HD 166 orV439 Andromedae (ADS 69 A) is a 6th magnitude star in the constellationAndromeda, approximately 45light-years away fromEarth. It is avariable star of theBY Draconis type, varying between magnitudes 6.13 and 6.18 with a 6.23 days periodicity.[3] It appears within one degree of the starAlpha Andromedae[10] and is a member of theHercules-Lyra associationmoving group.[4] It also happens to be less than 2 degrees from right ascension 00h 00m.
Alight curve for V439 Andromedae, plotted fromTESS data,[11] with the 6.23 day rotation period shown in red
HD 166 is aK-type main sequence star, cooler and dimmer than theSun, and has astellar classification of K0Ve[3] where thee suffix indicates the presence ofemission lines in the spectrum. The star has a proper motion of 0.422 arcseconds per year in a direction 114.1° from north. It has an estimated visual luminosity of 61% of theSun,[7] and is emitting like ablackbody with aneffective temperature of 5,327K.[8] It has a diameter that is about 90% the size of the Sun[7] and a radial velocity of −6.9 km/s.[4] Age estimates range from as low as 78 million years old based on its chromospheric activity,[8] up to 9.6 billion years based on a comparison with theoretical evolutionary tracks.[7] X-ray emission has been detected from this star, with an estimated luminosity of8.5×1028 erg s−1.[12]
Aninfrared excess has been detected around HD 166, most likely indicating the presence of acircumstellar disk at a radius of 7.5 AU. The temperature of this dust is 90 K.[13]
Eric J. Gaidoset al. first detected variability in HD 166 in the year 2000.[9] It was given itsvariable star designation, V439 Andromedae, in 2006.[14]It has been found that the periodicity in the photometric variability of HD 166 is coincident with the rotation period.[9] This leads to its classification as aBY Draconis variable, where brightness variations are caused by the presence of largestarspots on the surface and by chromospheric activity.
^Micela, G.; Favata, F.; Sciortino, S. (October 1997), "HIPPARCOS distances of X-ray selected stars: implications on their nature as stellar population",Astronomy and Astrophysics,326:221–227,Bibcode:1997A&A...326..221M
