when molecular clouds – large clumps of gas and dust within galaxies – become unable to support their own weight they collapse inwards into a cascading series of overdensities and clumps which then become stars. even if they can support themselves, collisions between clouds, supernovae, or other dramatic events can cause dynamical disruptions in molecular clouds which trigger star formation.
as these clumps continue to collapse, they shed gravitational potential energy in the form of radiation. the center of a given clump will reach an equilibrium, but infalling material will collide with the newly stabilized “first hydrostatic core,” heating it further. as the core heats, it contracts until it has swallowed enough heated gas that the gas pressure can support it against gravity. reaching this point, the clump has now become a protostar.
not all the material in a clump will make it onto the protostar; some of it will begin to exchange angular momentum and flatten into a disk surrounding the protostar that will continue to feed the growing object; the rest of the halo will be blown away by the newly accreting object. once a protostar sheds its optically thick envelope, it has become a Pre-Main-Sequence (PMS) star. These stars emit light due to their ongoing contraction, rather than undergoing nuclear fusion like adult, Main Sequence stars. Nuclear fusion is a constant, dominant process that confines stars to a fairly well described function called the main sequence, but before its nuclear reactor can ignite, the PMS star has to undergo a few more growing pains.
For the next few million years the PMS star will continue to siphon material from its circumstellar disk as it contracts, building up enough heat and pressure to begin fusing its hydrogen into helium. during this process the object’s circumstellar disk will evolve as well, slowly building protoplanets that will form the star’s own solar system.
a molecular cloud will collapse into hundreds or thousands of individual protostars. associations of Young Stellar Objects (YSOs, a blanket term that encompasses protostars and PMS stars) will then evolve over the course of a few million years into groups of main sequence stars. these groups of stars are referred to as open clusters, which have a variety of masses and temperatures determined by the composition, distribution, and mass of the molecular cloud they formed from.
YSOs are the sites of many dramatic and beautiful phenomenon; propylids, Herbig-Haro objects, jets and outflows, circumstellar disks, and more. These complex but striking phenomena highlight the beauty of stellar childhood. Of particular interest to this post is the disruption of the parent molecular cloud by the largest and most active members of a YSO association. there are many examples of this effect: the Orion Nebula, the Rosette Nebula, and the Cocoon Nebula, among many others.
The Cocoon Nebula, also referred to as IC 5146, is the remains of a molecular cloud with a bright, central star ” [BD +46°3474] that formed near the near surface of the present cloud and evacuated a blister cavity out of which gas and dust are now flowing through a funnel-shaped volume in the approximate direction of the Sun. It is suggested that the IC 5146 cluster stars formed in a dense foreground section of the molecular cloud that was dissipated following the appearance of +46°3474″ (Herbig, Dahm 2002).
I observed this target briefly last November from the Amherst College observatory, and recently threw together a CMD and three color image for my observation. Note that while about half of the stars in the FOV and therefore in the CMD are foreground/background targets, and therefore not members of the YSO association, there is a significant population of stars to the right of the main sequence (indicating that they are cooler than fully-formed stars) which is typical of pre-main-sequence objects. Hopefully I can go back sometime and take much deeper images of the nebula, it is strikingly beautiful.