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I see Adhara is cited in many places (for example, here) as being the brightest source of ultraviolet light in the sky. The particular source uses the modifier "extreme", but most other statements about it do not, for example:

If people could see in ultraviolet light, Adhara would be the brightest star in the sky.

Yet Adhara's U magnitude is given as 0.36 in the SIMBAD database. In comparison, for example, Sirius has a U magnitude of -1.51.

Is there a specific meaning for "extreme ultraviolet" in the claim? And if so, where can I find a catalog of the magnitudes of various stars in that band?

The claim may refer to the band labeled U4 in the Celescope Catalog of Ultraviolet Stellar Observations, which refers to a filter with a spectral response centered around 1500 Angstroms and sensitive over a range from about 1200 to 1900 Angstroms. Of the four ultraviolet filters used in that catalog, U4 covered the "most extreme" ultraviolet span.

However, that catalog lists four stars (Hadar, Acrux, Alnitak, and Mintaka Aa) that have a greater apparent magnitude in the U4 range, but each of those is (now) known to be a multi-star system (as is Adhara). Perhaps it's true that ε Canis Majoris A taken singly outshines each of the brightest stars in the other multi-star systems when taken singly, but I can find no data on that breakdown.

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Those making the claims should define exactly what they mean.

U-band magnitudes are well defined. The Johnson U-band covers from about 300-400 nm with a peak at 360 nm. The peak wavelength would be just blueward of the cut-off in sensitivity to the human eye (for most people), so could be termed "ultraviolet" (certainly is classed as UV-A in terms of skin exposure). However, in astronomy, ultraviolet is often reserved for wavelengths shortward of the atmospheric cut-off at about 330 nm and would then extend to wavelengths as short as 121 nm (the Lyman alpha line). Wavelengths shorter than this and perhaps down to 10 nm would be classed as "extreme ultraviolet (EUV)".

Since Adhara is much hotter than Sirius A it is quite plausible that it would be brighter than Sirius at wavelengths shorter than about 300 nm.

Adhara is the brightest EUV source in the sky - depending on exactly what you mean by EUV.

At very short wavelengths (11-20 nm) that honour falls to the hot white dwarf known as Hz 43, as judged by the ROSAT Wide Field Camera EUV instrument (in the S2 band 11-20 nm - Pounds et al. 1993 give a catalogue of EUV sources).

On the other hand, in the EUVE catalogue of EUV sources (Malina et al. 1994), Adhara is the brightest source in the sky at wavelengths between 35-75 nm and given its temperature and distance is likely to be the brightest source at all wavelengths longer than that up to about 300 nm.

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    $\begingroup$ Perfect, thank you! $\endgroup$ Mar 25, 2023 at 17:08
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    $\begingroup$ @L.ScottJohnson you are hasty. Make sure you saw the edits. $\endgroup$
    – ProfRob
    Mar 25, 2023 at 17:09
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    $\begingroup$ Quick, perhaps, rather than hasty. :-) 35-75nm, yeah, I saw that. $\endgroup$ Mar 25, 2023 at 17:10

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