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.