Star Formation

While most stars that we see in our Galaxy were born long time ago (hundreds or thousands of millions of years), in some places we can observe stars that are currently being born. Understanding how this process occurs is one of the major challenges of modern astronomy, and a goal to which considerable observational and theoretical effort is being devoted.

Thanks to an intense research effort carried out over the past few decades, we now have an approximate knowledge of the main events that take place during the relatively short process of stellar birth (about a million years).

Stars are born in giant clouds of interstellar gas, which are accumulations of diffuse material from previous generations of stars that expelled their envelopes when they died. This material slowly gathers over tens of millions of years in a complex process that involves turbulent motions, magnetic fields, and gravitational interactions. As a result of this process, the material acquires a chaotic and filamentary appearance that reminds that of the water vapor clouds in the Earth's atmosphere.

In some cloud locations, the gas becomes so dense that its self-gravity overcomes the combined stabilizing action of the magnetic field, the turbulence, and the internal gas pressure. When this occurs, the gas loses its equilibrium state and begins to collapse rapidly to form a dense object: a protostar.

The protostar consists of an approximately spherical central condensation surrounded by a disk of rotating matter. This disk is made out of material that initially had such high velocity that it could not contract to a point due the physical law of conservation of angular momentum. This material constitutes the precursor of a future planetary system around the new star.

Perpendicular to the disk, the protostar ejects a powerful bipolar outflow that starts to slow-down the collapsing material and dissipate the original gas condensation. While this occurs, the central object contracts slowly until its interior reaches the temperature and density required to ignite nuclear reactions. At this stage, we consider that the star-formation process is finished, and that the star has reached its adult stage, or more technically, the so-called main sequence. This is the phase in which most stars that we know, including our Sun, are now.