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.