R SCT (R Scuti). R Scuti, the first variable found in the constellation Scutum, the Shield (variables named with single or double Roman letters), holds personal memories from thy writer's youth. While roaming the Milky Way with binoculars, I came upon a prominent oval of stars that lies at the boundary of northern Scutum and southern Aquila, one member of which was this wonderful reddish (to my eyes) star. Over the years, sometimes it was quite prominent, other times almost not there. Nearby lies one of the most compact of all open clusters, Messier 11, all near Scutum's amazingly bright star-clouds. Varying from an easily visible fourth magnitude (4.45) to eighth (8.20), this class K (K0) supergiant averages a naked-eye visual magnitude of 5.2.
The light curve of R Scuti reveals a 71 day period between alternating deep and shallow minima, the deep minima taking place every 142 days. The behavior sometimes goes away, but inevitably returns. The Julian Date is a running count of days since January 1, 4713 BC, of the Julian Calendar. JD 2450000 corresponds to October 9, 1995. Janet Mattei (American Association of Variable Star Observers), from an article by M. Matsuura et al. in Astronomy and Astrophysics
R Scuti is a member of the still-mysterious class of "RV Tauri stars," which (rather like Delta Cephei and Mekbuda) are pulsating variables, but are characterized by alternating deep and shallow minima with periods in the upper 10s of days. Unlike Cepheids, they have low masses (more solar) in spite of their distended natures. Their evolutionary status is unclear, but the feeling is that they are -- like 89 Herculis -- really done with their giant or supergiant behavior, and with dead carbon-oxygen cores are in a state of transition in which they will slough off their outer envelopes, expose their cores, possibly turn into planetary nebulae (in which the cores illuminate the fleeing envelopes), and become white dwarfs. Or maybe not. At a rather great distance of 1400 light years, with a temperature that goes from 4750 to 5250 Kelvin during the variation cycle, R Scuti can drop to class M (M3). The period between R Scuti's faint brightness minima is a rather steady 142 days. In between the deep minima is a "harmonic" brighter minimum that gives an actual 71 day cycle in the variation. Over longer intervals, this alternating behavior disappears, only to return. Application of the distance to the average visual magnitude coupled with uncertain corrections for infrared radiation and for absorption of starlight by intervening interstellar dust (which dims the star by about a magnitude) give a luminosity between 4000 and 5000 times that of the Sun. With a diameter at least 85 times solar (about the size of Mercury's orbit), the star is surrounded by some kind of extended atmosphere rich in water vapor, and an extended dust shell thousands of times bigger than the star itself, though the current mass loss rate is not very high. Contrary to general opinion about these stars, R Scuti may yet be a supergiant in the act of its development, and in a state more like Mira, in which a carbon core is surrounded by a shell of helium fusing to carbon that is in turn surrounded by a shell of hydrogen fusing to helium, the two alternating activity, the star currently in its helium-fusing state. R Scuti, bright and beautiful, remains a puzzle.

Update: The new Hipparcos reduction gives a much smaller distance of 870 light years (give or take perhaps 20 percent), which reduces the already uncertain luminosity to 1500-2000 Suns, the radius to around 60 solar radii (0.25 Astronomical Units) or less. Coupled with theory, these parameters give a mass of around 6 times that of the Sun.
Written by Jim Kaler 10/15/04; updated 4/29/11. Return to STARS.