Engraved Hourglass Nebula

Lore & Background

The Engraved Hourglass Nebula was discovered by Annie Jump Cannon and Margaret W. Mayall during their work on an extended Henry Draper Catalogue, which was built between 1918 and 1924. At that time, it was designated simply as a small faint planetary nebula. Much improved telescopes and imaging techniques allowed the hourglass shape of the nebula to be discovered by Romano Coradi and Hugo Schwarz in images taken during 1991–1992 at the European Southern Observatory.

It is conjectured 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. The vivid colours given off by the nebula are the result of different 'shells' of elements being expelled from the dying star, in this case helium, nitrogen, oxygen and carbon. The central star of the nebula has a strong magnetic field.

The Hourglass Nebula was photographed by the Wide Field and Planetary Camera 2 of the Hubble Space Telescope. A less-famous 'Hourglass Nebula' is located inside the Lagoon Nebula.

Reader's Guide

The Engraved Hourglass Nebula's significance lies in its distinctive shape and the insights it provides into the late stages of stellar evolution. Its hourglass form, discovered in the early 1990s, is conjectured to result from the interaction of a fast stellar wind with a slowly expanding cloud that is denser at the equator than at the poles. This mechanism helps astronomers understand how asymmetrical planetary nebulae form. The vivid colors, produced by expelled shells of helium, nitrogen, oxygen, and carbon, reveal the chemical composition of the dying star's ejecta. The detection of a strong magnetic field in the central star adds another layer to the understanding of how magnetic fields may shape nebulae. The nebula's imaging by the Hubble Space Telescope's Wide Field and Planetary Camera 2 provided high-resolution views that have become iconic. Its legacy includes serving as a key example of a bipolar planetary nebula and inspiring further study of stellar winds and magnetic fields in such objects.

Discovery & Early Cataloguing

The Engraved Hourglass Nebula, catalogued under the designation MyCn 18, first entered the astronomical record as a modest and unremarkable object. During the construction of an extended version of the Henry Draper Catalogue—a major reference project carried out between 1918 and 1924—astronomers Annie Jump Cannon and Margaret W. Mayall identified the object among the stars of the southern constellation Musca. At that stage of observational capability, the nebula presented itself merely as a small, faint planetary nebula, indistinguishable in shape from countless similar objects scattered across the southern sky. For nearly seven decades it remained essentially a footnote in the catalogue, a dot of diffuse light with no distinguishing morphology. It was not until the capabilities of ground-based instrumentation advanced dramatically that the object's true character began to emerge from the data. The fact that two of the most prolific stellar classifiers of the early twentieth century could have logged it without noting anything unusual speaks volumes about how much the nebula's structure was hidden from view by the limitations of the era's optics and photographic plates.

The Hourglass Revelation

For nearly seventy years after its initial cataloguing, MyCn 18 sat in the literature as a featureless smudge. The breakthrough came in the early 1990s when Romano Coradi and Hugo Schwarz examined images captured between 1991 and 1992 at the European Southern Observatory. The significantly upgraded telescopes and imaging techniques available at that facility finally resolved the nebula's true morphology: a striking hourglass figure, with material pinched at the waist and flaring outward at both ends. This was not a new object but a new understanding of an old one. The revelation that a previously small and faint entry in a catalogue could conceal such a geometrically precise structure underscored how much the apparent simplicity of a nebula could depend entirely on the resolving power of the instrument doing the looking. Coradi and Schwarz's identification of the hourglass geometry transformed MyCn 18 from a catalogue footnote into a subject of genuine astrophysical interest, prompting questions about what physical processes could sculpt expanding gas into such a symmetrical bipolar shape.

Physical Structure & Elemental Composition

MyCn 18 is classified as a young planetary nebula, meaning the central star has only recently shed its outer layers and the expelled gas is still in the early stages of expansion. The prevailing explanation for the hourglass geometry involves two interacting flows: a fast stellar wind driven by the central star pushing outward through a surrounding cloud that is expanding more slowly. Crucially, this outer cloud is not uniform in density—it is thicker and more concentrated along its equatorial plane than at its poles. The fast wind therefore carves a narrow channel through the denser equatorial material while expanding more freely along the polar axes, producing the characteristic pinched-waist, flared-ends silhouette. The vivid colours visible in images of the nebula arise from multiple shells of different chemical elements being ejected as the dying star sheds its layers. The elements identified include helium, nitrogen, oxygen, and carbon, each contributing its own distinctive hue to the overall glow. Adding to the object's astrophysical character, the central star possesses a strong magnetic field.

Modern Observation & Distinction

The Engraved Hourglass Nebula has been imaged by the Hubble Space Telescope, specifically through its Wide Field and Planetary Camera 2 instrument, placing it among the objects that have benefited from space-based observation free from atmospheric distortion. This Hubble coverage has allowed astronomers to study the nebula's structure with a clarity that ground-based facilities, even those as capable as the European Southern Observatory, cannot always match. It is worth noting, however, that the name Hourglass Nebula is not unique to MyCn 18. A less well-known object bearing the same common name resides within the Lagoon Nebula, a much larger and more famous nebula. This naming overlap can create confusion in popular astronomy writing, where the two objects are sometimes conflated. The Engraved Hourglass Nebula's position in the southern constellation Musca also means it is best observed from southern-hemisphere sites, a practical detail that has historically limited the number of observatories able to study it in detail. Together, its youth, distinctive bipolar morphology, rich elemental palette, and the magnetic field of its central star make MyCn 18 a compact but information-dense window into the final stages of stellar evolution.