, would start to contract as described above. He derived a formula for calculating the mass and size that a cloud would have to reach as a function of its density and temperature before gravitational contraction would begin. This critical mass is known as the Jeans mass. It is given by the following formula:

Heating due to gravitational energy
As the cloud continues to contract it begins to increase in temperature. This is not caused by nuclear reactions but by the conversion of gravitational energy to thermal kinetic energy. As a particle (atom or molecule) decreases its distance from the centre of the contracting fragment this will result in a decrease in its gravitational energy. The total energy of the particle must remain constant so the reduction in gravitational energy must be accompanied by an increase in the particle's kinetic energy. This can be expressed as an increase in the thermal kinetic energy, or temperature, of the cloud. The more the cloud contracts the more the temperature increases.

Collisions between molecules often leave them in excited states which can emit radiation as those states decay. The radiation is often of a characteristic frequency. At these temperatures (10 to 20 kelvins) the radiation is in the microwave or infrared range of the spectrum. Most of this radiation will escape, preventing the rapid rise in temperature of the cloud.

As the cloud contracts the number density of the molecules increases. This will eventually make it more difficult for the emitted radiation to escape. In effect, the gas becomes opaque to the radiation and the temperature within the cloud will begin to rise more rapidly.

The fact that the cloud becomes opaque to radiation in the infrared makes it difficult for us to observe directly what is happening. We must look to longer wavelength radio radiation which does escape even the densest clouds. In addition, theory and computer modelling are necessary to understand this phase.

As long as the surrounding matter is falling onto the central condensation, it is considered to be in protostar stage. When the surrounding gas/dust envelope disperses and accretion process stops, the star is considered as pre-main sequence star. In HR diagram then it appears to be on the stellar birthline.



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