Hourglass nebula

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Hourglass nebula
MyCn18 Hourglass Nebula.png
Observational Data
Designation PN MyCn18
Right ascension 13h 39m 35.116s
Declination -67° 22′ 51.45″
Constellation Musca
Type of object Planetary nebula
Dimensions 0.201x0.201'[1]
Magnitude Apparent Mag: +13[1]
Absolute Mag: +1.1[2]
Redshift -0.000226[3]
Distance from Earth 8,000 ly
Radial velocity -67.8 km/s[3]
Proper motion RA: −9.9 mas/yr
Dec.: 1.2 mas/yr[3]

The Hourglass nebula, also called the Engraved Hourglass nebula or the Etched Hourglass nebula, is a planetary nebula in the constellation of Musca.[1] Unusually 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, which 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.


The nebula was first observed by Annie Jump Cannon and Margaret W. Mayall, some time between 1918 and 1924.[1] 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 telescopes 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 of 110,000 k.[1] 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.[3] 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.[4] Some have suggested that an additional companion star could explain this, but none has been observed.


  1. 1.0 1.1 1.2 1.3 1.4 Hourglass nebula from constellation-guide.com
  2. From definition of absolute magnitude, using apparent magniitude and distance given here.
  3. 3.0 3.1 3.2 3.3 PN MyCn 18 from simbad.u-strasbg.fr
  4. Hubble Finds an Hourglass Nebula around a Dying Star from jpl.nasa.gov