RW Cephei

RW Cephei is a K-type hypergiant and a semirregular variable star in the constellation Cepheus, at the edge of the Sharpless 132 H II region and close to the small open cluster Berkeley 94. It is among the largest stars known with a radius of almost 1,000 times that of the Sun (R), nearly as large as the orbit of Jupiter.

RW Cephei
CHARA/MIRC-X image of RW Cephei in the H-band during the "great dimming", revealing a box-like shape and a dark patch on the western side of the star
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cepheus
Right ascension 22h 23m 07.01521s
Declination +55° 57 47.6244
Apparent magnitude (V) +6.65 (6.0–7.6)
Characteristics
Evolutionary stage Red or yellow hypergiant
Spectral type K2 0-Ia (G8 - M2Ia-0)
Apparent magnitude (K) 1.88
U−B color index 2.38
B−V color index 2.22
Variable type SRd
Astrometry
Radial velocity (Rv)−56.00 km/s
Proper motion (μ) RA: −3.606 mas/yr
Dec.: −2.881 mas/yr
Parallax (π)0.1140 ± 0.0342 mas
Distance3,400+220
−200
 pc
Absolute magnitude (MV)−8.0  −9.4
Details
Radius900–1,760, 940 R
Luminosity300,000 L
Surface gravity (log g)0.2 cgs
Temperature4,200–4,400, 3,900 (during the dimming) K
Metallicity [Fe/H]+0.17±0.20 dex
Other designations
HD 212466, HIP 110504, SAO 34387, BD+55°2737, AAVSO 2219+55A
Database references
SIMBADdata

In 2022, the star underwent a "great dimming" event similar to Betelgeuse.

The temperature intermediate between the red supergiants and yellow hypergiants has led to it being variously considered as a red hypergiant or yellow hypergiant.

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