Which galaxy type is typically smooth and lacks spiral arms?

Which galaxy type is typically smooth and lacks spiral arms?

• A. Spiral
• B. Lenticular
• C. Irregular
• D. Elliptical

Answer: (D)

The idea being tested is how a galaxy’s structure reflects its stellar content and gas. Spiral galaxies have well-defined arms because their gas-rich disks form density waves that light up with new, bright stars in those arm regions. Elliptical galaxies, on the other hand, are mostly made of older stars on random orbits and contain very little cold gas or dust, so there isn’t ongoing star formation or a mechanism to create organized spiral patterns. This leads to a smooth, featureless brightness profile that appears rounded or ellipsoidal. Lenticulars have a disk but no spiral arms, and irregulars lack a smooth shape due to chaotic regions of star formation or interactions. So, the galaxy type that is typically smooth and lacks spiral arms is the elliptical.

The idea being tested is how a galaxy’s structure reflects its stellar content and gas. Spiral galaxies have well-defined arms because their gas-rich disks form density waves that light up with new, bright stars in those arm regions. Elliptical galaxies, on the other hand, are mostly made of older stars on random orbits and contain very little cold gas or dust, so there isn’t ongoing star formation or a mechanism to create organized spiral patterns. This leads to a smooth, featureless brightness profile that appears rounded or ellipsoidal. Lenticulars have a disk but no spiral arms, and irregulars lack a smooth shape due to chaotic regions of star formation or interactions. So, the galaxy type that is typically smooth and lacks spiral arms is the elliptical.