MyCn18: An Hourglass Nebula . ... Credit Copyright: R. Sahai and J. Trauger ( JPL ), WFPC2 , NASA Explanation: The sands of time are running out for the central star of this hourglass-shaped planetary nebula . ... Astronomers have recently used the Hubble Space Telescope (HST) 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". ...