A student asked about this question recently, and wondered whether it was safe to assume that stars had different luminosities.
If we look at this, though, we must EITHER assume that they all have the same luminosity, or assume that they might NOT have the same luminosity. The one that requires the smallest logical leap is the one we will choose.
In this case, we will assume that they don't all have the same luminosity - not that they all are identical.

Veritas Help

Makes sense thank you!