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{{Night Sky
| image =
| designation =PN MyCn18
| rightascension =13<sup>h</sup> 39<sup>m</sup> 35.116<sup>s</sup>
| declination =-67° 22′ 51.45″
| distance =8,000 ly
| constellation =[[Musca]]
| type =[[Planetary nebula]]
| dimensions = 0.201x0.201'<ref name=constellation-guide>[http://www.constellation-guide.com/hourglass-nebula/ Hourglass nebula] from constellation-guide.com</ref>
| magnitude = Apparent Mag: +13<ref name=constellation-guide/><br/>Absolute Mag: +1.1<ref>From definition of absolute magnitude, using apparent magniitude and distance given here.</ref>
| redshift =-0.000226<ref name=simbad>[http://simbad.u-strasbg.fr/simbad/sim-id?Ident=PN+MyCn+18&submit=submit+id PN MyCn 18] from simbad.u-strasbg.fr</ref>
| radvelocity =-67.8 km/s<ref name=simbad/>
| propmotion =RA: −9.9 mas/yr<br/>Dec.: 1.2 mas/yr<ref name=simbad/>
| parallax =
}}
The '''Hourglass nebula''', also called the the Engraved Hourglass nebula or the Etched Hourglass nebula, is a [[planetary nebula]] in the [[constellation]] of [[Musca]].<ref name=constellation-guide/> Unusual for a planetary nebula, it has two large rings and one smaller one. The [[star]], a white dwarf, at the centre of the nebula is somewhat offset from the exact centre also puzzles astronomers. This throws into question the current belief of how planetary nebulae form, as this would mean the star should be along the axis of symmetry of the nebula.
==History==
The nebula was first observed by Annie Jump Cannon and Margaret W. Mayall, some time between 1918 and 1924.<ref name=constellation-guide/> They recorded it as a small and faint nebula as no structure was apparent with the equipment available at their time. The structure of the nebula was not established until 1996, when better [[telescope]]s and advanced imaging techniques became available. The structure was first recognized by Raghvendra Sahai and John Trauger of the [[Jet Propulsion Laboratory]].
==Properties and Structure==
Then nebula is composed of two large lobes that meet at a central [[star]], giving it a shape like an hourglass. The star at the centre is a [[Wolf-Rayet star]], thought by some to have a temperature or 110,000 k.<ref name=constellation-guide/> The radiation it emits causes the nebula to glow. The nebula itself is expanding at around 10 km/s and is in fact approaching us at around 67.8 km/s.<ref name=simbad/> The lobes also have an intricate pattern of etchings.
The asymmetrical placement of the central star, away from the axis of symmetry, throws into question the current belief of how planetary nebulae form. The current belief that [[planetary nebulae]] form from the ejected material of stars would require the resulting nebula to by symmetric about some axis, but this is not the case.<ref>[https://www.jpl.nasa.gov/spaceimages/details.php?id=pia14442 Hubble Finds an Hourglass Nebula around a Dying Star] from jpl.nasa.gov</ref> Some have suggested that an additional companion star could explain this, but none has been observed.
==References==
{{reflist}}
{{Nebulae}}
[[Category:Astronomy]]
[[Category:Nebulae]]
| image =
| designation =PN MyCn18
| rightascension =13<sup>h</sup> 39<sup>m</sup> 35.116<sup>s</sup>
| declination =-67° 22′ 51.45″
| distance =8,000 ly
| constellation =[[Musca]]
| type =[[Planetary nebula]]
| dimensions = 0.201x0.201'<ref name=constellation-guide>[http://www.constellation-guide.com/hourglass-nebula/ Hourglass nebula] from constellation-guide.com</ref>
| magnitude = Apparent Mag: +13<ref name=constellation-guide/><br/>Absolute Mag: +1.1<ref>From definition of absolute magnitude, using apparent magniitude and distance given here.</ref>
| redshift =-0.000226<ref name=simbad>[http://simbad.u-strasbg.fr/simbad/sim-id?Ident=PN+MyCn+18&submit=submit+id PN MyCn 18] from simbad.u-strasbg.fr</ref>
| radvelocity =-67.8 km/s<ref name=simbad/>
| propmotion =RA: −9.9 mas/yr<br/>Dec.: 1.2 mas/yr<ref name=simbad/>
| parallax =
}}
The '''Hourglass nebula''', also called the the Engraved Hourglass nebula or the Etched Hourglass nebula, is a [[planetary nebula]] in the [[constellation]] of [[Musca]].<ref name=constellation-guide/> Unusual for a planetary nebula, it has two large rings and one smaller one. The [[star]], a white dwarf, at the centre of the nebula is somewhat offset from the exact centre also puzzles astronomers. This throws into question the current belief of how planetary nebulae form, as this would mean the star should be along the axis of symmetry of the nebula.
==History==
The nebula was first observed by Annie Jump Cannon and Margaret W. Mayall, some time between 1918 and 1924.<ref name=constellation-guide/> They recorded it as a small and faint nebula as no structure was apparent with the equipment available at their time. The structure of the nebula was not established until 1996, when better [[telescope]]s and advanced imaging techniques became available. The structure was first recognized by Raghvendra Sahai and John Trauger of the [[Jet Propulsion Laboratory]].
==Properties and Structure==
Then nebula is composed of two large lobes that meet at a central [[star]], giving it a shape like an hourglass. The star at the centre is a [[Wolf-Rayet star]], thought by some to have a temperature or 110,000 k.<ref name=constellation-guide/> The radiation it emits causes the nebula to glow. The nebula itself is expanding at around 10 km/s and is in fact approaching us at around 67.8 km/s.<ref name=simbad/> The lobes also have an intricate pattern of etchings.
The asymmetrical placement of the central star, away from the axis of symmetry, throws into question the current belief of how planetary nebulae form. The current belief that [[planetary nebulae]] form from the ejected material of stars would require the resulting nebula to by symmetric about some axis, but this is not the case.<ref>[https://www.jpl.nasa.gov/spaceimages/details.php?id=pia14442 Hubble Finds an Hourglass Nebula around a Dying Star] from jpl.nasa.gov</ref> Some have suggested that an additional companion star could explain this, but none has been observed.
==References==
{{reflist}}
{{Nebulae}}
[[Category:Astronomy]]
[[Category:Nebulae]]