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.