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.