What did Hubble need to ensure about the Cepheids observed in Andromeda besides just measuring their period?
Answer
Confirming they were embedded within Andromeda, not foreground stars.
A crucial step, often overlooked in the summary, was confirming the physical location of the variable stars detected. Because the line of sight towards M31 passes through the plane of our own Milky Way galaxy, Hubble needed absolute certainty that the Cepheids he was measuring were actually situated *within* the Andromeda structure itself. If they had been foreground stars merely lying along the same line of sight, their measured distance would correspond only to the edge of our own galaxy, rendering the entire calculation meaningless regarding M31's true location.
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