I have to ask again, what's up with that too-bright but too-faint orange star, HD 23463, smack in the middle of the Pleiades? Its Gaia parallax, 7.5458 ± 0.0461, places it at a distance of some 432 light-years, which is "the perfect distance" to the Pleiades!!
So is this star a member of the Pleiades, then? But how can it be? It is way too bright to be a K-type main sequence star, and it is way too faint to be a massive Pleiades B-type star that has evolved into a red giant. Indeed, if this star was a K-type dwarf at the distance of the Pleiades, it would barely be visible in this image, and certainly not be surrounded by obvious diffraction spikes!
And if it was a recently evolved red giant, the product of a B-type main sequence star that had just run out of hydrogen in its core, we would expect it to be at least more or less as bright as the brightest blue stars in the cluster.
Take a look at the image of NGC 4755 and its red giant star, DU Crucis. Yes, this orange star is indeed fainter than the blue giants in the cluster, some 1½ magnitudes fainter than the brightest blue giant of the cluster (at upper right). Yes, but the red star in the center of the Pleiades is more than 4½ magnitudes fainter than the luminary of the Pleiades, Alcyone!
The only explanation I can think of for the faintness of HD 23463 is that this red star is not
a true member of the Pleiades after all. Instead, I think it is an old and very moderate red giant, perhaps similar to Pollux, but a little redder. This star has to be older than the Pleiades, because we expect the most massive stars to use up their core hydrogen and turn into red giants first, and massive blue stars should give rise to bright red giants. Indeed, it wouldn't surprise me if the red star of the Pleiades is perhaps an old Altair, a star with less than twice the mass of the Sun, turned red giant. Alcyone, by contrast, is believed to have a mass of 5.9-6.1 M☉
. And if Alcyone hasn't evolved into a red giant yet, we certainly don't expect an equally young but much more lightweight star to have used up its core hydrogen already.
So what is this old relatively lightweight star doing in the center of the Pleiades if it is not a member of the cluster? Well, perhaps it just blundered into the cluster as the Pleiades and the star were orbiting the center of the Milky Way at different speeds. HD 23463 does seem to orbit slower than the average proper motion of the Pleiades.
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