What is the primary difference in stellar population between Ellipticals and Spirals?
Answer
Ellipticals are dominated by old, red stars; Spirals have a mix including young, blue stars
The defining contrast lies in the presence of young stellar populations. Spiral galaxies, like the Milky Way, maintain ongoing star formation concentrated in their spiral arms, evidenced by the consistent presence of young, hot, blue stars. Conversely, elliptical galaxies are past this phase, meaning their light is integrated from long-lived, evolved red giants and smaller stars, resulting in a net yellowish-red color and an almost complete absence of the blue light indicative of recent stellar birth. The spiral light is thus a mixture, whereas the elliptical light profile is dictated by an ancient, static population.
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