The Sadr Region

At the heart of Cygnus, around the brilliant star Sadr that marks the centre of the Northern Cross, lies one of the largest star-forming regions of the northern sky. The red glow filling the entire field is ionized hydrogen: the nebula IC 1318, also known as the Gamma Cygni Nebula. Embedded within it shines a small, tight group of blue stars, the open cluster NGC 6910.

NGC 6910 is a young cluster, only a few million years old, and the core of the stellar association Cygnus OB9. Its hottest, most massive blue members are among the reference objects for the study of Beta Cephei pulsating stars, whose surfaces swell and contract over just a few hours. Its exact age is still debated: depending on the method used, astronomers find between 4 and 11 million years.

The red glow of IC 1318 is not lit by Sadr, despite what its name suggests, but by young, hot O-type stars largely hidden by dust. Their ultraviolet radiation ionizes the gas and makes it shine. The broad dark bands that carve the nebula into lobes are not empty gaps: they are foreground clouds of dust, the most prominent catalogued as LDN 889, absorbing the light of the gas behind them.

Sadr, the brightest star in the image, is in fact an interloper: this supergiant lies about 1,800 light-years away, far closer than the nebula and cluster, which sit around 5,600 light-years away, behind the Great Cygnus Rift. The three line up by mere perspective. This image gathers 10 h 23 min of exposure from Texas, through a dual narrowband filter (ionized hydrogen and oxygen).