The infancy of stars

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When a region of the cloud becomes dense and hot enough, nuclear reactions will be able to start locally : hydrogen will transform into helium by means of nuclear fusion. The necessary energy to start and maintain these reactions is coming from the gravitational collapse of the cloud.

The biggest part of the surrounding cloud of gas and dust will be ejected by the powerful winds initiated by the protostar.
This ejection appears mainly as polar jets, perpendicular to the disk of matter which still surrounds the star.
According to current theory, these jets are partly generated by the distortion of magnetic field lines within the cloud, when the disk begins to appear.

HH30, forming protostar
A view of HH30, a forming protostar : if the future star is invisible,
it lights up the surrounding disk of matter (in green).
One can clearly see the two polar jets, coming from the central area, in red color.

 

Source : NASA / HST

This ejection of matter becomes very significant for the continuation of the star forming. It allows the excess of angular momentum to dissipate, and so, it lowers the centrifugal force which could prevent the gravitational collapse.

In a few million years, the main part of the disk has been ejected. One can see directly the young star in the middle of the cloud.
It's called a T-Tauri star, this name coming from a prototype star in the constellation of Taurus.
A significant amount of the remaining gas can stay trapped by the magnetic field of the star, and can get hot enough to radiate in the X-ray range of the spectrum.

At this time, the surrounding cloud of gas is able, in some cases, to condense and form planets.

Orion Proplyds
The Orion proplyds, young T-Tauri stars,
with proto-planetary disks clearly visible around them.

 

Source : NASA / HST

This process lasts no more than 40 million years. Quite a short time, in fact, compared with the rest of the star's life...

 

The forming process of a middle-sized star. (Pictures NASA/HST)
forming process
forming process


This process, that we've just described here, applies for stars whose final mass will be equal to, or less than a few solar masses.
The formation of heavy stars is not completely understood at the present time. It could partially be explained by the fusion of several protostars.

 


References :
Feedback from Protostellar Outflows in Star and Star Cluster Formation (C.Matzner)
Primordial stellar evolution - The protostar phase ( S.W.Stahler &al.)