OMI AQL (Omicron Aquilae). Within the Milky Way, fifth magnitude (5.11) Omicron Aquilae sits in Aquila 1.5 degrees almost due north of first magnitude Altair, making it quite easy to find. A class F (F8) dwarf, Omi Aql is almost sunlike, though with some notable exceptions, one of major importance. The distance of 62.6 light years is known to remarkable accuracy, within just 0.4 light years. Though in the Galactic disk, the star's closeness precludes significant dimming by interstellar dust, and a well-determined temperature of 6125 Kelvin places almost all the radiation in the visible band of the electromagnetic spectrum. From these considerations, we get a total luminosity of 2.73 times that of the Sun and a radius of 1.47 times solar. Oddly, there is no measure of rotation speed. Theory then gives a mass of 1.2 to 1.25 Suns. With an age of 3 billion years, the star has less than a billion to go before it gives up core hydrogen fusion and turns into a subgiant on its way to gianthood.

There's nothing much unusual here, so why bother with it? First, Omi Aql is not metal-poor (a common condition), but metal-rich, with an iron-to-hydrogen ratio 25 percent greater than the Sun's, suggesting that it came from somewhere in the Galactic interior where more metals are available to birth clouds thanks to a denser stellar population and more supernovae. Yet the star's velocity of 25 kilometers per second is not all that high, just double normal. Metal-rich stars are breeding grounds for planets, but Omicron Aquilae does not even seem to have a debris disk around it that would at least be evidence for a planetary system (none at this time obviously detected or inferred). It's also quintuple. OK, not quite. Omi Aquilae C, D, and E, respectively of 14th, 13th, and 12th magnitudes, are separated from bright "A" by 22, 50, and 82 seconds of arc. Alas, their motions relative to Omi Aql A over only short timelines reveal all to be just in the line of sight and not actual companions. But then there is Omi Aql B. Now 19.5 seconds away from "A," the separation has changed by only a second of arc over the past century even though the AB pair has shifted relative to its surroundings by nearly a full half minute of arc, some 30 times the stars' separation. Moreover, Omi B's absolute brightness is consistent with it being classified as an M3 red dwarf, so the two stars are almost certainly partners. If so, the companion is at least 375 AU away from its much brighter mate, and would take 6000 years or more to make a full circuit. Assuming the above distance, from Omicron A, Omi B would appear five times brighter than our Venus, while from Omi B, Omi A would be would shine as a point of light glowing with 10 full Moons. By far the most absorbing feature of Omicron Aquilae is that in 1979-80 it burst forth with two superflares in which it brightened by 0.09 magnitudes, nearly nine percent! The first flare could have lasted up to five days, while the second stayed up for nearly two weeks. Red dwarfs are supposed to behave this way, not those more or less like the Sun. Omi Aql thus joins an exclusive club that includes 5 Serpentis and Groombridge 1830 (HR 4550). Reasoning from ordinary solar and red dwarf flares, imagine what the X-ray intensity must have been. Imagine too what it would be like if the Sun were to join the group. Why such stars go off this way is a mystery, as is how frequently such flares go off. Without a planet, or so it appears anyway, there is nobody there to report back, which is probably a good thing, as it sure would be a dangerous place to be.

Written byJim Kaler 8/08/14. Return to STARS.