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 should occur in any region of material dense enough for 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. Star formation therefore only occurs in a few areas. Galaxies are primarily located in molecular clouds – expansive, cool clouds composed primarily of hydrogen and helium. Molecular clouds are on average too diffuse to contract gravitationally, but within a cloud are regions of locally higher density, which are the sites of active star formation. It is not clear what causes molecular clouds and star-forming regions to be distributed as they are. However, this appears to be related to the spiral arm structure of spiral galaxies, which is thought to be the result of density waves passing through the disk, compressing matter and igniting star formation in their wake, leaving trails of young, hot , blue stars in their wake which are the main characteristic of spiral galaxies. The distribution of gas in these regions is likely sufficiently erratic that once sufficient compression occurs across a large number of separate regions, they will contract individually. When a sufficiently dense region begins to contract, the process becomes self-sustaining, because contraction only increases the density and makes the contraction faster. Any net rotation in the region will be flattened by conservation of angular momentum as the cloud contracts and rotates faster, making the contracting region disk-shaped. The center of mass of the cloud will, of course, be the location of highest density, and when material rains down on the center it will quickly heat up to very high temperatures. This "protostar" will emit strong radiation and winds which, although not immediate enough to stop the influx of material from the disk, generate powerful outflow jets along the poles of the system, which may also be influenced by magnetic force coming from the disk. Throughout the process, the temperature, density and pressure in the protostar at the center increase and soon approach figures comparable to those of normal stars. Once the center temperature