Blue planetary — Planetary Nebula in Centaurus

NGC 3918 is a bright planetary nebula in the constellation Centaurus, about 4,900 light-years away, often called the 'Blue Planetary' for its brilliant robin's-egg color — among the most vividly blue planetary nebulae visible from Earth. It is the southern counterpart to C22 (NGC 7662, the 'Blue Snowball') in the north. The nebula is young by planetary-nebula standards, only a few thousand years old, and its intense ultraviolet radiation from the central white dwarf (surface temperature around 140,000 K) drives strong emission in the doubly-ionized oxygen line that gives the nebula its striking color. Its angular diameter is only about 12 arcseconds — small but strikingly bright.

A Southern Hemisphere showpiece. From mid-southern latitudes it rises high overhead during austral autumn. At magnitude 8, a 3-inch telescope easily shows it as a blue-tinted fuzzy star at low power; higher magnifications (200x and up) reveal its disk-like shape and vivid color. A 6- to 8-inch telescope at 300x shows a clearly non-stellar object with slight structural hints — the nebula has a bright inner shell and a fainter outer halo. No filter is needed; the color is one of the attractions. Best observed April through July from southern latitudes. From northern sites above 30°N it rises too low for good observation.

Discovered by John Herschel from the Cape of Good Hope on April 8, 1834, during his systematic southern-sky survey. Herschel immediately noted its strong blue color and compared it to the Blue Snowball he had seen in the north. Spectroscopic studies in the 1960s and Hubble Space Telescope imaging in the 1990s revealed a complex multi-shell structure with fast collimated outflows — suggesting the central star shed its outer layers in multiple episodes rather than a single smooth expansion.