Astronomy Practice Exam 2025 – Complete Preparation Guide

Spiral galaxies are characterized by several distinct features that distinguish them from other galaxy types. One of the hallmark features of spiral galaxies is the presence of a central bulge, which comprises older stars and is found at the galaxy's center. Additionally, they are known for their spiraling arms that extend outward from the central bulge, which often contain young, bright stars, gas, and dust, contributing to the galaxy's overall structure and appearance. The presence of these young stars is indicative of ongoing star formation, which is prevalent in the arms of spiral galaxies.

The elongated structure resembling a bar is associated with a different type of galaxy, specifically barred spiral galaxies. While many spiral galaxies may exhibit some degree of elongation in their morphology, the defining characteristic of them is the presence of the central bulge and the spiraling arms. The bar structure serves as a classification criterion for a subgroup of spiral galaxies but is not a necessary feature of all spiral galaxies. Therefore, the correct option accurately identifies a feature that is not essential or typical for the classification of spiral galaxies.