The basic idea of ​​star formation is gravitational collapse – the contraction of a region of gas under the influence of gravity. This is a simple process that would be expected to occur in any region of material dense enough to allow collisions between atoms to radiate energy. However, the gas must be dense enough for collisions to occur and the temperature must be low enough that atomic velocities cannot escape the system's gravity, so star formation occurs in only a few areas. Star formation sites in Galaxies are mostly found within molecular clouds: extended, cold clouds composed primarily of hydrogen and helium. Molecular clouds are on average too diffuse to gravitationally contract, but within a cloud there are regions of locally higher density, which are the sites of active star formation. It is not known exactly what causes molecular clouds and star-forming regions to be distributed as they are. However, it appears to be related to the spiral arm structure of spiral galaxies, which is believed to be the result of density waves passing through the disk, compressing matter and triggering star formation in their wake, leaving traces of young, hot , blue stars in their wake that are the main feature of spiral galaxies. The distribution of gas in these regions is likely irregular enough that, once sufficient compression occurs, large numbers of separate regions will be triggered to contract individually. Once a sufficiently dense region begins to contract, the process becomes self-sustaining, since contraction only increases the density and makes the contraction faster. Any net rotation in the region will be flattened by the conservation of angular momentum as the cloud contracts and rotates more rapidly, causing the contracting region to reach the shape of a disk. The center of mass of the cloud, of course, will be the location of the highest density, and as matter rains into the center, it will rapidly heat up to very high temperatures. This "protostar" will emit strong radiation and winds which, although not immediately sufficient to stop the influx of matter from the disk, generate powerful outflow jets along the poles of the system, which can also be influenced by the magnetic force originating in the disk. Throughout the process the temperature, density and pressure in the central protostar increase, and soon approach values ​​comparable to those of normal stars. Once the core temperature is reached