A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties:
- A considerable total angular momentum
- Composed of a central bulge surrounded by a disk
- The bulge resembles an elliptical galaxy, containing many old, so-called “Population II” stars, and usually a supermassive black hole at its center.
- The disk is a flat, rotating assembly consisting of interstellar matter, young “Population I” stars and open star clusters.
Spiral galaxies are also named due to the bright arms of star formation within the disk that extend—roughly logarithmically—from the bulge. Though sometimes difficult to discern, such as in flocculent spirals, these arms distinguish spiral galaxies from their lenticular counterparts, which exhibit a disk structure but no evident spiral.
The disks of spiral galaxies tend to be surrounded by large spheroid halos of Population II stars, many of which are concentrated in globular clusters that orbit the galactic center.
Our galaxy, the Milky Way, has long been thought to be a spiral, with a Hubble sequence classification of Sbc (possibly SBb); recent research from the Spitzer Space Telescope, however, confirms that it is in fact a barred spiral.