The evolution of circumstellar dust discs appears to be a natural by-product of star formation. This conjecture is outlined here; Dust formed in stellar environments is primarily created in shells around stars in the red giant phase of their evolution. Following ejection from the star, by radiation pressure, the dust grains may form abundant expanses in the interstellar medium. Molecular hydrogen is then formed by the meeting of two hydrogen atoms on the surface of a dust grain, they become adsorbed on the grain surface, wander around and find one another, before evaporating off. It is these large molecular clouds, consisting of 
and dust, which finally collapse, through a process called hierarchical fragmentation, to form protostars; Should the cloud become too large and gravitationally unstable, but still remain isothermal, it will start to collapse. While the cloud collapses as a whole, there will also be smaller sub-section collapses within the cloud. These sub-sections compress down onto themselves and cause the original cloud to fragment. This fragmentation process can occur again within the smaller sub-sections, and then repeatedly within each of these sub-sections. Fragmentation only stops when the fragments become opaque to their own cooling radiation. Opaqueness occurs when the gas and dust is unable to radiate away energy as fast as it is released by gravitational collapse (the fragments are no longer isothermal). At this stage, the fragments heat up until they are close to hydrostatic equilibrium. At this stage of events, the fragments are deemed protostars. They will have both a dust envelope and a dust disc surrounding them. The disc appears to be a natural by-product of this star formation process.
A typical four-stage star formation schematic can be seen in FIGURE 4. (a) Cores form within molecular cloud envelopes, as magnetic and turbulent support is lost through ambipolar diffusion. (b) A protostar with a surrounding nebular disc forms at the centre of a cloud core collapsing from inside out. The binding energy of the accreted matter is removed by radiation, which is absorbed by dust, and re-radiated in the far infrared. (c) A stellar wind breaks out along the rotational axis of the system, creating a bipolar flow, or jet. (d) The infall terminates revealing a newly formed, hydrogen burning star with a rotating circumstellar disc. (Shu, Adams and Lizanao, 1987.)
There are however, problems with this conjecture. The original un-fragmented cloud should collapse on a free-fall time scale that is greater than the speed with which the smaller fragments will contract. Any fragmentation is therefore likely to be short lived. Solutions to this problem invoke an initially rotating cloud. This rotation encourages a flattening of the cloud to a disc like structure. Rotational fragmentation spreads the fragments out, rather than them being centrally condensed, the sub-sections have now have less chance of interacting with each other and re-amalgamating. The fragments will also have more chance of survival due to the disc structuring of the original cloud, which will have a collapse time-scale less than that of the cloud which does not rotate.
Submillimetre observations together with the existence of bipolar outflows (jets) tend rather to point to circumstellar geometry somewhat akin to a “cored apple” structure. In this model, [we] envisage the outflow having cleared a cavity along the axis of the apple, while infall occurs from all other directions. Perhaps this is the precursor phase to disc formation, and either a significant disc will eventually grow at the centre of the apple, or the apple will itself collapse down to a thin disc. (André, Ward-Thompson, & Barnsony, 1993)
The dust nearest the protostar rotates with such velocity that it collapses down to form a disc or torus around the star. This accretion disc contributes to the growth and evolution of the young star by supplying mass.
As can be seen from FIGURE 5, although disc growth has already begun by the protostellar stage, the dust atmosphere still appears to dominate the circumstellar mass.
The process of disc accretion is not well understood in any astrophysical system. The most popular theory involves the inward transport of mass and the outward transport of angular momentum by the friction associated with some form of viscosity in a differentially rotating disc (Lynden-Bell and Pringle 1974). In the case of nebulae discs that surround young stellar objects, Lin and Papaloziou, 1985, have identified thermal convection in the disc as a plausible source of the turbulent viscosity.
FIGURE 6 The M16 nebula shown above is composed of a young star cluster and an emission nebulae (Emission nebulae are clouds associated with stars of spectral type O or B0. Their nebular spectra show strong emission lines and a weak background continuum.) lined with distinct regions of interstellar dust. Most of the stars in the cluster can be seen offset just above and to the right of the centre of the photograph. This type of star cluster is called an "open" or "galactic" cluster and typically has a few hundred young bright members. The redness of the surrounding emission nebula gas is caused by electrons recombining with hydrogen nuclei, while the dark regions are dust lanes that absorb much of the radiation that enters it. The dust absorbs so much light it allows astronomers to determine which stars are inside the nebula, and which are in the foreground.
It is not yet known how stellar jets form. Many theoretical models have been put forward and the field is rapidly developing. The supersonic material in jets may originate as winds. The origin of these winds could be the circumstellar disc, the boundary between the disc and the star or the star itself.
During star formation a powerful wind breaks out along the axis of rotation of the protostar, reversing the flow of inward falling material by sweeping up the matter over the poles into two outwardly expanding shells of gas and dust. (Stage c in FIGURE 4) Current consensus is that magneto-hydrodynamic forces drive the winds.
The star masked by a dust cloud at the left of the FIGURE 8 is expelling an energetic beam of charged particles into interstellar space. This jet, moving from left to right, has burrowed through much interstellar material, and now expands out into interstellar space. Although jet particles move at nearly 
, we still do not see any daily movement because of the enormous distances involved. In fact, the jet is trillions of kilometres long. This stellar jet occurs in a system called HH-47, which is located near the edge of the Gum Nebula.
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