SKAT (Delta Aquarii). Some stars are just not what they at first appear to be, and are a bit of a surprise when you dig into them. Rather well to the south (15 degrees or so) of Aquarius's Water Jar, the proper name "Skat" comes to us from an Arabic word that means "The Shin," a clear reference to the star's position within its constellation. Third brightest in this relatively dim figure (following third magnitude Sadalmelik and Sadalsuud, the Alpha and Beta stars), Skat (better known by its Greek letter name of Delta Aquarii), shines from a distance of 161 light years still at third magnitude, though at 3.27 just a bit fainter than those two. It's usually called a class A (A3) dwarf, though an alternative is an A2-4 giant. As we will see below, both seem to be wrong, though an average of sorts may be right on the mark. Other than that, and its relative brightness and prominence in its constellation, it has, as they say, few distinguishing characteristics. There seem to be no companions. The data from Hipparcos (the parallax satellite) suggest a binary, but the orbit makes no physical sense. Nor does sophisticated interferometry reveal any gravitional neighbors. There is also no evidence of any surrounding disk as there are for so many class A stars (for example Vega and Fomalhaut), which suggests that there is no planetary system either. That lack may go along with a modestly low heavy element content (70 percent solar iron, 40 percent solar oxygen), as stars with planets tend to have more heavy stuff relative to that found in the Sun. What we do have is a star on the verge of seriously aging. A distance and a temperature of 8525 Kelvin lead to a luminosity of 95 solar and a radius 4.5 times that of the Sun. A fairly fast projected equatorial spin speed of 76 kilometers per second puts the rotation period at under 3.0 days. Theory then shows the star to be right on the verge of shutting down its core hydrogen fusion, if it has not done so already, the mass falling between 2.5 and 2.7 Suns, depending on the exact state, the age between 500 and 600 million years. From theory, Delta Aqr is quite clearly physically a "subgiant," a star just starting on its trek to becoming a true giant. Too bad we can't actually watch it happen. By the time the star's collapsing core hits the temperature at which it begins to fuse core helium into carbon and oxygen, it will have tripled its total luminosity (most of which will be in the infrared).
Written by Jim Kaler 10/30/09. Return to STARS.