RHO AQL (Rho Aquilae). A rare star indeed. Not for physical reasons, but for its position. In older times, celestial map-makers drew arbitrary boundaries around their depictions of constellations, and everyone had a somewhat different point of view. The situation was made worse by a proliferation of modern constellations, not all of which were accepted by everyone else. Finally, in 1930, all was put right when Eugene Delporte, working with the International Astronomical Union, drew definitive rectangular boundaries, the IAU adopting an official set of 88 constellations. But since stars had already been named using older boundaries, a few were orphaned into the wrong constellations. Eta Aquilae, for example, is in modern Scutum, 30 Serpentis is in Libra, and so on. None of this involves any real confusion. Rho Aquilae was properly in Aquila, across the border from Delphinus. But all stars move. Orbiting the center of the Galaxy, all on slightly different (even wildly different) paths, they move relative to each other. Over millions of years the constellations fall apart to create new ones. There are two aspects to stellar motions, ACROSS the line of sight (the "proper motion") and along it (along which we observe the "radial velocity" from the Doppler shift). Put the two together, and we know the complete motion of a star relative to the Sun. Get data on a lot of stars and we know how the Sun is moving relative to the local swarm (the "apex of the Sun's way" toward Hercules and Lyra). Rho Aquilae has a rather ordinary proper motion of 55 thousandths of a second of arc per year to the east. It was, however, positioned just to the west of the boundary with Delphinus. In 1999, it crossed it. Now in Delphinus, the star orphaned itself! Not that that will change the name of the star. Calling it Rho Delphini (which does not exist) would just add to the confusion, so Rho Aql it remains. Physically, Rho Aql now of Delphinus is a rather ordinary class A (A2) fifth magnitude (4.95) dwarf with a temperature of 8870 Kelvin. Positioned in the Milky Way at a distance of 150 light years, it suffers from a 15 percent dimming by interstellar dust. When all this information is taken into account, the star shines with the light of 21 Suns, from which we deduce a radius of 1.95 solar, and (from a very fast equatorial rotation velocity of 165 kilometers per second) a rotation period of under 14 hours. The fast motion stirs up the stellar gases, preventing weird abundance anomalies often seen in this class of star that are caused by gravitational settling and radiative levetaion of different elements. The theory of stellar structure then leads to a mass 2.1 times that of the Sun, and shows the star to be not just a dwarf, but a very young one with an age between 50 and 165 million years, far short of its projected hydrogen-fusing lifetime of a billion years. There is some suggestion from the spectrum of a companion, but it's never been confirmed. Like a number of other stars of its class (Vega, Fomalhaut), Rho Aql is surrounded by a (probable) disk of dust, that may (like Fomalhaut) contain a buried planet, though none has ever been detected. (Thanks to Joseph Jarrell for suggesting this star.)
Written by Jim Kaler 7/10/09. Return to STARS.