SU_Cygni
SU Cygni
Variable star system in the constellation Cygnus
SU Cygni is a triple star system in the northern constellation of Cygnus, abbreviated SU Cyg. The primary component of the system is a classical Cepheid variable with a period of 3.8455473 days. The changing luminosity of this star causes the system to vary in brightness from a peak apparent visual magnitude of 6.44 down to magnitude 7.22 over the course of its cycle.[3] The distance to this system is approximately 3,200 light years based on parallax measurements.[2] It is a member of the Turner 9 open cluster of stars.[10]
The variable luminosity of this star was announced by G. Müller and G. Kempf in 1898.[12] The following year, M. Luizet determined a period of 3.846 days.[13] In 1906, the radial velocity of this star was found to be variable by J. D. Maddrill, with its cycle matching the luminosity period but trailing in phase by half a day.[14] By 1916 it was classified as a Cepheid variable, with spectrographic studies showing that the spectral type varied over the course of each cycle. It ranged from a class of A6 near peak luminosity down to F7 at minimum.[15]
A companion star, designated component B, was detected photometrically by B. F. Madore in 1977,[16] with the colors suggesting a B-type main-sequence star with a class of B6–7V. This finding was supported by observations of J. D. Fernie in 1979, who determined a class of B6V. In 1984, M. Imbert measured an orbital period of 549.2 days for the pair, with an orbital eccentricity (ovalness) of 0.35.[8] Ultraviolet observations with the IUE spacecraft in 1985 showed that the companion is a close binary system with a period of 4.7 days. The members of this pair have classes of B8 and A0.[17] In 1998, the B-type companion was found to be a chemically peculiar HgMn star.[18]
There is a candidate comoving companion star located at an angular separation of 24″ to the west of SU Cyg. It is an A-type main-sequence star with a class of A2V. This star does not appear to be gravitationally bound to SU Cyg.[10]