Credit : lumenlearning
Although regions like Orion give us clues as to how star formation begins, the later stages are still mysterious (and a lot of dust). There is a big difference between the density of the core of a molecular cloud and the density of younger stars that might be detected. Direct observations of this collapse at higher densities are almost impossible for two reasons: First, the dust-covered interiors of molecular clouds in which star births take place cannot be observed in visible light. Second, the timescale for the initial collapse (thousands of years) is astronomically very short.
Since each star only spends such a small part of its life at this point in time, relatively few stars go through the collapse process at any given point in time. However, through a combination of theoretical calculations and the limited observations available, Astronomers have created a picture of the likely early stages of stellar evolution.
The first step in star formation is to form dense cores in a clump of gas and dust. It is widely believed that all of the star’s material comes from the core, The largest structure surrounds the forming star. Over time, the gravitational force of infalling gas becomes strong enough to overcome the pressure exert by cold material that forms the dense-core.
Then the material collapses quickly and the density of the core increases enormously. In the time when a dense core contracts into a true star, But before protons begin to fuse into helium, let’s call the object a protostar.
The natural turbulence within a clump-tends to give any part of it an initial spinning-motion (even if it is very slow). As a result, each collapsing core is expected to rotate. According to the law of conservation of angular momentum a rotating body spin faster with decreasing size.
In other words, If the object can rotate its material around a smaller circle, it can move that material faster, like a figure skater who rotates faster when she brings her arms closer to her body. This is exactly what happens when a core contracts into a protostar: When it shrinks, its rotation speed increases.
But all directions on a rotating sphere are not the same. Since the protostar rotates, the material falls much more easily directly onto the poles (which rotate more slowly) than it does onto the equator (where the material moves around faster). Thus, gas and dust falling towards the equator of the protostar are “held back” by the rotation and form an extended whirling disk around the equator.
You may have observed the same “equator effect” while driving in an amusement park, standing with your back to a cylinder that is spinning faster and faster. As it rotates very quickly, it is pushed against the wall with such force that it cannot possibly fall towards the center of the cylinder. However, gas can easily fall onto the protostar from a direction away from the star’s equator.
At this stage the protostar and the disk are embedded in a shell of dust and gas from which the material still falls onto the protostar. This dusty shell blocks visible light, but infrared radiation can penetrate. As a result, the protostar itself emits infrared radiation in this phase of its evolution and can therefore only be observed in the infrared range of the spectrum.
After almost all of the available material has been accumulated and the central protostar has almost reached its final mass, it is given a special name: it is called star T Tauri, the name of one of the most studied and brightest members of this class. of stars, which was discovered in the constellation Taurus. (Astronomers tend to name star types after the first example they discover or understand. It’s not a fancy system, but it works.) Only stars with masses less than or equal to the mass of the Sun become T-Tauri stars. Massive stars do not go through this stage, although they seem to follow the formation scenario.
