RHO PEG (Rho Pegasi). Less than a degree north of the border with Pisces, northwest of the constellation's famed Circlet, lies a rather special star, Rho Pegasi (in Pegasus, Perseus's steed that he rode to rescue Andromeda). A fifth magnitude (4.90) class A (A1) dwarf, after allowance for some ultraviolet light from a surface heated to 9586 Kelvin, the star shines with the light of 92 Suns, from which (with temperature) we derive a radius 3.5 times the solar value. It's exactly the radius derived from the estimated angular diameter that is used as a calibrator to determine stellar dimensions. Spinning rather quickly, with a projected equatorial velocity of 102 kilometers per second, Rho Peg rotates with a period of under 1.7 days, which is nothing unusual for its class. The high speed is consistent with a lack of spectral peculiarities that would suggest odd abundance anomalies caused by separation of elements that would take place in the atmosphere of a quiet, barely-rotating, star. Theory applied to luminosity and temperature tell of a star that carries a mass 2.7 times that of the Sun and that is perhaps three-fourths or so of the way through its hydrogen-fusing lifetime. With a speed of 40 kilometers per second relative to the Sun, it's moving some 2.5 times faster than average, but that is nothing terribly unusual either. So what makes it so special? It's not companionship, as in the case for nearby Sigma Pegasi, which lies just a degree to the northwest, as Rho is decidedly single. By astronomical definition, Rho Pegasi is pure white. Standard stellar magnitudes are as seen in the visual realm of the spectrum. Appropriate to the yellow part of the spectrum, where the eye is most sensitive, they are formally known as "V" magnitudes. The use of photography in the nineteenth century defined a "photographic magnitude" as would be seen by eyes sensitive to blue light (to which early emulsions were sensitive). In the electronic era, they became known as "blue" or "B" magnitudes. Moving deeper into the spectrum we can also define "ultraviolet," or "U," magnitudes. The long-used UBV system of magnitudes has been supplemented, if not replaced, by vast numbers of other systems that define not just ultraviolet but red and infrared magnitudes. Rho Pegasi is one of the few stars for which all three magnitudes, U, B, and V, are the same. That is, the magnitude differences (B-V and U-B), which are the defined as the "colors" of the star, are all zero. Formal star colors are often used as proxies for spectral class and are of immense importance in calculating the amount of obscuring dust that might lie in the line of sight, since as stars are dimmed they are also proportionately reddened. The trick is to find the proportion, the search for which has the reputation of driving astronomers mad.
Written byJim Kaler 10/23/15. Return to STARS.