Zeta1 Scorpii

For other stars with this Bayer designation, see Zeta Scorpii.

Zeta¹ Scorpii (Zeta¹ Sco, ζ¹ Scorpii, ζ¹ Sco) is a binary star^[7] in the constellation of Scorpius, composed by an B-type hypergiant star as the primary, and a secondary of which little is known. It has an apparent visual magnitude which varies between 4.66 and 4.86.^[3] It is a member of the Scorpius OB1 association, and potentially of the open star cluster NGC 6231,^[11] also known as the "Northern jewel box" cluster.

ζ¹ Scorpii

Location of ζ1 Scorpii (circled)
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Scorpius
Right ascension	16h 53m 59.72713s[1]
Declination	−42° 21′ 43.3073″[1]
Apparent magnitude (V)	4.705[2] (4.66 to 4.86)[3]
Characteristics
Spectral type	B1.5 Ia+[4]
U−B color index	−0.567[2]
B−V color index	+0.480[2]
Variable type	Luminous blue variable[5]
Astrometry
Radial velocity (Rv)	−26.0[6] km/s
Proper motion (μ)	RA: −0.094 mas/yr[1] Dec.: −3.368 mas/yr[1]
Parallax (π)	0.5855±0.1176 mas[1]
Distance	approx. 6,000 ly (approx. 1,700 pc)
Absolute magnitude (MV)	−8.5[4]
Details
Mass	36 to 53[4] M☉
Radius	159+83 −34[7] R☉
Luminosity (bolometric)	(1.0–1.6)×106[7] L☉
Surface gravity (log g)	1.7[4] cgs
Temperature	17,000–19,000[7] K
Rotational velocity (v sin i)	60[8] km/s
Age	6.5±0.1[9] Myr
Other designations
ζ1 Sco, CD−42 11633, CPD−42 7545, GC 22730, HD 152236, HIP 82671, HR 6262, PPM 322342, SAO 227375.[10]
Database references
SIMBAD	data

Characteristics

A visual band light curve for Zeta¹ Scorpii, adapted from Sterken et al. (1997).^[12] The inset plot shows the long term variability. The purple arrow shows the brightness reported by al-Sufi in A.D. 962.

ζ¹ Scorpii's primary is a luminous blue variable according to its luminosity and spectral appearance,^[13] yet is has not shown the characteristic types of variability, hence is classified as a dormant LBV.^[5] It has around 36 times as massive as the Sun^[4] and is one of the most luminous stars known in the Galaxy, with an estimated bolometric luminosity between 1 and 1.6 million times that of the Sun and a radius around 160 times that of the Sun.^[7] The stellar wind from this supergiant is expelling matter from the star at the rate of 1.55 × 10⁻⁶ solar masses per year, or roughly the equivalent to the Sun's mass every 640,000 years.^[4]

The secondary has been detected using interferometry, and its discovery was announced in 2021. As of 2021^[update], it has an angular separation of 11.54±0.10 mas along a position angle of 283.22°+0.76°
−0.34°. It is 6.3 magnitudes fainter than the primary.^[7]

ζ¹ Scorpii alongside the brighter ζ² Scorpii to the south of NGC 6231

ζ¹ Scorpii forms a naked eye double with ζ² Scorpii, but the stars are merely coincidentally near in the line of sight from Earth. ζ² is a mere 135 light-years distant and much less luminous in real terms. ζ¹ Scorpii can also be distinguished from ζ², due to the latter's orange hue especially in long-exposure photographs.