Final answer:
Spiral galaxies have a disk with spiral arms and both young and old stars, with ongoing star formation indicated by bright emission nebulae. Elliptical galaxies lack the disk structure and consist of old stars, with very little star formation due to a lack of interstellar matter. The disk/spheroidal components and star formation activities explain why elliptical galaxies do not have hot, young stars.
Step-by-step explanation:
Spiral galaxies and elliptical galaxies are both distinct types of galaxies that are primarily differentiated by their shapes and their stellar components. Spiral galaxies, such as our own Milky Way, typically have a flat disk with a bulging central nucleus and spiral arms. These galaxies contain a mix of old and young stars, with the young, hot stars and areas of star formation being particularly noticeable in the spiral arms. Interstellar matter, such as gas and dust, is prevalent in the disk, which facilitates ongoing star formation. This is why spiral galaxies can exhibit bright emission nebulae, indicative of the birth of new stars.
In contrast, elliptical galaxies lack the flat disk and spiral structure that characterizes their spiral counterparts. They range in shape from nearly spherical to slightly elongated and consist almost entirely of old stars. Elliptical galaxies have very little interstellar matter left, which results in a near-absence of star formation. It's believed that elliptical galaxies converted most of their interstellar matter into stars billions of years in the past, leaving them with a population of older, cooler stars and lacking the hot, young stars that are a hallmark of active star-forming regions found in spiral galaxies.
The distinctions in the disk and spheroidal components of these galaxies can be associated with the mass-to-light ratio. This ratio is generally smaller in spiral galaxies because the light from hot, young stars is more abundant due to ongoing star formation, while elliptical galaxies, with their older star population and lower star formation rates, tend to have a higher mass-to-light ratio.