Which statement about spiral galaxies is true?

Which statement about spiral galaxies is true?

• A. They contain no gas or dust.
• B. They are always larger than elliptical galaxies.
• C. They have stars and gas concentrated mainly in spiral arms.
• D. They cannot form new stars.

Answer: (C)

Spiral galaxies are defined by a rotating disk that contains gas, dust, and stars, with a pattern of spiral arms where much of the activity happens. The gas and dust settle into those arms, and along them you see regions where new stars are formed. That ongoing star formation in the arms is why this statement is true: stars and gas are concentrated mainly in the spiral arms. Gas and dust are essential for making new stars, so the idea that spiral galaxies contain none is incorrect. They also aren’t necessarily larger than elliptical galaxies; there are both large and small spirals, just as there are large and small ellipticals. And since the disk holds gas, dust, and young stars, spiral galaxies can and do form new stars rather than being unable to.

Spiral galaxies are defined by a rotating disk that contains gas, dust, and stars, with a pattern of spiral arms where much of the activity happens. The gas and dust settle into those arms, and along them you see regions where new stars are formed. That ongoing star formation in the arms is why this statement is true: stars and gas are concentrated mainly in the spiral arms.

Gas and dust are essential for making new stars, so the idea that spiral galaxies contain none is incorrect. They also aren’t necessarily larger than elliptical galaxies; there are both large and small spirals, just as there are large and small ellipticals. And since the disk holds gas, dust, and young stars, spiral galaxies can and do form new stars rather than being unable to.