Alpha Trianguli

Alpha Trianguli (Alpha Tri, α Trianguli, α Tri) is the Bayer designation for a binary star in the constellation Triangulum. It has the traditional names Ras al Muthallah (or Mothallah) and Caput Trianguli.

Estimates of the combined stellar classification for this system range from F5III to F6IV, with the luminosity class of 'IV' or 'III' indicating the primary component is a subgiant or giant star, respectively. It is a member of a close binary system—a spectroscopic binary—whose components complete an orbit about their center of mass once every 1.736 days. Because the primary star is rotating rapidly, it has assumed the shape of an oblate spheroid. The ellipsoidal profile of the star, as viewed from Earth, varies over the course of an orbit causing the luminosity to vary in magnitude during the same period. Such stars are termed ellipsoidal variables. Within a few million years, as the primary continues to evolve into a red giant star, the system may become a semi-detached binary with the Roche lobe becoming filled to overflowing.

The mean apparent magnitude of +3.42 for this pair is bright enough to be readily seen with the naked eye. It forms the second brightest star or star system in this generally faint constellation, following Beta Trianguli. Parallax measurements place this system at a distance of 63.3 light-years (19.4 parsecs) from the Earth. The effective temperature of the primary's outer envelope is 6,288 K, giving it a yellow-white hue typical of F-type stars. It has a mean radius about three times the radius of the Sun. The system is an estimated 1.6 billion years old.