Garnet Star

The Garnet Star; captured 2023 in Krefeld, Germany with a Skywatcher 150/750; ZWO ASI 2600MC Pro Color and an Optolong L-Extreme filter; total exposure time of 4h (5min per single light frame), CC BY NiKo 2023

The name ‘garnet star’ comes from Wilhelm Herschel, as μ Cephei (SIMBAD) appears garnet red in telescopes due to its spectral class M2. With 4.08 Vmag, the star is visible to the naked eye, but the eye cannot recognize the colour without a telescope because colour vision cuts off at roughly 2 mag.

Etymology and History

μ Cephei became famous through observations by Wilhelm Herschel (1738 - 1822), who described it in an essay in 1783 listing stars that were not listed in Flamsteed's star catalogue. Herschel's German term is "granatrot" (‘garnet red’):

Einen sehr ansehnlichen Stern, der nicht von Flamsteed markiert wurde, kann man in der Nähe des Kopfes von Cepheus finden. Er hat eine schöne, tief granatrote Farbe und ist ein äußerst schönes Objekt, ganz besonders, wenn man zuvor einen weißen Stern betrachtet, bevor man das Teleskop auf ihn richtet.

English: A very handsome star that was not marked by Flamsteed can be found near the head of Cepheus. It has a beautiful, deep garnet red colour and is an extremely beautiful object, especially if you look at a white star before pointing the telescope at it.

This description earned the star the name ‘Herschel's Garnet Star’, in German simply "Granatstern" (without Herschels name in the star name). The star's variability between 3.4 and 5 mag is not mentioned by Herschel, but only suspected by J. R. Hind in 1848 and confirmed by Argelander. Like many red supergiants (e.g. Betelgeuze, Antares), the Garnet Star has two periods, one of 850 and another 4400 days.

The description of mu Cephei as "deep red" evokes the image of a vividly red star. In fact, the star's color index (B – V) of 2.35 magnitudes makes it the reddest one visible for the naked eye (although the colour is only visible in telescopes). This intense hue is largely due to the interstellar dust between us and the star. Corresponding to its spectral type, without the reddening effect of this dust, its color index would be (B – V) = 1.71 magnitudes.

Allen (1899) writes that Piazzi in his Palermo Star Catalogue copied Herschel's name "Garnet Star". Flamsteed omits the star in his star list (possibily due to its variability), and it reoccurs in star catalogues only after the discovery of its variability.

IAU Working Group Star Names

In 2023, the WGSN discovered that SIMBAD has two proper names for the star, "Erakis" and "Herschel's Garnet Star", and decided to study their history. Which of the names should be preferred?

The Arabic reading group agreed that "Erakis" is a misspelling of "Alrakis", a name which is applied to another star already (in 2017, WGSN applied "Alrakis" to μ Dra with the reason by Kunitzsch and Smart that the star name had been misapplied to this star earlier). Therefore, the Arabic alternative should be dropped. We also suggest to use the shorter (and non-eponymous) version "Garnet Star".

The name was discussed and approved by the IAU WGSN in 202x. μ Cephei is a semi-regularly variable star and was the prototype of the no longer used class of "mu Cephei stars".

As this star is already named ..., the WGSN chose ... (not to apply/ to apply the name to a neighbouring star/ to ...) in the IAU-CSN.