Final answer:
Galaxies are categorized morphologically mainly into elliptical, spiral, and irregular types. Elliptical galaxies are mostly old stars forming spheroidal shapes, while spiral galaxies have distinct arms and ongoing star formation. Irregular galaxies have less structured forms and appearances may change due to cosmic events.
Step-by-step explanation:
Types of Galaxies by Morphology The types of galaxies based on morphological (appearance/shape) classifications include elliptical, spiral, and irregular galaxies. Elliptical galaxies are characterized by their spheroidal or ellipsoidal shapes, consisting mostly of old stars, and having very little interstellar matter. They range from massive giants to dwarf galaxies and lack spiral arms. Spiral galaxies, such as our own Milky Way, feature a central bulge, disk, halo, and conspicuous spiral arms filled with interstellar material, where ongoing star formation occurs. Irregular galaxies have disorganized shapes and are less common than elliptical and spiral types. Over time, galaxies can change appearance due to collisions or changes in star formation rates. Galaxies are vast systems of stars, gas, dust, and dark matter bound together by gravity. The three main morphological types are elliptical, spiral, and irregular. Elliptical galaxies are shaped like ellipsoids, often containing older stars. Spiral galaxies, like our Milky Way, have a central bulge and spiral arms with ongoing star formation. Irregular galaxies lack a defined structure, showcasing a mix of features. Option (a) correctly identifies these morphological types. On the other hand, options (b) and (c) refer to other astronomical objects, such as star clusters, nebulae, supernovae, pulsars, and quasars, which are not types of galaxies. Option (d) mislabels galaxies as "celestial," "terrestrial," and "aquatic," which are terms more relevant to Earth-based classifications.