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SKAT (Delta Aquarii). Some stars are just not what they at firstappear to be, and are a bit of a surprise when you dig into them. Rather well to the south (15 degrees or so) ofAquarius's Water Jar, the proper name "Skat" comes tous from an Arabic word that means "The Shin," a clear reference tothe star's position within itsconstellation. Third brightest in thisrelatively dim figure (following third magnitudeSadalmelik andSadalsuud, the Alpha and Beta stars),Skat (better known by itsGreek lettername of Delta Aquarii), shines from a distance of161 light years still at third magnitude, though at3.27 just a bit fainter than those two. It's usually called a class A (A3) dwarf,though an alternative is an A2-4giant. As we will see below,both seem to be wrong, though an average of sorts may be right onthe mark. Other than that, and its relative brightness andprominence in its constellation, it has, as they say, fewdistinguishing characteristics. There seem to be nocompanions. The data fromHipparcos (the parallax satellite) suggest a binary, but the orbitmakes no physical sense. Nor does sophisticated interferometryreveal any gravitional neighbors. There is also no evidence of anysurrounding disk as there are for so many class A stars (forexampleVega andFomalhaut), which suggests that there isnoplanetary system either. That lack may goalong with a modestly low heavy element content (70 percent solariron, 40 percent solar oxygen), as stars with planets tend to havemore heavy stuff relative to that found in the Sun. What we dohave is a star on the verge of seriously aging. A distance and atemperature of 8525 Kelvin lead to a luminosity of 95 solar and aradius 4.5 times that of theSun. A fairlyfast projected equatorial spin speed of 76 kilometers per secondputs the rotation period at under 3.0 days. Theory then shows thestar to be right on the verge of shutting down its core hydrogenfusion, if it has not done so already, the mass falling between 2.5and 2.7 Suns, depending on the exact state, the age between 500 and600 million years. From theory, Delta Aqr is quite clearlyphysically a "subgiant," a star just starting on its trek tobecoming a true giant. Too bad we can't actually watch it happen. By the time the star's collapsing core hits the temperature atwhich it begins to fuse core helium into carbon and oxygen, it willhave tripled its total luminosity (most of which will be in theinfrared). Written byJim Kaler 10/30/09. Return toSTARS.
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