Which statement correctly defines a spiral galaxy?

Which statement correctly defines a spiral galaxy?

• A. It contains no gas or dust.
• B. It is a galaxy in which the stars and gas clouds are concentrated mainly in one or more spiral arms.
• C. It is always smaller than elliptical galaxies.
• D. It has a spheroidal shape with no distinct arms.

Answer: (B)

Spiral galaxies are defined by a flattened, rotating disk that features winding spiral arms where stars, gas, and dust concentrate. These arms are the sites of ongoing star formation, producing many young, bright stars along the arms. So a statement that describes stars and gas clouds being concentrated mainly in spiral arms directly captures the characteristic structure and star-forming regions of spirals. In contrast, saying there’s no gas or dust points to an older, gas-poor elliptical galaxy; claiming they’re always smaller than ellipticals isn’t a reliable rule, since sizes vary; and describing a spheroidal shape with no distinct arms matches elliptical galaxies, not spirals.

Spiral galaxies are defined by a flattened, rotating disk that features winding spiral arms where stars, gas, and dust concentrate. These arms are the sites of ongoing star formation, producing many young, bright stars along the arms. So a statement that describes stars and gas clouds being concentrated mainly in spiral arms directly captures the characteristic structure and star-forming regions of spirals. In contrast, saying there’s no gas or dust points to an older, gas-poor elliptical galaxy; claiming they’re always smaller than ellipticals isn’t a reliable rule, since sizes vary; and describing a spheroidal shape with no distinct arms matches elliptical galaxies, not spirals.