Phi Andromedae

Phi Andromedae (φ Andromedae, φ And) is the Bayer designation for a binary star system near the border of the northern constellation of Andromeda. This system has a combined apparent visual magnitude of 4.25 and is visible to the naked eye. Based upon parallax measurements made during the Hipparcosmission, this star system is located at a distance of about 720 light-years (220 parsecs) from Earth. With χ And it forms the Chinese asterism 軍南門 (Keun Nan Mun, Mandarin jūnnánmén), "the South Gate of the Camp".

The 4.46 magnitude primary component is a Be star with a stellar classification of B7 Ve, indicating that it is a B-type main sequence star that shows prominent emission lines of hydrogen in its spectrum. These emission lines come from a flattened decretion disk of hot gas that is orbiting the host star. The star is rotating rapidly with a projected rotational velocity of 75 km/s. The pole of the star is inclined around 20° to the line of sight from the Earth.

The 6.06 magnitude companion star is a B-type main sequence star with a classification of B9 V. On average the two stars are separated by about 0.6 arcseconds and have an orbital period of roughly 554 years. Based upon their orbital elements, the system has a combined mass of around 6.5 ± 2.8 times the mass of the Sun.