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The Engraved Hourglass Nebula, also known as the Hourglass Nebula and as MyCn 18, is a young planetary nebula that is in the southern constellation Musca. The nebula is about 8,000 light years, or 2442.8 parsecs, from the Earth.
It was discovered by Annie Cannon and Margaret Mayall while they were working on an extended Henry Draper Catalog, but they were not able to see its shape clearly. It was designated simply as a small faint planetary nebula. On January 18, 1996, due to much improved telescopes and imaging techniques, Raghvendra Sahai and John Trauger, from the Jet Propulsion Laboratory, were able to distinguish its distinct hourglass shape. It is believed by many scientists that MyCn 18′s hourglass shape is produced by the expansion of a fast stellar wind within a slowly expanding cloud which is denser near its equator than its poles. There is a smaller, less known Hourglass Nebula inside the Lagoon Nebula. There is very little information available about this nebula.
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant phase branch phase of some types of stars late in their life. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years. At the end of the star’s life( the red giant phase) the outer layers of the star are expelled via pulsations and strong stellar winds Without these opaque layers, the hot, luminous core emits ultraviolet radiation that ionizes the ejected layers of the star. This energized shell radiates as the planetary nebulae that we see. Some scientists believe that some of these phenomena may actually have died out before their light was first seen by scientists on our planet.