What are the three major classes of galaxies?

• A. Spiral, Elliptical, and Irregular
• B. Spiral, Dwarf, Elliptical
• C. Elliptical, Disc, Irregular
• D. Spiral, Elliptical, and Nebular

Answer: (A)

Galaxy shapes are the primary way astronomers organize galaxies, separating them into the main structural families they exhibit. The three major classes are spiral, elliptical, and irregular. Spiral galaxies have flattened disks with winding arms and often a central bulge, showing ongoing star formation in their arms. Elliptical galaxies are more rounded or elongated without visible arms, generally smoother and with little gas or new star formation, consisting mainly of older stars. Irregular galaxies lack a distinct, regular shape, often showing chaotic structure and active star formation, often due to past interactions.

This trio—spiral, elliptical, and irregular—captures the broadest, most fundamental categories observed in the galaxy population. Other classifications exist as subtypes (like barred spirals or lenticular galaxies) or are based on size (dwarf galaxies), but they are not considered one of the three basic classes. The option that includes dwarf or uses terms like disc or nebular does not reflect the standard threefold split of galaxy morphology.