PHI PEG (Phi Pegasi). Several are the classic stellar tests for minimal as well as excellent vision. The best known is certainly Mizar and Alcor in the Big Dipper, the Arabs' "Horse and Rider?" Most people can rather easily make them out. How many stars can you see in the Pleiades, the "Seven Sisters?" Most people see six (hence the "Lost Pleiad), but keen-eyed observers can make out seven, even eight or nine or more. A much tougher test is to try to split the "Double-Double," Epsilon Lyrae, whose stars (each of which is double yet) are about three and a half minutes of arc apart. Among the lesser-practiced tests is to see how many stars are visible inside the nearly blank Great Square of Pegasus. There is no agreed-upon or absolute value of course, but good eyes should at least see the four that carry Bayer Greek letters, as these came out of Tycho's visual observations. Here we find fourth magnitude Upsilon Peg (at 4.40 the brightest), fifth magnitude (4.60) Tau Peg, fifth magnitude Psi (4.66) and fifth magnitude (5.08) Phi. To these we might add 71 Peg (mag 5.32) and 56 Peg(5.38). Of great significance is HR 8799, which has an orbiting planets that can be seen with direct imaging, though at sixth magnitude (5.99) the star's a tough naked-eye find about halfway between Beta and Alpha Peg. Near it is fifth magnitude (5.49) 51 Peg with the first-known exoplanet, but it's just outside the square and doesn't count. As a separate but curious issue, Phi Peg is just east of the equinoctial colure, the great circle that connects the equinoxes and the celestial poles. Because of the 26,000-year precession of the Earth's axis, the equinoxes and solstices are moving east against the stars. Phi Peg will cross over around the year 3030. (It's never too soon to plan a party). Phi Peg is actually a reddish class M (M2.5) giant. (The square seems to attract them: so are Psi and 71 Peg.) Very little is known about the star, which lies at a distance of 463 light years (give or take 18). A temperature of 3400 Kelvin adopted from the spectral class allows an estimate of the cool star's copious infrared radiation, yielding a total luminosity of 1860 Suns, which in turn gives a radius of 125 times that of the Sun, 0.58 Astronomical Units, 1.5 times the radius of Mercury's orbit. Given all the uncertainties, the star probably carries a mass of around twice that of the Sun. Some 1.5 billion years old, the best guess is that it has run out of helium fuel and is brightening for the second time with a dead carbon core and will before long shed its outer layers, allowing the core to turn into a white dwarf. There seems to be no evidence for any sort of companion to watch the action, the star circling the Galaxy on its own.
Written byJim Kaler 12/02/16. Return to STARS.