You'll be entering uncharted territory when you seek out this little known 'Shadow of the Veil' in Cygnus this summer.
Most star atlases don't plot it, but this cousin to the famed Veil Nebula supernova remnant in Cygnus has been lurking near the bright double star Albireo just waiting for you to show it some love.
When a giant star explodes as a supernova, it sends a powerful shock wave reverberating across space at up to 30,000 kilometers per second that compresses and heats the material ejected in the explosion along with interstellar dust and gases swept up along the way to millions of degrees.
As the material cools, it emits light in several colors including 'hydrogen red' and 'oxygen green'. Expanding outward like ripples in a pond lasting thousands of years, the shock wave fashions a shell-shaped supernova 'remnant' (SNR) many light years across. Some, like the bright Veil Nebula, span several degrees. That translates to a spread of 70 light years at its estimated distance of 1,500 light years.
Most amateur astronomers make it a point to stop by for a look at this magnificent supernova remnant at least once a summer. But did you know that at the other end of Cygnus, about 2.5° northeast of the double star favorite Albireo, there lurks a fainter counterpart? It goes by the name of Sharpless 2-091, or simply Sh 2-91.
To see it, you'll need a 10-inch or larger telescope (some claim a 6-inch can do it!) under a dark sky and a narrowband O III or similar filter to enhance the nebula's contrast. A moonless night this month or next when Cygnus flies high in the southern sky during convenient evening viewing hours make ideal viewing circumstances. Maybe you'll share my "where have you been all my life" reaction when I first trained my 15-inch reflector at 64x on the wispy streak. While dim, I was pleasantly surprised at how easy it was to see.
To find it, point your finderscope at Albireo and slide northeast 2.5 degrees to 4.5 magnitude 12 (Phi) Cygni. Measuring about 1′ wide by 20′ long, the arc-shaped strip passes directly in front of the 8th magnitude star HD 185735 (V2086) located 15′ due south of 12 Cygni. I found the western end and swath near the star a tad brighter and easier to discern. What will you see?
Sh 2-91 is but one shred of a much larger shell that includes the impossibly dim filaments Sh 2-94 and Sh 2-96 that together covers some 3°x4° of sky. At a distance of around 2,500 light years, this SNR's true size is something like 228 light years!
30,000 years ago when our ancestors were painting likenesses of animals in caves, a massive star exploded in the Milky Way. Did they witness the blast? We'll probably never know. But the remnant remains to remind us of a stellar trauma that happened so long ago.
Here are some additional resources on Sh 2-91 you might find helpful:
* A New Supernova Remnant in Cygnus: Sharpless 2-091 - Observing reports, maps
* Stephane Zoll's G 65.3+5.7 page - Lots of color and B&W photos
* Sharpless Objects Observing Catalog or download it free HERE.
Curious about objects like Sh 2-91? Let Sue French guide you through the night sky with her elegant and fanciful style in her book Deep-Sky Wonders.