{{Short description|Star in the constellation Draco}} {{Starbox begin | name=BY Draconis }} {{Starbox image | image = [[Image:BYDraLightCurve.png|250px]] | caption = [[Photometric_system#Photometric_letters|Visual band]] [[light curve]]s for BY Draconis from six different years, adapted from Pettersen ''et al.'' (1992)<ref name=aaass96/> }} {{Starbox observe 1s | epoch=J2000 | component1 = AB | ra={{RA|18|33|55.7728}}{{cn|date=May 2026}} | dec={{DEC|+51|43|08.905}}{{cn|date=May 2026}} | appmag_v=8.07{{cn|date=May 2026}} | constell=[[Draco (constellation)|Draco]] }} {{Starbox character | class=K4Ve + K7.5Ve + M5<ref name=SIMBAD/> | b-v=+1.19<ref name=rob176>{{cite journal | last=Epps | first=E. A. | title=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 | journal=[[Royal Observatory Bulletin]] | date=1972 | volume=176 | pages=77–115 | bibcode=1972RGOB..176..127E }}</ref> | u-b=+0.99<ref name=rob176/> | variable={{nowrap|[[BY Draconis variable|BY Dra]] + [[Flare star|UV Cet]]}}<ref name=gcvs>{{cite journal | title=General catalogue of variable stars: Version GCVS 5.1 | last1=Samus' | first1=N. N | last2=Kazarovets | first2=E. V | last3=Durlevich | first3=O. V | last4=Kireeva | first4=N. N | last5=Pastukhova | first5=E. N | journal=Astronomy Reports | volume=61 | issue=1 | pages=80 | year=2017 | doi=10.1134/S1063772917010085 | bibcode=2017ARep...61...80S | s2cid=125853869 }}</ref> }} {{Starbox astrometry | component1 = AB | radial_v={{val|−25.484|0.046}}<ref name=mnras419>{{cite journal | last=Hełminiak | first=K.G. | title=New high-precision orbital and physical parameters of the double-lined low-mass spectroscopic binary BY Draconis | journal=[[Monthly Notices of the Royal Astronomical Society]] | date = 2012 | volume = 419 | issue=2 | pages=1285–1293 | arxiv=1109.5059 | doi = 10.1111/j.1365-2966.2011.19785.x | doi-access=free | bibcode=2012MNRAS.419.1285H | s2cid=19002972 }}</ref> | prop_mo_ra=185.759{{cn|date=May 2026}} | prop_mo_dec=−325.590{{cn|date=May 2026}} | parallax=60.59 | p_error=0.28 | parallax_footnote={{cn|date=May 2026}} | absmag_v=7.48 / 8.63<ref name=apj547/> }} {{Starbox orbit |reference=<ref name=mnras419/> |primary=BY Dra A |name=BY Dra B |period_unitless=5.9751139 ± 0.0000046 d |periastron=JD 2453999.2144 ± 0.0021 |eccentricity=0.30014 ± 0.00062 |k1=28.394 ± 0.060 |k2=32.284 ± 0.061 |periarg=230.33 ± 0.17 |node=152.3 ± 0.1 |inclination=154.41± 0.29 |axis_unitless=4.4472 ± 0.0091 mas }} {{Starbox detail |source=<ref name=mnras419/> |component1=BY Dra A |mass=0.792 ± 0.026 |component2=BY Dra B |mass2=0.697 ± 0.023 }} {{Starbox catalog | names=BY Dra, [[Gliese-Jahreiss catalogue|GJ]] 719, [[Bonner Durchmusterung|BD]]+51 2402, [[Henry Draper catalogue|HD]] 234677, [[Luyten Two-Tenths catalogue|LTT]] 15477, [[Smithsonian Astrophysical Observatory|SAO]] 31048, [[Hipparcos catalogue|HIP]] 91009<ref name=SIMBAD>{{cite simbad | title=V* BY Dra | accessdate=2008-04-18 }}</ref> }} {{Starbox reference | Simbad = BY+Dra | sn=BY Dra AB | Simbad2 = NAME+BY+Dra+C | sn2=BY Dra C | ARICNS = 01493 }} {{Starbox end}}

'''BY Draconis''' is a [[multiple star system]] in the [[constellation]] [[Draco (constellation)|Draco]], consisting of at least three components. Components A and B are [[main sequence]] stars,<ref name=mnras419/> and form a close [[binary star]] system with a short [[orbital period]] of only 5.98 days. Their individual [[spectroscopy|spectroscopic]] classifications are dK5e and dK7e.<ref name=aaass96/> They form the prototype of a class of [[variable star]]s known as [[BY Draconis variable]]s.<ref name=apj547>{{cite journal | last=Boden | first= A. F. |author2=Lane, B. F. | title=A Preliminary Visual Orbit of BY Draconis | journal=[[The Astrophysical Journal]] | date=2001 | volume=547 | issue=2 | pages=1071–1076 | arxiv=astro-ph/0001138 | doi=10.1086/318394 | bibcode=2001ApJ...547.1071B| s2cid= 10704476 }}</ref>

The third component (C) is, by comparison, widely separated from the A-B pair by an [[angular distance]] of 17 [[arcsecond]]s, which corresponds to 260 [[Astronomical Unit|AU]] at the estimated distance of this star system&mdash;where an AU is the average distance from the [[Earth]] to the Sun. Component C is an [[spectral classification|M5 class]] [[red dwarf]] star. There may be a fourth component to the system, orbiting with a ≤1000‑day period, responsible for the eccentricity of the 5.98-day orbit, but this has not been visually confirmed.<ref name=mnras419/>

The variability of BY Draconis is caused by activity in the stellar [[photosphere]] called [[starspot]]s, which are comparable to [[sunspot]]s on the [[Sun]], in combination with [[stellar rotation|rapid rotation]] that changes the viewing angle of the activity relative to the observer. This variation has an average periodicity of 3.8285&nbsp;days, but the brightness also changes over the course of several years&mdash;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.<ref name=aaass96>{{cite journal | first1=B. R. | last1=Pettersen | first2=K. | last2=Olah | first3=W. H. | last3=Sandmann | title=Longterm behaviour of starspots. II - A decade of new starspot photometry of BY Draconis and EV Lacertae | journal=Astronomy and Astrophysics Supplement Series | date=1992 | volume=96 | issue=3 | pages=497&ndash;504 | bibcode=1992A&AS...96..497P}}</ref>

==References== {{Reflist}}

==External links== * {{cite web | url=http://www.daviddarling.info/encyclopedia/B/BY_Draconis_star.html | title=BY Draconis star | publisher=The Internet Encyclopedia of Science | access-date=2008-04-18 | archive-url= https://web.archive.org/web/20080304192144/http://www.daviddarling.info/encyclopedia/B/BY_Draconis_star.html| archive-date= 4 March 2008 | url-status= live}}

{{Stars of Draco}}

[[Category:Draco (constellation)]] [[Category:BY Draconis variables]] [[Category:K-type main-sequence stars]] [[Category:Objects with variable star designations|Draconis, BY]] [[Category:Henry Draper Catalogue objects|234667]] [[Category:Gliese and GJ objects|0719]] [[Category:Durchmusterung objects]] [[Category:Hipparcos objects|091009]] [[Category:Flare stars]] [[Category:Triple star systems]] [[Category:Emission-line stars]]