At magnitude 2.20, Sadr (or Gamma Cygni) lies prominently at the center of Cygnus's famed asterism, the Northern Cross. The name Sadr refers to the great celestial Swan, and comes from an Arabic phrase that means "the hen's breast."
Sadr lies in a magnificent portion of the Milky Way along Cygnus' long axis. It is at the northern end of the Great Rift, a dark lane that appears to divide the Milky Way in two, and extends down through Sagittarius and Scorpius. The Rift actually consists of a huge complex of fairly nearby dust clouds in which stars are being born.
Sadr is surrounded by a diffuse nebula called IC 1318, or the Gamma Cygni nebula, although this is not connected to it.
Physical Properties
The star itself is a fairly unusual class F8 supergiant. Most of supergiants are either fairly hot and blue-white, or quite cool and reddish. Few, like Sadr, are yellow-white, in the mid-temperature range, near 6500 K - not much hotter than the Sun.
Truly luminous as befits a supergiant, the star is around 65,000 times brighter than the Sun. It does not dominate its part of the sky only because of its rather large distance of 1500 light years; its light is also dimmed by nearly half a magnitude by interstellar dust absorption.
Evolution and Variability
The star is in the process of dying, having ceased fusing hydrogen in its deep core. But it is not possible to know just what state it is in, and whether it will heat or cool at its surface. From its current brightness and temperature, the star's original mass must have been around a dozen times the Sun's, close to the limit at which stars are believed explode as supernovae. However, Sadr's distance is not all that well known, and Sadr might be fainter and less massive than supposed.
Sadr's temperature and luminosity are close to a regime in which stars become unstable and pulsate. Though it is not obviously variable, Sadr does appear to pulsate somewhat in a complex way with a 74 day period.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]