BY Draconis

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BY Draconis AB
Observation data
Epoch J2000      Equinox J2000
Constellation Draco
Right ascension 18h 33m 55.7728s[1]
Declination +51° 43′ 08.905″[1]
Apparent magnitude (V) 8.07[1]
Spectral type K6Ve[1]
U−B color index +0.99[2]
B−V color index +1.19[2]
Variable type BY Dra
Radial velocity (Rv) −27±5[1] km/s
Proper motion (μ) RA: 186.62[1] mas/yr
Dec.: −324.90[1] mas/yr
Parallax (π) 60.90 ± 0.73[1] mas
Distance 53.6 ± 0.6 ly
(16.4 ± 0.2 pc)
Absolute magnitude (MV) 7.48 / 8.63[3]
Primary BY Dra A
Companion BY Dra B
Period (P) 5.975078 ± 0.00001 d
Semi-major axis (a) 4.399 ± 0.050 mas
Eccentricity (e) 0.2978 ± 0.0032
Inclination (i) 151.8 ± 3.5°
Longitude of the node (Ω) 115.1 ± 1.3°
Periastron epoch (T) JD 2451376.1232 ± 0.0045
Argument of periastron (ω)
232.21 ± 0.91°
Semi-amplitude (K1)
28.31 ± 0.14 km/s
Semi-amplitude (K2)
31.96 ± 0.15 km/s
BY Dra A
Mass 0.59 ± 0.14 M
BY Dra B
Mass 0.52 ± 0.13 M
Other designations
BY Dra, GJ 719, BD+51 2402, HD 234677, LTT 15477, SAO 31048, HIP 91009.
Database references
BY Dra C

BY Draconis is a multiple star system in the constellation Draco, consisting of at least three components. Components A and B form a close binary star system with a short orbital period of only 5.98 days. These may be pre-main sequence objects that are still in the process of collapsing. Their individual spectroscopic classifications are dK5e and dK7e.[4] They form the prototype of a class of variable stars known as BY Draconis variables.[3]

The third component (C) is, by comparison, widely separated from the A-B pair by an angular distance of 17 arcseconds, which corresponds to 260 AU at the estimated distance of this star system—where an AU is the average distance from the Earth to the Sun. Component C is an M5 class red dwarf star. There may be a fourth component to the system, orbiting with a 114‑day period, but this has not been visually confirmed.[3]

The variability of BY Draconis is caused by activity in the stellar photosphere called starspots, which are comparable to sunspots on the Sun, in combination with rapid rotation that changes the viewing angle of the activity relative to the observer. This variation has an average periodicity of 3.8285 days, but the brightness also changes over the course of several years—depending on the level of surface activity. Most observers believe that the primary star (A) is responsible for the variability as the secondary produces only a third of the total luminosity from the system. However, the spots may occur on both stars. Unlike the Sun, these spots may occur in the polar regions of the stars.[4]


  1. ^ a b c d e f g h "SIMBAD query result: V* BY Dra -- Variable of BY Dra type". Centre de Données astronomiques de Strasbourg. Retrieved 2008-04-18. 
  2. ^ a b Epps, E. A. (1972). "UBV photoelectric observations. I. Stars within 25 parasecs of the Sun. II. Stars in quasar, galaxy fields. III. Stars in Kapteyn selected areas. IV. Miscellaneous stars". Royal Observatory Bulletin. 176: 77–115. Bibcode:1972RGOB..176..127E. 
  3. ^ a b c d e Boden, A. F.; Lane, B. F. (2001). "A Preliminary Visual Orbit of BY Draconis". The Astrophysical Journal. 547 (2): 1071–1076. arXiv:astro-ph/0001138Freely accessible. Bibcode:2001ApJ...547.1071B. doi:10.1086/318394. 
  4. ^ a b Pettersen, B. R.; Olah, K.; Sandmann, W. H. (1992). "Longterm behaviour of starspots. II - A decade of new starspot photometry of BY Draconis and EV Lacertae". Astronomy and Astrophysics Supplement Series. 96 (3): 497–504. Bibcode:1992A&AS...96..497P. 

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