A theory to account for the spiral structure of galaxies: Supposing that such a density wave could be set up in the first place, the theory suggests that the spiral arms mark the positions of regions of higher than average density, which rotate around the galaxy.

Stars orbiting the center of the galaxy spend a considerable amount of time in the higher density regions before moving out, with the higher density also favoring the formation of young stars by fragmentation within it.

A waveform that has at least three states (for example, positive, off, and negative).

It has less harmonic content than a square wave.

A waveform that has at least three states (such as, positive, off, and negative).

It has less harmonic content than a square wave.

A form of electromagnetic radiation traveling at the speed of light whose frequency lies between about 10 kilohertz and about 100,000 megahertz: Radio waves are low frequency and, therefore, long wavelength electromagnetic radiations.

The radio part of the electromagnetic spectrum lies beyond the infrared region and comprises all radiation with a frequency lower than about 300,000 million hertz (300,000 MHz) or with a wavelength longer than about one millimeter.