28 Cygni

This article is about b² Cygni. For other stars with this Bayer designation, see b Cygni.

28 Cygni is a binary^[6] star in the northern constellation of Cygnus. It is a faint blue-white hued star but visible to the naked eye with an apparent visual magnitude of 4.93.^[3] The distance to 28 Cyg, as estimated from its annual parallax shift of 3.9 mas,^[2] is around 840 light years. It has an absolute magnitude of −2.56,^[3] which means that if the star were just 10 parsecs (33 light-years) away it would be brighter than Sirius, the brightest star in the night sky.

28 Cygni

A light curve for V1624 Cygni, plotted from TESS data[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Cygnus
Right ascension	20h 09m 25.61906s[2]
Declination	+36° 50′ 22.6340″[2]
Apparent magnitude (V)	4.93[3]
Characteristics
Spectral type	B2.5 V[4] or B2 IV(e)[5] + sdO[6]
B−V color index	−0.139±0.004[3]
Variable type	SX Ari[7]
Astrometry
Radial velocity (Rv)	−0.36±2.59[8] km/s
Proper motion (μ)	RA: +2.739 mas/yr[2] Dec.: +13.168 mas/yr[2]
Parallax (π)	3.8921±0.1129 mas[2]
Distance	840 ± 20 ly (257 ± 7 pc)
Absolute magnitude (MV)	−2.56[3]
Details
A
Mass	6.26±0.28[9] M☉
Radius	6.5 (equator) 5.7 (polar)[10] R☉
Luminosity	1,353.22[11] L☉
Surface gravity (log g)	3.983[12] cgs
Temperature	20,470[13] K
Rotational velocity (v sin i)	320[10] km/s
Age	22.1±2.8[14] Myr
B
Mass	0.76±0.28[9] M☉
Orbit[9]
Primary	A
Companion	B
Period (P)	359.260±0.041 days
Semi-major axis (a)	1.89±0.06 AU
Eccentricity (e)	0
Inclination (i)	118.7±0.2°
Longitude of the node (Ω)	146.0±0.3°
Argument of periastron (ω) (secondary)	90°
Semi-amplitude (K1) (primary)	5.4±1.7 km/s
Semi-amplitude (K2) (secondary)	44.9±1.0 km/s
Other designations
b2 Cygni, 28 Cygni, V1624 Cygni, BD+36°3907, HD 191610, HIP 99303, HR 7708, SAO 69518[15]
Database references
SIMBAD	data

This primary object is a B-type main-sequence star with a stellar classification of B2.5 V, per Lesh (1968).^[4] Slettebak (1982) found a class of B2 IV(e),^[5] which would suggest this is a more evolved subgiant star. It is a Be star, which means the spectrum displays emission lines due a disk of ejected gas in a Keplerian orbit around the star. The star displays short-term variability with two or more periods,^[10] and is classified as an SX Arietis variable by Samus et al. (2017).^[7] It is spinning rapidly with a projected rotational velocity of 320 km/s; estimated at round 80% of the critical rotation rate. This is giving the star an oblate shape with an equatorial bulge out to 6.5 times the Sun's radius, compared to 5.7 at the poles.^[10]

The companion is a subdwarf O star.^[6] After previous failed attempts to find the star,^[16] the companion was detected using interferometry.^[6][9] It has an orbital period of nearly a year and is separated by 1.9 astronomical units from its host.^[9]