Stars and Nebulae in the “Hen’s Chest”
Posted by Brian Ventrudo
As the Northern Cross rises in the late evening hours of July, look for the silver-white spray of the Great Cygnus Star Cloud running along the length of the Cross and into Aquila and Ophiuchus to the southeast. This is a splendid region of the Milky Way, with thousands of background stars visible in binoculars or a small telescope. Look also for the Great Rift, a complex of dark interstellar dust that splits the Cygnus Star Cloud down the middle.
At the northern end of the Great Rift and smack in the middle of the Northern Cross lies the star Sadr, which anchors a stunning region of stars and nebulosity, some of which is revealed to the patient stargazer with clear and dark sky.
Sadr and the rest of the Northern Cross are part of the constellation Cygnus, the Swan, but these stars were often linked to some type of bird by ancient stargazers. Sadr, in fact, takes its name from its Arabic for the “hen’s chest”. The star shines at a respectable magnitude 2.2, but it’s nearly 1,800 light years away, which means it’s intrinsically bright, about 30,000x brighter than the Sun, with a radius about 150x as large. The star is some 12 solar masses and has evolved off the main sequence to become a type-F8Iab supergiant with a surface temperature of about 6,100K. So far, it’s unclear whether the star will retain enough mass at the end of its life to detonate as a supernova.
Sadr is intriguing enough, but the best targets for small telescopes lie within a degree or two of this giant star. Just half a degree to the northwest of Sadr lies the open star cluster NGC 6910. This Y-shaped cluster features two 7th-magnitude yellow-white stars at the top of each tip of the “Y”. A somewhat fainter blue star lies below the “Y”.
NGC 6910 itself is embedded in a section of the wide patchwork of nebulosity known as IC 1318 (see image above). Two or three bright sections of nebula lie within 1.5o of Sadr, and the whole complex spans more than 3o. So use low a magnification to look for this nebulosity. This emission nebula is excited by a very energetic Wolf-Rayet star, a massive evolved star that’s loosing mass quickly and nearing an unstable phase near the end of its life. A nebula filter will help spot this visually challenging complex, as will clear and very dark sky.
Sadr and region is best placed for northern stargazers. But it peers just above the horizon for deep-southern stargazers after midnight in late July.