OMEGA PSC (Omega Piscium). Twenty-four Greek letters run from Alpha to Omega, the "Alpha star" of a constellation usually (but hardly always) its brightest. The "Omega star" is never actually the 24th brightest (the Greek letters assigned as much by position as brightness), nor is it usually of any great import, the massive globular cluster Omega Centauri a distinct exception. Mid- fourth magnitude (4.04) Omega Piscium is another, but one that is little-recognized. The first star to the east of the Circlet of Pisces, Omega Psc lies just south and a bit to the west of the eastern edge of the Great Square of Pegasus. Stars are positioned by "right ascension" (akin to terrestrial longitude), defined as their angles to the east of the Vernal Equinox, and by declination (akin to latitude), their angles north or south of the celestial equator. Right ascensions are expressed in time units (a full circle having 24 hours). Because of the 26,000-year wobble of the Earth's axis (which changes the pole star (Polaris now ours, Thuban that of the ancient Egyptians), these coordinates change with time, every star going through a full 360 degrees of right ascension. Counting proper names, Greek letters, and Flamsteed numbers, Omega Cen is the named star with the highest right ascension, as of the year 2000, 23 hours, 59 minutes, 18.7 seconds. It will therefore be the next named star to be passed by the Vernal Equinox, in the year 2013, when its right ascension will go from 23-plus hours to 0 hours. (The last named star to make the passage was Epsilon Tucanae in 2001.) Physically, Omega Psc is a rather confusing class F (F5) subgiant/dwarf (one that has probably just given up core hydrogen fusion) 106 light years away with a temperature of about 6600 Kelvin that may or may not be a close double star. Variations in the spectrum were interpreted as being caused by orbital motion of a binary with a period of 2.16 days, a claim later debunked as false, the variations coming from some intrinsic variability. If it is a single star, it radiates at a rate 20 times that of the Sun, the mass 1.8 solar. If double, the mass is less, and the two (if equal) are closer to being subgiants of 1.6 solar masses. Position and transition to the other side of the Equinox aside, if nothing else, Omega Psc illustrates that even modestly-bright naked eye stars are not necessarily all that well understood.
Written by Jim Kaler. Return to STARS.