RW Cephei

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RW Cephei
Observational Data
Designation Rw Cep
HD 21246
HIP 110504
Right ascension 22h 23m 07.0152s[1][2]
Declination +55° 57′ 47.6161″[1][2]
Constellation Cepheus
Type of object Supergiant star
Magnitude Apparent Mag: +6.44[3]
Absolute Mag: -3.18 / -4.02[3]
Distance from Earth 4026.71 ly[3]
Radial velocity -56.00 ± 5.7 km/s[1][4]
Proper motion RA: -3.616 mas/yr[1][2]
Dec: -2.349mas/yr[1][2]
Parallax 0.2357 ± 0.0944 mas[1][2]

RW Cephei (Rw Cep, HD 21246, HIP 110504) is a supergiant star in the constellation of Cepheus.[3] It is not part of the constellation itself, but does lie within the borders of Cepheus. The star is one of the largest stars known with a radius nearly 1,600 greater than the Sun's.[5] The star is sufficiently bright that it does not require binoculars or a telescope to observe it.

RW Cephei is situated roughly 4026.71 light years from Earth.[3] The star is truly huge, thought to have a radius of 1,535 solar radii and a mass of 13.7 solar masses.[5] The temperature of the star is thought to be low at 590 kelvin.[3] The prefix "RV" in the name identifies RW Cephei as a variable star, meaning its brightness changes over time. It is classified as a pulsating, slow regular variable. The Star slowly expands and contracts over time in a regular and predictable manner. As it does so, the brightness varies between +6.456 and +6.620 over a period of 0.170 days or four hours.[3][6]

The star is not known to have any exoplanets.


  1. 1.0 1.1 1.2 1.3 1.4 1.5 RW Cephei. Simbad Astronomical Database. Retrieved on June 22, 2020.
  2. 2.0 2.1 2.2 2.3 2.4 Gaia Collaboration (2018). "VizieR Online Data Catalog: Gaia DR2". VizieR Online Data Catalog I/345: I/345. Bibcode2018yCat.1345....0G.  arXiv:0708.1752
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 RW Cephei (Supergiant Star) Star Facts. Retrieved on June 22, 2020.
  4. Kharchenko, N. V.; Scholz, R. -D.; Piskunov, A. E. et al. (2007). "Astrophysical supplements to the ASCC-2.5: Ia. Radial velocities of 55000 stars and mean radial velocities of 516 Galactic open clusters and associations". Astronomische Nachrichten 328 (9): 889. doi:10.1002/asna.200710776. Bibcode2007AN....328..889K.  arXiv:0705.0878
  5. 5.0 5.1 Nola Taylor Redd (July 26, 2018). What Is the Biggest Star?. Retrieved on June 22, 2020.
  6. Irregular and Semi-Regular Variable Stars. Retrieved on June 22, 2020.