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This Wikipedia page contains information about the star Spica in the rightmost table. A small snapshot of this table is below.

table data

In this table, the absolute magnitude is given as −3.55 (−3.5/−1.5). Instead of seeing the value and error given as value ± error, I am thinking the error is given as it is because of absolute magnitude is given on a logarithmic scale (as opposed to linear scale). But I am not quite sure how to interpret this as an error reading. How does one interpret this reading?

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Spica (α Vir) is a binary star. The Wikipedia article cites Herbison-Evans et al. 1971, whose Table III says:

Absolute magnitude of primary (Mv1)   -3.5±0.1
Absolute magnitude of secondary (Mv2)   -1.5±0.2

So the confusing "-3.55 (-3.5 / -1.5)" can be interpreted as "combined (primary / secondary)."

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