NU AQL (Nu Aquilae). Just a bit more than ten degrees southwest of Altair in Aquila (and about 2 3/4 degrees south of Delta Aql) lies an anonymous-looking star, fifth magnitude (4.66, almost fourth) Nu Aquilae, that holds some surprises. First, it's an "equator star" located only a third or so of a degree north of the celestial equator. Precession, the 26,000-year wobble of the Earth's rotational axis, is at this location moving the equator south, so the star appears to be shifting to the north. Looking back, around 1830 Nu Aql crossed the divide from the southern celestial hemisphere into the northern. Second, it's pretty far away, some 2800 light years, give or take almost 700. In the Milky Way, it is thus subject to considerable dimming by interstellar dust. Were the pathway clear, Nu Aql would shine a full magnitude brighter. Third, and most important, it's a rare class F (F2) supergiant (albeit of the lesser variety). Factoring in distance, interstellar absorption, and the 6805 Kelvin surface temperature (which shows that most of the radiation is in the visual spectrum), Nu Aql is then seen to shine with the brilliance of 21,000 Suns. With temperature, the luminosity yields a radius of 104 solar radii, 0.48 Astronomical Units, 25 percent bigger than Merury's orbit. From a projected equatorial rotation velocity of 12 kilometers per second, the rotation period could be as long as 1.2 years. Most impressive is the mass, found through the theory of stellar struture and evolution. If the star is still swelling with a dead and contracting helium core, the mass comes in at around a dozen solar, while if it is already a helium burner (the core fusing to a mix of carbon and oxygen), which seems the more likely, the figure is more like 10 solar, which yields an age of some 20 million years (the star having begun life as a hot blue class B dwarf). It's pretty much on the lower border of stars that could explode to become supernovae. If it did tonight (very unlikely), it would increase its brightness by perhaps 50,000 times to become the brightest light in the sky after the Sun and Moon. If below the controversial limit of 8 to 10 Suns, Nu Aql would, after sloughing off its outer layers to near- expose the core, would turn into a massive white dwarf, perhaps one of the rare neon-oxygen variety. Lastly, just over three minutes of arc away lies a ninth-tenth (9.5) magnitude class A1 "neighbor." Its motion over the past 120 years suggests that it is just in the line of sight. If it were physically related, from its brightness it would be a mid-B dwarf of perhaps 3 solar masses, orbit at a distance of at least 175,000 AU, and take at least 20 million years to make a circuit, which makes real companionship all that more unlikely. Best to just take the star itself for what it is, a real celestial rarity, though given the inherent brilliance of such stars, it may not seem that way, as their visibility is all out of proportion to their real numbers.

Written by Jim Kaler 10/05/12. Return to STARS.