Object 117: Kappa Cassiopeiae

Podcast release date: 05 February 2024

Right ascension: 00:33:00.0


Epoch: ICRS

Constellation: Cassiopeia

Corresponding Earth location: The starting point of a trail in Storset, Norway, that leads up to the top of the mountain Norskå.

Kappa Cassiopeiae is a blue supergiant star estimated to be 34.5 times the mass of the Sun [1] located in the constellation Cassiopeia at a distance of roughly 3070 light years (940 pc) [2, 3]. The brighter stars in this constellation are all between magnitude 2.2 and 3.4 and form an easy-to-spot W shape [4], but Kappa Cassiopeiae is magnitude 4.2 [4], which means that it's only about 1-2 magnitudes brighter than the faintest stars that can be seen without a telescope and therefore really difficult to find without a star chart.

One of the notable things about Kappa Cassiopeiae is that it is a variable star. It belongs to a class of stars named Alpha Cygni type variable stars after Alpha Cygni, the first star where these types of variations were identified [5]. The brightness variations in Kappa Cassiopeiae, which are very small and very difficult to see without the use of modern telescope instrumentation, occur with multiple time periods, but the strongest variations take place on a period of 2.7 days [6]. These variations appear to be linked to instabilities in the star's outer layers.

While some astronomers are interested in the variability of Kappa Cassiopeiae, others are much more interested in the funky hemispherical shell-like structure of gas and dust located on one side of this star. This structure, which has an inner radius of about 2.4 light years [7], is called a bow shock, although some references call it an atmosphere, which I thought was as confusing as heck, so I am calling it a bow shock. It formed from the interaction of the stellar wind from Kappa Cassiopeiae with the interstellar gas and dust that it is moving through, and because interstellar dust radiates infrared light, this structure looks really bright in the infrared [8]. The infrared emission from the dust also has a very wispy, filamentary structure that has been very challenging to describe using physics models but which has provided interesting insights into the exact nature of the interactions between the stellar wind and the interstellar medium [1].

While the stellar winds from all stars, including the Sun, eventually collide with the interstellar medium surrounding those stars, those collisions do not normally produce bow shocks like the one found near Kappa Cassiopeiae. As best as I can tell, this is because of two reasons. First, this star is a really big, really bright, and really hot star, which means that it has very strong stellar winds to begin with. More importantly, though, this star is a runaway star [8, 9]. This means that Kappa Cassiopeiae was ejected from the location where it formed, most likely through some sort of gravitational interaction with the other stars in that location, and it is now moving at a relatively high velocity through a random part of the interstellar medium, thus producing the bow shock we see today.


[1] Yadav, Abhay Pratap et al., Instabilities and pulsations in models of the B-type supergiant κ Cassiopeiae (HD 2905), 2021, Monthly Notices of the Royal Astronomical Society, 500, 5515

[2] Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1

[3] Gaia Collaboration et al., Gaia Early Data Release 3: Summary of the contents and survey properties, 2020, arXiv e-prints, arXiv:2012.01533

[4] Ducati, J. R., VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system., 2002, VizieR Online Data Catalog

[5] American Association of Variable Star Observers, kap Cas, 2024, The International Variable Star Index

[6] Simón-Díaz, S. et al., Low-frequency photospheric and wind variability in the early-B supergiant HD 2905, 2018, Astronomy & Astrophysics, 612, A40

[7] Katushkina, O. A. et al., An astrosphere around the blue supergiant κ Cas: possible explanation of its filamentary structure, 2018, Monthly Notices of the Royal Astronomical Society, 473, 1576

[8] van Buren, Dave et al., An IRAS/ISSA Survey of Bow Shocks Around Runaway Stars, 1995, Astronomical Journal, 110, 2914

[9] Maíz Apellániz, J. et al., Search for Galactic runaway stars using Gaia Data Release 1 and HIPPARCOS proper motions, 2018, Astronomy & Astrophysics, 616, A149


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: CHARLIESMILER, DasKaesebrot, dronemachine, guitarguy1985, ivolipa, jameswrowles, Joao_Janz, jsbarrett, metrostock99, miloyd, and tutenchwimse at The Freesound Project

Image Viewer: Aladin Sky Atlas (developed at CDS, Strasbourg Observatory, France)