Sirrah is an alternate name for Alpheratz or α Andromedae, the brightest star in the constellation Andromeda. Both "Sirrah" and "Alpheratz" come from from the Arabic Al Surrat al Faras or "The Horse's Navel"; the star used to be considered the navel (or sometimes the shoulder) of the constellation Pegasus. Ptolemy considered it part of Andromeda, however, and the later Arabic astronomers adopted his assignment and called it Al Ras al Mar'ah al Musalsalah, "the head of the woman in chains". In Europe, it is represents Andromeda's hair. In the Vedic system, Sirrah and the other stars of the western Great Square of Pegasus formed the Proshtapadas -- the double nakshatra -- commonly called the "feet of the footstool".
Sirrah is a spectroscopic binary, quite bright at magnitude 2.2, and consists of the primary -- a luminous, blue star of spectral type B8IV -- and a much fainter secondary companion of magnitude 11.2. The pair are about 30 parsecs distant from us, are separated by an angular distance of about 80 arcseconds, and have an orbital period of 96.6960 days. The star is a variable star, but not because of eclipses. Adelman et al. (2002) suggest that the cause of variation is due to inhomogeneities in the primary star's surface composition, likely caused by magnetic fields. These differences in composition result in local inhomogeneities in the stellar photospheric opacity, resulting in differences in brightness around the surface of the star. Thus the apparent magnitude changes slightly as the star rotates (about once every three days).
The peak visibility season for Sirrah is centered on the autumnal equinox; it is located at
(J2000) α 00h 08m 23.26s, δ +29° 05' 25.6". It will be easily visible throughout Autumn to all observers in the Northern hemisphere, but is fairly low in the northern sky to observers in the Southern hemisphere. As madvid noted in his writeup in Alpheratz, it is one of the common stars used for celestial navigation. It is also very near the celestial meridian (the zero hour meridian in right ascension).
While Alpheratz is the preferred name (Hoffleit & Jaschek 1982), Sirrah
is also acceptable. The latter name was used in the song "Jack Luminous", from Voivod's 1993 release The Outer Limits:
And what about Hydra, Sirrah,
Pollux, and Gemma,
Adelman, S., 2002, Astrophysical Journal 575, 449
Allen, Richard, 1964, Star Names: Their Lore and Meanings (London: Dover Books, reprint of 1899 edition)
Hoffleit, D. & Jaschek, C., 1982, The Bright Star Catalogue (New Haven, CT: Yale University Obs.)
Kholopov, P.N., 1988, General Catalogue of Variable Stars (Moscow: Nauka)