A protostar is a large mass 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.
Ambartsumian’s research was in the so-called 'continuous emission', observed in the spectra of young stars of the T Tauri type and their associated neighbor stars. As opposed to the classical hypotheses suggesting that stars formed singly as a result of condensation of small masses of diffuse matter, the new hypothesis postulated the existence of massive star-forming bodies, “proto-stars”. The process of disintegration of proto-stars is responsible for the formation of multiple members in star associations.
Role in stellar evolution
Star formation begins in giant molecular clouds. These clouds are initially balanced between gravitational forces, which work to collapse the cloud, and pressure forces (primarily from the gas) which work to keep the cloud from collapsing. When these forces fall out of balance, such as due to a supernova shock wave, the cloud begins to collapse and fragment into smaller and smaller fragments. The smallest of these fragments begin contracting and become protostars.
As the cloud continues to contract, it begins to increase in temperature. The temperature increase is not caused by nuclear reactions but rather by the conversion of gravitational energy to thermal kinetic energy. As a particle (atom or molecule) falls towards the centre of the contracting fragment, its gravitational energy decreases. As the total energy of the particle must remain constant (due to conservation of energy), the reduction in gravitational potential energy results in an increase in the particle's kinetic energy. The kinetic energy of a group of particles is 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. At the temperatures of a protostar (10 to 20 kelvins) most of the radiation is in the microwave or infrared range of the spectrum. At this early stage of star formation, most of this radiation escapes, preventing a rapid rise in temperature of the cloud. This stage of protostar evolution is known as the isothermal phase.
As the cloud contracts the number density of the molecules increases, making 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 gas cloud still has much more gas at this stage, called a Class 0 protostar.
As the system evolves, more and more emission starts to come from the protostar rather than the surrounding dust and gas. In the Class I stage, the protostar is now about the same mass as the surrounding envelope.
The next stage of protostar evolution is the classic T Tauri star (a.k.a. Class II protostar). In this phase, the temperature increases substantially and this disk becomes substantially smaller than the protostar. In the final stage of protostar evolution, the temperature rises and the surrounding material becomes an order of magnitude smaller, becoming a Class III protostar ('weak' T Tauri star). 
Classes of Protostars
|Class||peak emission||duration (Years)||description|
|I||far-infrared||105||main accretion phase|
|II||near-infrared||106||classic T Tauri star|
|III||visible||107||'weak line' T Tauri star|
|Wikimedia Commons has media related to Protostars.|
- Herbig-Haro object
- Pre–main sequence star
- Protoplanetary disk
- NGC 7538, home of the largest discovered protostar which is about 300 times the size of our Solar System.
- Larson, R.B. (2003), The physics of star formation'', Reports on Progress in Physics, vol. 66, issue 10, pp. 1651–1697
- Planet-Forming Disks Might Put Brakes On Stars (SpaceDaily) Jul 25, 2006
- Planets could put the brakes on young stars Lucy Sherriff (The Register) Thursday 27 July 2006 13:02 GMT
- Why Fast-Spinning Young Stars Don't Fly Apart (SPACE.com) 24 July 2006 03:10 pm ET