56 PEG (56 Pegasi). Just barely fifth magnitude (4.74, almost fourth), 56 Peg (commonly designated by its Flamsteed number), lies practically on the outline of the Great Square of Pegasus about 2.5 degrees due south of Beta Peg (Scheat). At first it looks like just another class G or K helium-fusing giant that so abound (and make much of our constellations), though one a bit brighter than most. Originally classed as G8 lesser supergiant, it's also been called a K1 subgiant. We'll go here with the last one, though it is of little consequence. Common as it appears, the star harbors a secret, though one not all that rare. At a healthy distance of 592 light years (give or take 25), assuming there is no interstellar dimming (of which there is some evidence), and adopting a temperature of 4509 Kelvin (the average of several measures) from which we can assess the amount of infrared radiation), we come up with a luminosity of 570 times that of the Sun (less than we'd expect for a supergiant), a radius of 39 times solar, and a mass of 4 Suns, putting it smack into the so-called "clump" of helium-fusing giants with similar properties though of differing masses. A maximum interstellar dimming of 1.25 magnitudes raises the luminosity to 1800 Suns, the radius to 70 times solar, and the mass to 6 Suns. It's really just a good old K1 or so giant with a rotational period of a year of less. An interferometric measure of angular diameter gives 46 solar radii, not that far from the mean of the dimmed-undimmed values and about half the size of Mercury's orbit. At least we are more or less on the right track. 56 Peg is special because it's a "barium star" with an elevated abundance of barium as well high levels of other elements produced by the slow capture of neutrons. Such abundances cannot be innate to a simple giant, so it must have been contaminated by enriched gas flowing to it from a companion that was once a highly advanced giant with a dead helium core that is now lost on the glare of 56. Sure enough, 56 Peg is orbited by a white dwarf with a temperature of 32,000 Kelvin that makes the star shine brilliantly in the ultraviolet where it is easily detected. Doppler shifts in the spectrum of 56 proper reveal an orbital period of 111 days, which is next-to-shortest for barium-star white dwarfs after 80-day HD 77247. To have already evolved to a white dwarf, the smaller star must once have been the more massive of the two. Not too long ago, the current giant must have been surrounded by a planetary nebula, the older star's ejecta illuminated by what is now the white dwarf. The lost mass, rich in new chemical elements will eventually find its way into another generation of stars. (Thanks to R. F. Griffin in "The Observatory," February 2006.)
Written byJim Kaler 12/30/16. Return to STARS.