SADR (Gamma Cygni). Pronounced more like "sudder" or "sadder," the star is nowhere near as obscure as its proper name. Mid-second magnitude (2.20), Sadr (though second brightest) is the Gamma star of the constellation Cygnus, the Swan, and lies prominently at the center of Cygnus's famed asterism, the Northern Cross, at the crossing point of the staff and bar. The name, however, refers to the great ancient celestial bird, and comes from an Arabic phrase that means "the hen's breast." Sadr lies in a magnificent portion of the Milky Way as it runs along Cygnus' long axis. It is near the northern end of the famed Great Rift, a dark lane that appears to divide the Milky Way -- the disk of our Galaxy -- in two and extends down through Sagittarius and Scorpius. The Rift actually consists of a huge complex of fairly nearby cold dust clouds in which stars are being born. More detailed study of the region around Sadr show it to be filled with luminous interstellar clouds (similar to the Orion Nebula) as well as the remnant from an exploded star, none of which is directly connected with Sadr itself, though were the line of sight clear, Sadr, obscured by interstellar dust, would be half a magnitude brighter. (The region, bright in the radio spectrum, was long ago named "Cygnus X," not to be confused with " Cygnus X-1," which is a binary star that contains a black hole. Historically derived, astronomical nomenclature can be amusingly confusing.)

Sadr Sadr (at center), in a complex region of the Milky Way, is surrounded by reddish interstellar clouds that are excited to glow by the ultraviolet light of hot stars, IC 1318 up and to the right. Just up and a bit to the left of Sadr is the open cluster NGC 6910, while just to the left of center near the lower edge lies the better known cluster Messier 29. P Cygni is at the lower right edge. See the labelled picture for identifications.

Even within its remarkable surroundings, our star stands out as a fairly unusual class F (F8) supergiant. Most supergiants are either fairly hot or quite cool and reddish. Few, like Sadr, are yellow-white and in the mid-temperature range near 5800 Kelvin, close to that of the Sun (Sadr at an unsure 5870 K) such that most of the radiation pours out in the optical spectrum. From a healthy distance of 1830 light years, give or take a large uncertainty of 280 (second Hipparcos reduction), we find the star to shine with a total luminosity of around 60,000 Suns. Temperature and luminosity then lead to a large radius of 235 times solar, 10 percent bigger than Earth's orbit. However, direct measures of angular diameter through interferometry yield a radius of 183 Suns, 30 percent smaller, showing that something may be wrong with some of the input parameters, or perhaps pointing out that the meaning of a "surface" to such a distended star is ambiguous. Though Gamma Cyg is in the process of dying, having ceased hydrogen fusion in its deep core, it is not possible to know just what state the star is in, whether it will heat or cool at its surface or whether helium fusion will occur or has already taking place, any course taking longer than any human will see. From its current brightness and temperature, the original mass of the star at birth must have been around 14-16 times that of the Sun, above the limit at which stars should explode as supernovae. Though Sadr is not obviously variable, the star does appear to pulsate somewhat in a complex way with a 74 day period. At a separation of somewhat over two minutes of arc lies a ninth magnitude "companion," itself a double star, Gamma Cyg BC, which has as third member 41 seconds of arc away. The only connection though is in the name, as BC is moving relative to Sadr proper to be no more than another line of sight coincidence. (Thanks to Dean Moberg for pointing me back to this star.)
Written by Jim Kaler 9/4/98; revised 11/30/12. Return to STARS.