Astronomers observe galaxies and categorize them according to four different kinds of shapes: elliptical, spiral, barred-spiral, and irregular. In addition to shape, each of the four different galaxy types can be described by other common characteristics.
Match the following characteristics with their corresponding galaxy type.

a) Round, no disk, very little gas and dust, only old stars
b) Central bulge, flattened disk, spiral arms, gas, dust, young stars.
c) Elongated central structure, flattened disk, spiral arms, gas and dust, young stars.
d) Asymmetric, often with gas, dust, and young stars.

1. Spiral galaxy
2. Irregular galaxy
3. Elliptical galaxy
4. Barred-spiral galaxy