Spiral galaxies are the most common type of galaxy in the universe, making up 77 percent of all known galaxies. These galaxies have long arms that wrap around a bright bulging center.
The older stars lie close to the center of spiral galaxies. The arms of spiral galaxies consist mainly of gas and dust, which is the raw material of new stars. Younger stars also inhabit these spiral arms. Type-Sa spiral galaxies have tightly wound arms whereas Type-Sc galaxies have loosely wound arms.
A typical spiral galaxy consists of billions of stars. The Sun and its Solar System belong to a spiral galaxy, the Milky Way. At the center of many spiral galaxies are supermassive black holes that can have masses of more than a billion suns.