Spiral galaxies get their name from the shape of their disks. In a spiral galaxy, the stars, gas and dust are gathered in spiral arms that spread outward from the galaxy’s center. Spiral galaxies are divided into three main types depending on how tightly wound their spiral arms are: Sa, Sb and Sc. Sa galaxies have very tightly wound arms around a larger central nucleus. Sc galaxies have very loosely wound arms around a smaller nucleus. Sb’s are between, having moderately wound arms around an average sized nucleus. Spiral galaxies have a lot of gas, dust and newly forming stars. Since they have a lot of hot, young stars, they are often among the brightest galaxies in the universe. About 20% of all galaxies are spirals. We live in a spiral galaxy called the Milky Way.
The Spiral Structure .
The spiral structure is associated with active star-forming regions. As already noted in conjunction with the Milky Way, This is in fact why the spiral arms of these galaxies are so prominent: because they are regions of active star formation, there are many hot young blue and blue-white stars there, making the spiral arms extremely visible. The adjacent image shows a star-forming region in the spiral arms of the galaxy M31 (Andromeda) called NGC 206 (Ref). Notice the diffuse blue glow associated with many young stars.
In contrast the nucleus of a spiral galaxy is typically much more red in color, often resembling elliptical galaxies; this indicates the presence of many old stars in the nucleus of spiral galaxies. Here is a movie sequence of a galaxy morphing its perceived shape from spiral to almost elliptical as the wavelength is varied.
The image shown below illustrates rather clearly the spiral structure for several spiral galaxies classified according to their Hubble class. These images were taken in UV light with the Ultraviolet Imaging Telescope aboard the Space Shuttle Columbia. Because the spiral arms are dominated by hot young stars, they stand out particularly well when photographed at UV wavelengths, since only hot young stars can produce much UV
Spiral galaxies are rich in gas and dust, which is often visible as lanes of dust when viewed from the “top” or “bottom”, and as layers of dust when viewed from the side. In our own Milky Way the center of the galaxy is invisible from our vantage point because the interstellar dust between us and the center (in Sagittarius) is so thick.