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Location of Rho Cassiopeiae in the Cassiopeia constellation. |
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| Observation data Epoch J2000 Equinox J2000 |
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|---|---|
| Constellation | Cassiopeia |
| Right ascension | 23h 54m 23.0s |
| Declination | +57° 29′ 58″ |
| Apparent magnitude (V) | 4.1 to 6.2[1] |
| Characteristics | |
| Spectral type | G2Ia0e |
| U−B color index | 1.12 |
| B−V color index | 1.20 |
| Variable type | semiregular (SRD)[1] |
| Astrometry | |
| Radial velocity (Rv) | −43.1 km/s |
| Proper motion (μ) | RA: −4.54 mas/yr Dec.: −3.45 mas/yr |
| Parallax (π) | 0.28 ± 0.58 mas |
| Distance | approx. 12000 ly (approx. 4000 pc) |
| Absolute magnitude (MV) | −7.48 |
| Details | |
| Mass | 40 M☉ |
| Radius | 450 R☉ |
| Luminosity | 5.5 × 105 L☉ |
| Temperature | (7,500 ± 200) K |
| Metallicity | 110% Sun |
| Rotation | 29 km/s |
| Age | ? years |
| Other designations | |
| Database references | |
| SIMBAD | data |
Rho Cassiopeiae ( /ˌroʊ kæsiəˈpiː.iː/; ρ Cas, ρ Cassiopeiae) is a yellow hypergiant star in the constellation Cassiopeia. It is about 11,650 light-years (3,570 pc) away from Earth, yet can still be seen by the naked eye (in the Northern Hemisphere only), as it is 550,000 times as luminous as the Sun. On average, it has an absolute magnitude of −7.5, making it one of the most luminous stars known. Its surface diameter measures 450 times that of the Sun, or approximately 630,000,000 kilometers. Being a yellow hypergiant, it is one of the rarest types of stars, one of only seven in the Milky Way that are currently known, though it is not the only one in its constellation, which also includes V509 Cassiopeiae.[2] Rho Cassiopeiae is a single star, and is categorized as a semiregular variable.
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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.
Rho Cassiopeiae is somewhat unstable in its luminosity. Its apparent magnitude is currently about 4.5. In 1946 it dimmed to 6th magnitude and cooled approximately 4000 Kelvin, before returning to its previous brightness.[3] 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 produced one of the largest outbursts known, ejecting approximately 10,000 Earth masses,[3] or about 3% of a solar mass. During the summer of 2000 it was observed (by the William Herschel Telescope) to have cooled from 7,000 to 4,000 Kelvin in the course of a few months.
These observations have also shown evidence that Rho Cassiopeiae may have already become a supernova (though the light from the explosion has not yet reached us), or will in the near future, because it has consumed most of its nuclear fuel. Assuming its eruptions have continued to occur at the same rate as those observed recently, the star will have lost approximately 20 solar masses over 10,000 years.[4]
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