4 AQL (4 Aquilae). Shifting constellation
boundaries, not to mention "new" (now several hundred years old)
constellations, can play havoc with star
names. The most westerly Flamsteed star in Aquila, the Eagle, is fifth magnitude
(5.02) 4 Aquilae. Whatever happened to 1, 2, and 3 Aquilae? They
are to be found in the modern constellation of Scutum (invented by Hevelius in 1684), the Shield, just
off the Eagle's southwestern edge, which was ignored by Flamsteed.
The trio later became Alpha, Delta, and Epsilon
Scuti. At a healthy distance of 465 light years (give or take
15), 4 Aql nicely stands out in its own right not just as a class
B (B9) dwarf, but also as a
"B-emission" ("Be") star that is surrounded by a radiating disk.
Classic examples are Gamma Cassiopiae
and Zeta Tauri. Four Aql's disk seems
to be set more or less on edge, making it into a "shell star" (as
the light from the star must shine through the disk, which acts as
a sort-of "shell"). Though set into the Milky
Way, there is no evidence for any dimming by interstellar dust. With a
temperature of 10,970 Kelvin (needed to account for
]spectra.html#emspectrum">ultraviolet light), 4 Aquilae
radiates at a rate of 242 times that of the Sun, which with temperature gives a stellar
radius of 4.3 times solar. As is the case for all Be stars, 4
Aquilae is a fast rotator, spinning with an equatorial speed of at
least 265 kilometers per second (with a large uncertainty), which
gives it a rotation period of under 0.8 days (the link between the
disk and rotation not entirely clear). The theory of stellar
structure and evolution yields a mass of 3.4 times that of the Sun,
and shows the star to be nearing the end of its dwarf lifetime of
some 270 million years. It will eventually slough off its outer
layers and die as a white
dwarf with a mass of about three-fourths that of the Sun. The
Flamsteed numbers then spread across the constellation toward the
east, ending in 71 Aquilae, 6 and 9 Aquilae renamed as Beta and Eta Scuti. (Thanks to Morton
Wagman in his "Lost Stars").
Written by Jim Kaler 9/20/13. Return to STARS.