In 1845, British astronomer Lord Ross discovered the "spiral nebulas". The nature of such nebulas was ascertained only in the beginning of XX century. Spiral nebulas are large solar systems that resemble our Galaxy and exist millions of light years away from it.
To better understand the process of spiral galaxies' formation, consider two plates that are stacked together, or a biconvex lens. Spiral galaxies contain a halo and a massive stellar disk. The central part of the disk is known as the bulge because of its peculiar shape. Dark stripes along the disk are nontransparent layers of interstellar medium, i.e., cosmic dust.
Letter S designates spiral galaxies. They are distinguished by the extent of their spiral structure by adding letters a, b, or c to S-symbol. Sa is a spiral galaxy with poorly developed spiral structure and a strong core. Sc is a galaxy with a small core and a strongly developed spiral branches. Our Galaxy belongs to the intermediate type Sb. Stellar crosspiece, known as the bar, exists in the central part of certain spiral systems. For galaxies that have a stellar crosspiece, capital letter B is added to the "S" designation.
Spiral waves with compression-rarefaction density, resembling sonic waves, may propagate throughout the rotating disk that is formed by the stellar gas. They run through the entire galaxy for several hundred million of years with a constant angular velocity. These waves are responsible for the creation of spiral branches. Nearly all stars of the disk either belong to the spiral branches, or emanate from them. Shorting circumference is the only location where velocities of stars and branches coincide. In our Galaxy, the Sun is situated right next to the Shorting circumference. This is extremely beneficial for the Earth. Our planet exists in a relatively calm area of the Galaxy, and it has not been exposed to galactic cataclysms for billions of years.
This model represents the most prominent theory regarding the formation of spiral galaxies. According to this theory, spiral galaxies form from quickly rotating clouds. Rotation compresses clouds into disks. Press "Run" button to observe how, over the course of time, stars come into being from certain parts of the cloud, while the cloud itself gradually compresses. Spiral branches become progressively more prolific over time. The formation of stars practically stops after several billion years. This model stops after four billion virtual years.
A special input window allows you to observe galactic rotation from different angles of perspective. "Stop" and "Reset" buttons bring the model to a halt and return it to its initial state, respectively.