Upsilon2 Cassiopeiae

For other star systems with this Bayer designation, see υ Cassiopeiae

Upsilon² Cassiopeiae is a solitary star in the constellation of Cassiopeia, a few degrees to the south of Gamma Cassiopeiae. It has the proper name Castula /ˈkæstjʊlə/, which has been officially adopted by the IAU.^[8] The star has a yellow hue and is faintly visible to the naked eye with an apparent visual magnitude of +4.62.^[3] Based upon parallax measurements, it is located at a distance of approximately 189 light years from the Sun.^[2] The star is drifting closer with a radial velocity of −48 km/s.^[6]

υ² Cassiopeiae

Location of υ2 Cassiopeiae (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Cassiopeia[1]
Right ascension	00h 56m 39.90413s[2]
Declination	+59° 10′ 51.8006″[2]
Apparent magnitude (V)	+4.62[3]
Characteristics
Evolutionary stage	horizontal branch[4]
Spectral type	G8 IIIb Fe−0.5[5]
U−B color index	+0.68[3]
B−V color index	+0.96[3]
Astrometry
Radial velocity (Rv)	−47.73±0.12[6] km/s
Proper motion (μ)	RA: −92.085[2] mas/yr Dec.: −45.079[2] mas/yr
Parallax (π)	17.2875±0.1880 mas[2]
Distance	189 ± 2 ly (57.8 ± 0.6 pc)
Absolute magnitude (MV)	+0.62[7]
Details[6]
Mass	1.44±0.16 M☉
Radius	10.44+0.24 −0.50[2] R☉
Luminosity	55.3±0.7[2] L☉
Surface gravity (log g)	2.75±0.19 cgs
Temperature	4,937±14 K
Metallicity [Fe/H]	−0.35±0.03 dex
Rotational velocity (v sin i)	0.54±0.38 km/s
Age	2.20±0.62 Gyr
Other designations
Castula, υ2 Cas, 28 Cassiopeiae, BD+58°138, HD 5395, HIP 4422, HR 265, SAO 21855
Database references
SIMBAD	data

This is an aging giant star with a stellar classification of G8 IIIb Fe−0.5,^[5] where the suffix notation indicates a mild underabundance of iron in the spectrum. With the supply of core hydrogen exhausted, this star has cooled and expanded off the main sequence – at present it has 10^[2] times the girth of the Sun. It is a red clump giant, which means it is on the horizontal branch undergoing core helium fusion.^[4]

Upsilon² Cassiopeiae is a barium star, showing an excess of the element barium in its spectrum. This can occur from mass transfer from a more-evolved companion star that later became a white dwarf, although no companion has been detected.^[7] It is 2.2 billion years old with 1.44 times the mass of the Sun.^[6] The star is radiating 55^[2] times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 4,937 K.^[6]

Nomenclature

Upsilon² Cassiopeiae, Latinised from υ² Cassiopeiae, is the star's Bayer designation.

This star bore the name Castula in Bayer's Uranometria, which is Latin for a type of woman's tunic.^[9] In 2016, the IAU organized a Working Group on Star Names (WGSN)^[10] to catalog and standardize proper names for stars. The WGSN approved the name Castula for this star on 5 September 2017 and it is now so included in the List of IAU-approved Star Names.^[8]