The alpha star in a constellation has the highest brightness, but does that mean it has the highest flux density among the other stars in the constellation?
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3$\begingroup$ "The alpha star in a constellation has the highest brightness": often, but not always. Beta Orionis (Rigel) is brighter than Alpha Orionis (Betelgeuse), and there are other examples. $\endgroup$– James KCommented Nov 4, 2021 at 6:24
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1$\begingroup$ It's a misconception that the Bayer Designations go in order of brightness. The Wikipedia entry has more info: en.wikipedia.org/wiki/Bayer_designation $\endgroup$– Greg MillerCommented Aug 1, 2022 at 14:10
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2 Answers
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"Brightness" can mean two different things. The apparent brightness of the point of light in the sky as viewed from Earth and the absolute brightness of the star as a physical object, viewed from a standard distance.
For example Eta Carina is a very very very bright star in absolute terms, but only an averagely bright point of light. Sirus is the brightest star in the sky, but far less powerful. Sirius appears brighter than Eta Carina because it is closer.
The Bayer designations (Alpha, Beta etc) are based, in part, on the apparent brightness. The flux density is a measure of the apparent brightness. So Sirius has the highest flux density.
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Stars were historically grouped into constellations due to their (angular) proximity, as seen from Earth, but can have vastly different distances. As apparent magnitude depends on distance, a star's brightness is not an indication of its absolute magnitude.
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$\begingroup$ I don't think this is the answer to the question asked. I don't think "flux density" means "absolute magnitude" $\endgroup$– James KCommented Nov 4, 2021 at 21:30
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$\begingroup$ I think the author interpreted brightness as absolute magnitude, and flux density as apparent magnitude? $\endgroup$– TosicCommented Dec 4, 2021 at 16:19