Final answer:
Galaxies come in different shapes and sizes. The majority of bright galaxies are either spirals or ellipticals, while a small percentage are classified as irregulars. Galaxies can change their appearance over time due to collisions or changes in star formation.
Step-by-step explanation:
Galaxies come in different shapes and sizes. The majority of bright galaxies are either spirals or ellipticals, while a small percentage are classified as irregulars. Spiral galaxies, like our own Milky Way, have both old and young stars, as well as interstellar matter. Elliptical galaxies consist almost entirely of old stars and have shapes that are spheres or ellipsoids, with no trace of spiral arms. Galaxies can also change their appearance over time due to collisions with other galaxies or changes in the rate of star formation.