= Solution
Typical present <elliptical galaxies> are dominated by old <stellar populations>: their brightest short-lived <stars> have disappeared, leaving faint long-lived <stars> and <stellar remnants>, so luminosity per unit stellar mass is low. Ongoing <star formation> in many <spiral galaxies> and <irregular galaxies> supplies luminous young <stars>, giving \b[a smaller stellar mass-to-light ratio in a fixed band]; <dark matter>, the <initial mass function> and aperture must also be specified if the quoted ratio is dynamical.
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