NGC 6826, also known as Caldwell 15, is a planetary nebula approximately 2200 light years away, and it lies in the constellation of Cygnus (Ngc 6826 is at the top right quadrant of the sky map below). It is commonly referred to as the Blinking Nebula, and it is so named because when viewed through a telescope the nebula appears to blink or disappear. This is because the nebula is faint compared to the inner relatively bright white dwarf star.
At the centre, shining with a bright visual magnitude of 10.7, the star’s temperature hovers around a coolish 47,000 Kelvin with a luminosity of 1300 times that of our Sun giving a low mass around 0.55 times solar. Still heating at a constant luminosity, when the star hits around 100,000 Kelvin, it will begin to cool and fade, the ultimate result of a common lower-mass white dwarf. The nebula is between 0.4 and 0.5 light years across, the result of a relatively low expansion velocity of 11 kilometers per second. NGC 6826, however, is one of the few nebulae surrounded by a relatively bright outer giant halo, more than two light years wide, the result of earlier episodes of stellar mass loss illuminated by ultraviolet radiation from the star leaking into it.
A distinctive feature of this nebula are the two bright patches of blood-red FLIERs, or Fast Low-Ionization Emission Regions, that lie horizontally across the image. The surrounding faint green ‘white’ of the eye is believed to be gas that made up almost half of the star’s mass for most of its life. The hot remnant star (in the centre of the green oval) drives a fast wind into older material, forming a hot interior bubble, which pushes the older gas ahead of it to form a bright rim. Their red color comes from radiation from ionized nitrogen, whereas the greenish cast of the rest of the nebula is from twice- ionized oxygen.
Credits: Anne’s Astronomy, Astro Images, Hubble, Wikipedia.