Final answer:
A spiral galaxy is characterized by a flat shape with rotating arms of young stars and interstellar matter, ongoing star formation, and sometimes a bar-shaped structure at the center. The Milky Way is a typical example.
Step-by-step explanation:
A characteristic of a spiral galaxy is that it is a flattened, rotating galaxy with pinwheel-like arms of interstellar material and young stars, which wind out from its central bulge. Notably, spiral galaxies have ongoing star formation, marked by bright emission nebulae and hot, young stars especially present in the spiral arms. Furthermore, about two-thirds of nearby spiral galaxies have distinctive boxy or peanut-shaped bars of stars running through their centers, leading them to be known as barred spirals. The presence of both young and old stars, and a mixture of gas and dust, indicate that they are active sites of star formation, unlike elliptical galaxies which have mostly old stars and little to no star formation.
Our own Milky Way is an example of a large spiral galaxy, showcasing a central bulge, a halo, a disk, and distinct spiral arms. New star formation, which takes place particularly within these arms, is a hallmark characteristic of spiral galaxies and can be observed due to the presence of hot young stars and emission nebulae. The disks of spiral galaxies often display dust which is prominently visible when viewed almost edge-on, and their shape can be easily distinguished from elliptical galaxies.