Key Takeaways:
- NGC 6826, colloquially known as the Blinking Planetary, exhibits an apparent "blinking" effect attributed to human visual perception rather than intrinsic variability.
- This perceptual phenomenon arises from the differential sensitivity of the eye's retinal receptors: direct vision (cones) reveals the central star and brighter inner nebulosity, while averted vision (rods) allows the fainter outer shell to become discernible.
- Structurally, NGC 6826 is a classic double-shell planetary nebula, featuring a brighter inner shell immediately surrounding a 10th-magnitude central star, enveloped by a fainter outer shell.
- Distinct morphological features within the nebula are identified as FLIERS (Fast Low-Ionization Emission Regions), characterized as knots of condensed red gas that appear to have been ejected from the central star at elevated velocities.
As we peer through our telescopes, many of us have momentarily blinked while trying to view a challenging object. We don’t expect them to blink back at us. But 9th-magnitude NGC 6826 in Cygnus does — hence its popular nickname, the Blinking Planetary.
Of course, it’s not actually blinking at us. The human eye is responsible for that effect. The center of the eye’s retina is made up of bright-light receptors, called cones, as well as some low-light sensors called rods. Together, they allow us to look directly at stars. To see faint, diffuse objects, we need to look for them out of the corner of our eyes using averted vision. Doing so directs a target’s feeble light onto the peripheral area of the retina, which is rich in rods.
NGC 6826 is a classic example of a double-shell planetary nebula. Its fainter outer shell surrounds a brighter inner shell that is directly surrounding the 10th-magnitude central star. When we look directly at it, we are seeing the progenitor and some of the brighter nebulosity in the planetary’s inner shell. But when we use averted vision, we can also make out the fainter outer shell. Glance back directly and it disappears, giving the illusion that the oval, eye-shaped planetary is blinking.
The Blinking Planetary appears to have two weird knots of condensed red gas that seem to have blasted away from the hot central star at a faster rate than the surrounding nebula. These features have been dubbed FLIERS (Fast Low-Ionization Emission Regions).
