An Unexpected Shape in the Dying Light
In 1996, NASA’s Hubble Space Telescope captured an image that stunned astronomers: a vibrant, hourglass-shaped nebula, pierced with arcs and swirls, unlike anything seen before.
This structure, named MyCn18 and located about 8,000 light-years away, is a planetary nebula—formed during the final stages in the life of a sun-like star. But what Hubble revealed in this case challenged long-standing theories about how such cosmic remnants are shaped.
A Nebula Like No Other
Earlier ground-based images showed MyCn18 as a simple set of concentric rings. But when Hubble’s Wide Field and Planetary Camera 2 (WFPC2) observed it in ionized nitrogen (red), hydrogen (green), and doubly ionized oxygen (blue), the detailed image showed an intricate hourglass form with etched walls.
The camera combined separate exposures to reveal subtle structures hidden in previous observations. These included sharp, curved etchings and unexpected internal rings. The supposed central bright ring turned out to be a lumpy, potato-like structure, misaligned with the main hourglass shape.
Theories Under Strain
According to a prevailing theory, hourglass-shaped nebulae form when a fast stellar wind expands into a slower, denser cloud that is thicker at its equator. This model predicts symmetry, with the star located at the center. But MyCn18 broke that pattern.
The illuminating star, expected to lie at the hourglass’s core, is clearly off-center. This detail raised new questions. How could a dying star create such an intricate, asymmetric structure? Theories involving binary stars, collimated outflows, and flow instabilities were suggested, but the exact mechanisms remained uncertain.
More Surprises Within the Hourglass
Within the central region, Hubble also found a second, smaller hourglass structure with intersecting elliptical rings. The arc-like etchings along the nebula’s walls might be remnants of earlier mass ejections or the result of shock waves.
Their regularity hinted at organized physical processes, yet their origin could not be definitively identified. One proposal was that a jet or narrow beam of matter from the star or its surroundings carved the pattern into the expanding shell. Another possibility involved an unseen companion star shaping the outflows through gravitational influence.
A Mystery Still Unfolding
MyCn18 remains one of the most detailed and puzzling examples of a planetary nebula ever recorded. Its structure diverges from textbook expectations, and the complex forms it displays continue to be a subject of active study.
Every feature Hubble recorded added to the challenge of understanding how stars like our Sun die—not quietly, but with chaotic beauty written in gas and light.
In 1996, NASA’s Hubble Space Telescope captured a view that stunned astronomers: a glowing hourglass-shaped nebula filled with arcs and swirls.
Named MyCn18, it defied theories about how dying stars shed their outer layers…🧵👇 pic.twitter.com/JS6IcSdplL
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