Rho Cassiopeiae

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**Rho Cassiopeiae**
Observation data Equinox J2000
Constellation	Cassiopeia
Right ascension	23^h 54^m 23.0^s
Declination	+57° 29′ 58″
Apparent magnitude (V)	4.52
Characteristics
Spectral type	G2Ia0e
U-B color index	1.12
B-V color index	1.20
Variable type	semiregular
Astrometry
Radial velocity (R_v)	−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 (M_V)	−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
7 Cassiopeiae, HR 9045, BD+56°3111, HD 224014, SAO 35879, FK5 899, HIP 117863, GC 33160
Database references
SIMBAD	data

Rho Cassiopeiae (ρ Cas / ρ Cassiopeiae) is a yellow hypergiant in the constellation Cassiopeia. It is about 8150 light-years away, 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. Its surface diameter is 450 times that of our sun's. On average, it has an absolute magnitude of −7.5, making it one of the most luminous stars known.

Rho Cassiopeiae is somewhat unstable in its luminosity. Its normal magnitude is at about 4.5, but in 1946 it went down to 6th magnitude, before going back to normal. The same thing happened in 2000–2001, when it produced one of the largest outbursts known, ejecting roughly 3% of its mass, the equivalent of 10,000 Earths. During the summer of 2000 it was observed (by the William Herschel Telescope) to have cooled from 7000 to 4000 degrees in the course of a few months. It seems to undergo these eruptions approximately once every 50 years (data suggests previous eruptions in 1893 and 1945).

Astronomers think Rho Cassiopeiae may go supernova in the near future because it will soon have consumed most of its nuclear fuel.