XY LYR (XY Lyrae). Here is an ignored star whose principal claim to any sort of attention (and it is a good one) involves more its position rather than anything else. Just a degree north of Vega in the northern constellation Lyra and a degree west of the famed "double- double" Epsilon Lyrae (the three making a pretty good right triangle), XY allows one to locate and admire a fine example of a mid-class M (M4-5) bright giant (some say M4 supergiant) and irregular variable star (as indicated by its Roman-letter name). Hovering at sixth magnitude, XY varies (according to one source) between magnitudes 5.7 and 6.6, which takes it out of naked eye visibility. Further study shows some regularity, but there is no actual known period. The star's placement and general neglect make it a target for observation by amateur astronomers, the variation rather easily seen through comparison with its surroundings. XY's faintness is the result of its quite-large and not very precise distance of 1700 light years, known to an accuracy of about 300 light years. Beyond that, the parameters are pretty much up for grabs. The maximum temperature seems to be around 3350 Kelvin. Most of the stellar radiation then comes out in the invisible infrared, allowance for which yields a luminosity of just under 17,000 times that of the Sun. The radius (which is variable in an unknown way) then comes in at 380 times that of the Sun, or 1.8 Astronomical Units. Direct measure of angular diameter with an interferometer (which makes use of the interfering properties of light coming from different parts of the star) working in the infrared combined with distance gives the same value, so at least we seem to know more about the star than we at first suspected. Put it in place of our Sun and XY Lyr would extend past the orbit of Mars and into the inner edge of the asteroid belt. Adoption of a cooler temperature could easily double the luminosity and send the radius of the star out to as much as three AU. The most interesting property of XY Lyrae, though, its mass, is indeterminate but probably reasonably high, though likely below the limit at which stars explode as supernovae. After shedding its outer layers, which it is probably doing now, it will die as a massive white dwarf. If nothing else, XY Lyr shows the mystery that is still attached to so many of the stars of the cosmos.
Written by Jim Kaler 8/24/12. Return to STARS.