A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium

Interstellar medium

In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

. 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

T Tauri star

T Tauri stars are a class of variable stars named after their prototype – T Tauri. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines.-Characteristics:...

, which then develops into a main sequence star. This is heralded by the T Tauri wind

T Tauri wind

The T Tauri wind — so named because of the young star currently in this stage—is a phenomenon indicative of the phase of stellar development between the accretion of material from the slowing rotating material of a solar nebula and the ignition of the hydrogen that has agglomerated into...

, a type of super solar wind

Solar wind

The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

 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

Gravitational binding energy

The gravitational binding energy of an object consisting of loose material, held together by gravity alone, is the amount of energy required to pull all of the material apart, to infinity...

 of the cloud is balanced by the thermal pressure of the cloud's constituent molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s and dust particles.