Epoch J2000 Equinox J2000
|Right ascension||23h 54m 23.0s|
|Declination||+57° 29′ 58″|
|Apparent magnitude (V)||4.1 to 6.2|
|U−B color index||1.12|
|B−V color index||1.20|
|Variable type||semiregular (SRD)|
|Radial velocity (Rv)||−47 km/s|
|Proper motion (μ)||RA: −4.54 mas/yr
Dec.: −3.45 mas/yr
|Parallax (π)||0.28 ± 0.58 mas|
|Absolute magnitude (MV)||–-9.5|
|Surface gravity (log g)||0.1 cgs|
Rho Cassiopeiae (/ /; ρ Cas, ρ Cassiopeiae) is a yellow hypergiant star in the constellation Cassiopeia. It is about 8,200 light-years (2,500 pc) away from Earth, yet can still be seen by the naked eye (in the Northern Hemisphere only), as it is 500,000 times more luminous than the Sun. On average, it has an absolute magnitude of −9.5, making it one of the visually brightest stars known. Its surface diameter measures 450 times that of the Sun, or approximately 630,000,000 kilometers. As a yellow hypergiant it is one of the rarest types of stars. Although only around a dozen are known in the Milky Way, it is not the only one in its constellation, which also includes V509 Cassiopeiae. Rho Cassiopeiae is a single star, and is categorized as a semiregular variable.
The Bayer designation for this star was established in 1603 as part of the Uranometria, a star catalog produced by Johann Bayer. The star catalog by John Flamsteed published in 1712, which orders the stars in each constellation by their right ascension, gave this star the Flamsteed designation 7 Cassiopeiae.
Like other yellow hypergiants, Rho Cassiopeiae is somewhat unstable. Its apparent magnitude is usually about 4.5, but in 1946 it unexpectedly dimmed to 6th magnitude and cooled by over 3,000 Kelvin, before returning to its previous brightness. A similar eruption was recorded in 1893, suggesting that it undergoes these eruptions approximately once every 50 years.
This happened again in 2000–2001, when it was observed (by the William Herschel Telescope) to have cooled from 7,000K to 4,250K in the course of a few months. This was one of the largest outbursts observed for any star, ejecting approximately 3% of a solar mass or 10,000 Earth masses at rates comparable to Eta Carinae during its great outburst. Such mass loss rates are thousands of times higher than the rates between eruptions and would be the dominant mechanism for mass loss in this type of star, possibly providing a mechanism for such stars to lose enough mass to explode as a supernova.
In Chinese, 螣蛇 (Téng Shé), meaning Flying Serpent, refers to an asterism consisting of ρ Cassiopeiae, α Lacertae, 4 Lacertae, π2 Cygni, π1 Cygni, HD 206267, ε Cephei, β Lacertae, σ Cassiopeiae, τ Cassiopeiae, AR Cassiopeiae, 9 Lacertae, 3 Andromedae, 7 Andromedae, 8 Andromedae, λ Andromedae, κ Andromedae, ι Andromedae, and ψ Andromedae. Consequently, ρ Cassiopeiae itself is known as 螣蛇十二 (Téng Shé shíèr, English: the Twelfth Star of Flying Serpent)
- "GCVS Query=Rho Cas". General Catalogue of Variable Stars @ Sternberg Astronomical Institute, Moscow, Russia. Retrieved 2010-11-24.
- Klochkova, V. G.; Panchuk, V. E.; Tavolganskaya, N. S.; Usenko, I. A. (2013). "Instability of the kinematic state in the atmosphere of the hypergiant Rho Cas outside outburst". arXiv:1312.6922v1 [astro-ph.SR].
- Jura, M.; Kleinmann, S. G. (1990). "The probable dust formation episode around Rho Cassiopeiae". The Astrophysical Journal 351: 583. Bibcode:1990ApJ...351..583J. doi:10.1086/168496.
- Gorlova, N.; Lobel, A.; Burgasser, A. J.; Rieke, G. H.; Ilyin, I.; Stauffer, J. R. (2006). "On the CO Near‐Infrared Band and the Line‐splitting Phenomenon in the Yellow Hypergiant ρ Cassiopeiae". The Astrophysical Journal 651 (2): 1130. arXiv:astro-ph/0607158. Bibcode:2006ApJ...651.1130G. doi:10.1086/507590.
- Gesicki, K. (1992). "A Modelling of Circumstellar BAII Lines for the Hypergiant Rho-Cassiopeiae". Astronomy and Astrophysics 254: 280. Bibcode:1992A&A...254..280G.
- Israelian, G.; Lobel, A.; Schmidt, M. R. (1999). "The Yellow Hypergiants HR 8752 and ρ Cassiopeiae near the Evolutionary Border of Instability". The Astrophysical Journal 523 (2): L145. arXiv:astro-ph/9908308. Bibcode:1999ApJ...523L.145I. doi:10.1086/312283.
- Lobel, A.; Dupree, A. K.; Stefanik, R. P.; Torres, G.; Israelian, G.; Morrison, N.; De Jager, C.; Nieuwenhuijzen, H.; Ilyin, I.; Musaev, F. (2003). "High‐Resolution Spectroscopy of the Yellow Hypergiant ρ Cassiopeiae from 1993 through the Outburst of 2000–2001". The Astrophysical Journal 583 (2): 923. arXiv:astro-ph/0301238. Bibcode:2003ApJ...583..923L. doi:10.1086/345503.
- (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 7 日