RW Cephei is a hypergiant

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...

in the constellation Cepheus, at the edge of the Sharpless 132

HII region An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds ...

and close to the small

open cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...

Berkeley 94. One of the largest stars known, RW Cephei's radius is more than 1,000 times that of the Sun (), thus larger than the orbit of

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

RW Cephei is also a semi-regular

of type SRd, meaning that it is a slowly varying yellow giant or supergiant. The visual magnitude range is from 6.0 to 7.6, while the photographic range is from 8.6 - 10.7. The General Catalogue of Variable Stars gives a period of approximately 346 days, but other studies suggest different periods and certainly no strong periodicity. The spectral has been classified as early as G8 and as late as M2, but it isn't clear that there has been actual variation. In the first MK spectral atlas, it was listed M0:Ia. RW Cephei was later listed as the standard star for spectral type G8 Ia, then as the standard for K0 0-Ia. Based on the same spectra it was adjusted to the standard star for type K2 0-Ia. Molecular bands characteristic of M-class stars are seen in infrared spectra, but not always in optical spectra. The temperature is equally uncertain, with contradictory excitation strengths in the spectrum. A simple colour correlation temperature fit gives temperatures around 3,749 K, while a full spectrum fit gives a temperature of 5,018 K. The distance to RW Cephei has been estimated on the basis of its spectroscopic luminosity and is assumed to be a member of the

Cepheus OB1 External links Cepheus OB1 Stellar associations Cepheus (constellation) Star-forming regions ...

association at 3,500 parsecs, consistent with the parallax from

Gaia Data Release 2 The ''Gaia'' catalogues are star catalogues created using the results obtained by ''Gaia'' space telescope. The catalogues are released in stages that will contain increasing amounts of information; the early releases also miss some stars, especia ...

, but not the

Gaia Data Release 3 The ''Gaia'' catalogues are star catalogues created using the results obtained by ''Gaia'' space telescope. The catalogues are released in stages that will contain increasing amounts of information; the early releases also miss some stars, especia ...

parallax. The temperature intermediate between the red supergiants and

yellow hypergiant A yellow hypergiant (YHG) is a massive star with an extended atmosphere, a spectral class from A to K, and, starting with an initial mass of about 20–60 solar masses, has lost as much as half that mass. They are amongst the most visually lumino ...

s, and itself varying considerably, has led to it being variously considered as a red hypergiant or yellow hypergiant.

Great dimming

In December of 2022, the star was reported by two astronomers to be going through a great dimming, reaching a fainter than usual magnitude of 7.6. It is believed to be similar to the great dimming of

Betelgeuse Betelgeuse is a red supergiant of spectral type M1-2 and one of the largest stars visible to the naked eye. It is usually the tenth-brightest star in the night sky and, after Rigel, the second-brightest in the constellation of Orion ...

that happened in late 2019.

References

{{DEFAULTSORT:RW Cephei K-type hypergiants Cephei, RW Cepheus (constellation) 212466 BD+55 2737 110504 Semiregular variable stars G-type hypergiants M-type supergiants M-type hypergiants