EPS SCT (Epsilon Scuti). Between Aquila and Sagittarius lies
the modern constellation Scutum (the
Shield), which is renown for an especially bright patch in the
Milky Way called the Scutum star cloud
whose brightest part is bordered rather raggedly at the northern end by Beta Scuti and on the southern side by
fifth magnitude (4.90) Epsilon Scuti. Oddly, Epsilon and Beta Sct
share a property: both are relatively rare class G "bright giants,"
a type of star intermediate between ordinary giants and supergiants, Beta subclass G5,
Epsilon a G8, which, at a temperature of 4920 Kelvin, is about that
expected. An additional claim to fame (such as it is) is that
along with Alpha, Beta, Delta, and Eta Sct (restricting the list to
Greek letter names), Epsilon lies within "The
Box," a remarkable 6 X 8 (EW) degree rectangle defined by Lambda Aquilae near its northeast
corner that contains six open
clusters named in the New General Catalogue (NGC), four
variable stars (including Delta and R Scuti), two planetary
nebulae, and one NGC globular cluster. As augured
by its spectral class (as a G8 bright giant), Epsilon itself is no
weakling. After accounting for a distance of 538 light years (give
or take 57), temperature (to assess the amount of infrared light), and not quite
half a magnitude of dimming by interstellar dust, we find a
luminosity of 460 times that of the Sun,
which gives a radius of 30 Suns, 35 percent the size of Mercury's
orbit. A projected equatorial rotation speed of not quite six
kilometers per second gives a rotation period that could be as long
as 255 days. Theory then tells that Eps Sct carries a substantial
mass of 4 times that of the Sun. Some 180 million years ago, the
star began life as a blue-white class B6 hydrogen-fusing dwarf.
That phase ended 15 million years ago when the core hydrogen ran
out, leaving Epsilon now as an ordinary, though massive, helium-
fusing bright giant, the helium fusing to a mix of carbon and
oxygen. After the helium runs out, the star will brighten and then
die as a fairly massive (about 0.8 solar) white dwarf. The star is
surrounded by a rather large number of faint "companions" that range from Eps
Sct B through "F." "D" and "E," at 15 and 43 seconds separation,
have but one measure and we can for now reject them. "B" is moving
far too rapidly (from 20 to 14 seconds separation in 24 years) to
be anything other than in the line-of-sight. The closest to
belonging are Eps C and F. If 14th magnitude Eps Sct C, 39 seconds
of arc distant, were "real," it would be a K6 dwarf with a minimum
6500-AU-wide, 150,000-year orbit. But the separation increase can
be nicely accounted for by Epsilon A's motion across the line of
sight. The same can be said for F, leaving no other option but to
think the star is in fact all alone.
Written by Jim Kaler 9/16/11. Return to STARS.