Kappa Scorpii

With an apparent visual magnitude of 2.4, this star system is readily visible to the naked eye. Parallax measurements place it at an estimated distance of roughly 480 light-years (150 parsecs) from the Earth.

κ Scorpii

Location of κ Sco (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Scorpius
Right ascension	17h 42m 29.27520s
Declination	−39° 01′ 47.9391″
Apparent magnitude (V)	2.41 - 2.42
Characteristics
Spectral type	B1.5 III
U−B color index	−0.914
B−V color index	−0.228
Variable type	β Cephei
Astrometry
Radial velocity (Rv)	−14.0 km/s
Proper motion (μ)	RA: −6.05 mas/yr Dec.: −25.54 mas/yr
Parallax (π)	6.75 ± 0.17 mas
Distance	480 ± 10 ly (148 ± 4 pc)
Absolute magnitude (MV)	−3.46
Orbit
Period (P)	195.65 days
Eccentricity (e)	0.488±0.005
Periastron epoch (T)	92.7±0.5
Semi-amplitude (K1) (primary)	48.0±0.3 km/s
Details
κ Sco A
Mass	17 M☉
Radius	6.8 R☉
Luminosity	6,911 L☉
Surface gravity (log g)	4.01 cgs
Temperature	23,400 K
Rotation	1.90 days
Rotational velocity (v sin i)	105 km/s
Age	25.1±1.3 Myr
κ Sco B
Mass	12 M☉
Radius	5.8 R☉
Surface gravity (log g)	4.00 cgs
Temperature	18,800 K
Other designations
Girtab, κ Sco, CD−38°12137, CPD−38°6992, FK5 660, HD 160578, HIP 86670, HR 6580, SAO 209163
Database references
SIMBAD	data

This is a spectroscopic binary, which is a type of binary star system in which the two stars are so close together that they have not been individually resolved with a telescope. The pair orbit each other with a period of about 196 days and an eccentricity of nearly 0.5. The combined spectrum of this pair matches a star with a stellar classification of B1.5 III. The 'III' luminosity class indicates the presence of a giant star that has exhausted the supply of hydrogen at its core and is in a late evolutionary stage.

 

The primary component of the pair, κ Sco A, is a variable star of Beta Cephei type. It is undergoing radial pulsations with a dominant frequency of 4.99922 cycles per day, or 4.8 hours per cycle. There are overlapping secondary pulsation frequencies of about 4.85 and 5.69 cycles per day. This star has about 17 times the mass of the Sun and is nearly 7 times the Sun's radius. The effective temperature of the outer envelope is 23,400 K, giving it a blue-white hue. It is rotating rapidly, with an estimated period of only 1.9 days and an axis of rotation that is inclined by about 40° to the line of sight from the Earth.

The secondary component, κ Sco B, is smaller than the primary, but still much larger than the Sun. It has about 12 times the mass of the Sun and nearly six times the Sun's radius. The effective temperature of 18,800 K is also higher than the Sun's, which is at 5,778 K.

κ Scorpii has been called Girtab /ˈɡɜːrtæb/, which is the Sumerian word for 'scorpion'. The name has survived through the Babylonian star catalogues, and was originally applied to an asterism comprising this star, Lambda Scorpii, Upsilon Scorpii, and Iota Scorpii.

In Chinese, 尾宿 (Wěi Xiù), meaning Tail, refers to an asterism consisting κ Scorpii, μ¹ Scorpii, ε Scorpii, ζ¹ Scorpii and ζ² Scorpii, η Scorpii, θ Scorpii, ι¹ Scorpii and ι² Scorpii, λ Scorpii and υ Scorpii. Consequently, the Chinese name for κ Scorpii itself is 尾宿七 (Wěi Xiù qī), "the Seventh Star of Tail".

κ Scorpii appears on the flag of Brazil, symbolising the state of Paraíba.