Our Young Stellar Object continues to contract and densify until the temperature at its core reaches the point where nuclear fusion takes over and it radiates energy. It has become a bona fide star. Radiation pressure may well force some of the dusty material away from the areas near its surface until the star decouples from its surrounding ring of gas and dust, which joins the remaining material to orbit in a ring around the star. This decoupling causes the star to spin faster, setting up a magnetic field which may well produce high volumes of starspots on its surface. If the spots are large enough, they can cause the star to change in brightness as we see different areas of the star come into view as it rotates.
The little animation here shows how the star's variations would show up on a lightcurve. Since not only the number of spots is constantly changing, but also their area and position, then the resultant lightcurve is far from regular. In cases where the main cause of variation is starspotting, the amount of variation is not likely to be very great. Most stars of this type vary by only about half a magnitude, and many vary by much less. Of course, this a schematic, an idealised condition. We are assuming, for instance, that we are looking pole-on to the star so that the circumstellar ring does not cut off any of the star's light. In practice, this "ring" may be of varying degrees of flatness depending on the region of the cloud, speed or rotation of the star, etc. It could be more of a shell than a flat disc, for example.
Broadly speaking, if the star that is forming is between a certain range of masses (roughly that of the Sun) this type of stellar configuration produces the type of variation seen in what are probably the youngest of the Nebular variables, the T Tauri stars with their fairly gentle, sinusoidal light-curves. Associated with these stars is what is known as the T Tauri wind, and there is nothing gentle about that - a star can throw off up to half a Solar mass of material in this way - that's (wait for it, deep breath) 1,000,000,000,000,000,000,000,000,000 tons - a whole lot of gas, I think you'll agree! As material funnels down onto the star from the inner disc, it causes the star to fill what is known as its equipotential surface. Comparing the star with a bath, the bath is full up; it can take no more - and the excess has to go somewhere. The path of least resistance (the overflow if you like) is at the star's poles - since that is where the gravitational force of the whole system is weakest - and the excess gas is blasted out into the surrounding cloud, causing shockwaves which plough into the gas and illuminate it in a cone shape. That is why many T Tauri stars are attended by small fan-shaped nebulae. Many of these objects were catalogued by the astronomers Herbig and Haro, and are called - wait for it - Herbig-Haro Objects (HH). Here are two examples of these nebulae. The one on the left is a beautiful nebula known as HH 215, connected with the variable PV Cephei, while the other is NGC 2261, known as Hubble's variable nebula attending R Monocerotis.
Note that if you look carefully at the HH 215 example how there is a little, dim orange coloured "mirror image" of the main fan, on the other side of the star. This an effect of perspective caused by the fact that one pole of the star is pointing towards us, as in the diagram.
The above is very much an idealised scheme of things. Indeed, even within the T Tauri class there are two types of stars - the Weak and Classical types. Classical stars are those with a sizable circumstellar disc remaining, whereas the weak stars show evidence that they have no discs, or depleted ones. This seems to represent a difference of evolutionary state. The material in the weak stars' discs has been dissipated - but where has it gone to?
It is this last possibility that is the one that excites many astronomers, showing that planetary systems should be the rule, rather than the exception. A planetary system may be simply a by-product of a star's formative years. And in some cases, we believe that we can actually observe a planetary system forming, although the time scales involved are of course far too long for us to actually see any planets being born, as it were. We can do this when the disc is at an angle to us such that the clumps in the disc (future "Solar" System objects such as planets or comets, possibly) have grown to such a size that they periodically pass between us and the star, and cause its light to diminish.
Both the T Tauri and Herbig type stars form in definite groups in the sky. These are called T-associations and OB-associations respectively. An open cluster like the Pleiades - hot, young stars - will have been formed in a huge OB association several million years ago, and we can even still see some of the remaining nebular material. T associations tend to be fainter, simply because as we saw earlier the T Tauri stars go on to become less powerful objects. Maybe the nebular material from which they form does not exist in sufficient quantities to form large numbers of massive stars. Another reason may have something to do with not the birth of stars, but their death.
Stars like the Sun are born, grow to middle age, swell up and slowly fade away. Those that are several times the Sun's mass don't do this. Instead they end their lives in a Supernova explosion, which throws most of the star's mass out into interstellar space. If this mass encounters a gas cloud - and the massive stars that produce Supernovae are concentrated along the Milky Way, which is also where the star-forming gas clouds congregate - it sets up shock waves which can initiate more bursts of star formation. We can see this happening, strangely enough, in other galaxies, as we have an "outside looking in" view of them. So let's now look at the massive variables - the Herbig Ae-Be stars.