DELTA SCT (Delta Scuti). Stars look so rock-solid in the nighttime
sky, the constellations they make wheeling perpetually overhead the
same ones Homer saw when he composed the Iliad and Odyssey. Closer
look, however, shows that a great many of them vary noticeably in
brightness, the classic cases Mira and its
kind and the Cepheids, epitomized by Delta
Cephei, Mekbuda, and Eta Aquilae. Many, many are the other kinds
of variables, some so subtle as to be missed by the eye alone.
Modest Delta Scuti (in Scutum, the Shield),
shining only at 5th magnitude (4.71) in a
relatively obscure modern constellation best known for its
placement in the Milky Way, stands out as the prototype of one of
these, the pulsating "Delta Scuti stars," of which Caph in Cassiopeia is
the brightest. Delta Scuti, a class F (F2) peculiar giant, has no
proper name. It was, however, still being placed among the stars
of Aquila when the so-called "Flamsteed numbers" were
assigned, and is therefore also of all things "2 Aquilae," a name
no longer in use. Its distance of 187 light years and surface
temperature of 6860 Kelvin tells of a luminosity 33 times that of
the Sun. These results give this metal-rich star (its iron content
3 times solar) a mass 2.2 to 2.4 times that of the Sun, the radius 4.1 or so solar. Its minimum
rotation speed of 32 kilometers per second give it a rotation
period of less than 6.4 days. Delta Scuti has stopped fusing
hydrogen in its core, and will shortly be on its way to becoming a
true, much larger red giant. The variations are small, only 0.2 or
so magnitudes (about 20 %), just above the limit of 0.1 magnitudes
that can be recorded by eye. It is really a lower-mass version of
the Cepheid variables. However, unlike most of these, Delta and
its kind have multiple pulsation periods. Delta's chief period is
4.65 hours, while the secondary is 4.48 hours. To these are added
periods of 2.79 hours, 2.28 hours, 2.89 hours, and
20.11 hours. All of these "beat" against one another to produce a
very complex pattern of variation (you can hear such "beats" when
two slightly out of tune guitar strings are played together.)
Delta has two companions. The twelfth magnitude (12.2) "B"
component of the system (Delta proper the "A") lies 15.2 seconds of
arc away from "A", the ninth magnitude (9.2) "C" component 52.2
seconds away. Delta-B was at one time thought to be simply a line-
of-sight coincidence, but the two maintained the same separation
over a couple decades, and are probably really tied together. From
their brightnesses, Delta-B must be a class K8 star, Delta-C class
G7, not much less massive than the Sun. B orbits at a minimum
distance of 870 Astronomical Units, and must take at least 15,000
years to revolve, while "C" is at least 3000 AU out and must take
over 85,000 years to make a circuit. At those separations, from A
the two stars would shine 30 some times brighter than our Venus,
while from "B", "A" would look 42 times brighter than our full Moon
and from "C" 3.5 times brighter.
Update: The new Hipparcos reduction gives a somewhat higher
distance of 202 light years (with an uncertainty of just 4), which
brings the luminosity up to 39 Suns. From theory, the mass lies
between about 2 and 2.2 solar (depending on the exact state of
evolution), the star best described as a subgiant near or at the
end of its hydrogen-fusing lifetime.
Written by Jim Kaler 9/05/03; revised 6/29/07;
updated 4/29/11. Return to STARS.