Emission line image of M27

This excellent image of the Dumbbell Nebula (M27, NGC6853), a planetary nebula in the constellation of Vulpecula, was taken at the 3.5-meter WIYN telescope using the mini-mosaic imager (described in an NOAO newsletter article). The nebula was formed when an evolved, red giant star ejected its outer envelope near the end of its lifetime. The expanding cloud of gas becomes visible once the hot core of the star, visible near the center, is exposed and the high-energy, ultraviolet light from the core ionizes the cloud. This makes the nebula emit mostly a strong emission-line spectrum. This color image was obtained by combining three separate pictures, each one taken through a narrow filter centered at the emission wavelengths of Hydrogen (H alpha, red, 656 nm), doubly ionized Oxygen (OIII, green, 501 nm), and neutral Oxygen (OI, blue, 630 nm). It is physically very interesting that neutral gas can survive in a region right next to highly ionized gas, and this image shows how, as the blue component is concentrated in very dense clumps which act to protect the neutral oxygen from the intense ultraviolet radiation which has ionized the other gas components. We can compare this to a picture combining three images all of ionized gases, (also from the WIYN telecope), and to the more usual color image from the Mayall 4-meter telescope, or the nice deep image from the 2.1-meter telescope. The Dumbbell nebula is about 850 light-years away from Earth and about 1.5 light-years in diameter (although both distance and size are very poorly constrained).