"Hourglass"

NEBULA  MYCN 18 - AS ART
This image has been widely reproduced in settings as diverse as the cover of National Geographic [April 1997] and the cover of Pearl Jam's "Binaural" CD. 

The hourglass-shaped nebula has an intricate pattern of "etchings" in its walls. A planetary nebula is the glowing relic of a dying, sun-like star. Scientists theorize the hourglass-shape of this particular nebula is produced by the expansion of a fast stellar wind within a slowly expanding cloud of interstellar gas, which is denser near its equator than its poles.

A Chameleon SkyThe sands of time are running out for the central star of this the Hourglass Nebula. With its nuclear fuel exhausted, this brief, spectacular, closing phase of a sun-like star's life occurs as its outer layers are ejected and its core becomes a cooling, fading white dwarf. In 1995, astronomers used the Hubble Space Telescope to make a series of images of planetary nebulae, including the one above. Here, delicate rings of colorful glowing gas (nitrogen-red, hydrogen-green, and oxygen-blue) outline the tenuous walls of the 'hourglass.' The unprecedented sharpness of Hubble's images revealed surprising details of the nebula ejection process and may resolve the outstanding mystery of the variety of complex shapes and symmetries of planetary nebulae. 

Image Credit: NASA, WFPC2, HST, R. Sahai and J. Trauger (JPL)