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 5.3 mas,[2] is around 620 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.
Observation data Epoch J2000 Equinox J2000 | |
---|---|
Constellation | Cygnus |
Right ascension | 20h 09m 25.61909s[2] |
Declination | +36° 50′ 22.5961″[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: +2.728[2] mas/yr Dec.: +15.562[2] mas/yr |
Parallax (π) | 5.2947 ± 0.2255 mas[2] |
Distance | 620 ± 30 ly (189 ± 8 pc) |
Absolute magnitude (MV) | −2.56[3] |
Details | |
Mass | 9.5±0.3[9] M☉ |
Radius | 5.7[10] R☉ |
Luminosity | 1,353.22[11] L☉ |
Surface gravity (log g) | 3.983[12] cgs |
Temperature | 11,338[11] K |
Rotational velocity (v sin i) | 320[10] km/s |
Age | 22.1±2.8[9] Myr |
Other designations | |
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 central star is orbited by a secondary companion star, which is a subdwarf O star.[6] After previous failed attempts to find the star,[14] the companion was detected using interferometry, but the orbital parameters are unknown.[6]