29 CYG (29 Cygni). Good things in small packages have nothing on interesting things in faint stars seemingly lost in the Milky Way. In this case it is near-dead-on fifth magnitude (4.97) 29 Cygni, which lies in an especially rich portion of Cygnus's Milky Way not quite four degrees southwest of Sadr (Gamma Cyg) and just over a degree south of the luminous blue variable P Cygni.
29 Cyg This wonderfully-rich segment of the Milky Way in Cygnus (roughly 3.5 degrees wide) holds a number of treasures that start with Sadr (Gamma Cygni), then down and to the left the luminous blue variable P Cygni, and at bottom center, the Lambda Bootis/Delta Scuti star 29 Cygni. Reddish interstellar clouds flood the image. The compact open cluster Messier 29 lies near the left edge.
Though seemingly an ordinary class A2 dwarf, Flamsteed's 29 Cyg has a pair of features to offer, a weird chemical composition and subtle variability. Odd chemistry can make for mis-classification. The temperature of the star, 7920 Kelvin, is way too low for its purported class, and is more appropriate to A7 (A2 more like 9200 Kelvin). At a distance of 139 light years, the star radiates at a rate only 14.5 times that of the Sun, analysis of subtle jigglings in the brightness (through the science of "asteroseismology") suggesting a very satisfying 13.2 solar. Luminosity and temperature then combine to give a radius of almost exactly double that of the Sun. The measures of rotation velocity are in rough shape, values given ranging from 37 kilometers per second to over 150. The best estimate is probably 65 km/s, which gives a rotation period under 1.5 days. Theory then tells of a mass of 1.8 times that of the Sun and that 29 Cyg is about a third to a half way through its 1.5 billion year dwarf (hydrogen-fusing) lifetime. Now to the heart of it: 29 Cygni is a rare "Lambda Bootis" star, one that has very low metals (an iron content just 6 percent solar, magnesium down to four percent) but with more normal carbon and oxygen. There is no firm theory for such stars. The usually accepted concept is that their surfaces have been affected and contaminated by accretion of gas from metal-poor surrounding interstellar clouds. These metals had previously been absorbed into the clouds' dust grains, but the dust could not fall onto the star compliments of its outgoing radiation, just the metal-depleted gas. Or not. Nobody really knows. Making 29 Cyg even rarer, it's a "Delta Scuti" star. Such stars fall in an unstable temperature region in which they subtly but rapidly vary with multiple periods (the region among dwarfs analogous to that which produces the supergiant Cepheid variables). Our star here has a major period in which it varies by 0.016 magnitudes (about 1.6 percent) over a 38.5 minute period. At least a dozen other periods are known. Off in the distance lie three so-called companions, which are probably just in the line of sight. The major one, seventh magnitude 29 Cyg B over three minutes of arc away, would have to be a mid-class F star, and if really part of a double star would be at least 9000 Astronomical Units away and orbit with a period of at least half a million years, making it a "fragile binary" indeed.
Written by Jim Kaler 11/6/09. Return to STARS.