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The Hourglass (NGC 2346)
Planetary Nebula
aka NGC 2346, PNG 215.6+03.6, PK 215+03.1, M 1-10, ARO 80
RA: 07h09m22.5s Dec: -00°48'24" (Monoceros)
Integrated Visual Magnitude: 12.5
Angular Diameter: 1'
Mean Surface Brightness: 21.1 Mag/arc-secē
Distance 4300 ly

Minimum requirements to detect: 6-inch under dark skies


The Hourglass  is a little-known planetary nebula in Monoceros.  There is some confusion over the name of this nebula.  NGC 2346 has been referred to as the Hourglass, but this name has been more recently applied by astronomers at the Hubble Space Telescope Science Institute to MyCn 18.  The spectacular planetary MyCn 18 is also commonly referred to as the "Etched Hourglass."  To make things even more confusing, the press release for the HST image of NGC 2346 is entitled "Butterfly Nebula."

Although an extraordinarily interesting object, visually this planetary is small, faint, and difficult to observe due to the presence of a bright central star.  It is listed as 12.5 magnitude, with a diameter of 1', but it  may not appear that large in the eyepiece.

In 6 to 8-inch telescopes look for an 11th magnitude star surrounded by a small, faint halo.  Users of larger instruments should look for a slight pinching, which gives this nebula its colorful names in photographs.  Observers report that both UHC and OIII filters really help bring it out.


The field in a 6-inch at 50x. North is down and east is to the left.

I observed the Hourglass with my 18-inch in January 2000 and again in February 2002.  This little-known planetary nebula in Monoceros appeared as an obvious blurred star at 100x.  At 165x it appeared as a bright star with a trace of haze around it.  I went to 425x for the best view without the filter.  It looked like a round haze surrounding the star, with a slight darkening in two opposite directions.  This is most likely the pinching seen in photographs that gives this nebula its name.  The nebula would probably be easier to discern if it weren't for the bright 11th magnitude central star, which tends to drown it out. 

The OIII filter dimmed the central star while enhancing the nebula.  At 250x it appeared as a round, diffuse glow and with averted vision I could make out broad, diffuse wings.  The center looked quite "soft", as if the central star was more of a bright knot than stellar.


This image from the DSS shows a 20' x 20' field. North is down and east is to the right.

The Hourglass derives its shape from the cosmic dance of the two stars which lie deep within its center.  This is the eclipsing variable V651 Mon, which consists of an A5 main sequence star and a hot subdwarf.  In the eyepiece we see the combined  light of these two stars as the 11th magnitude central star.  What makes this system unique is that the eclipses involve the main sequence star and passing dust clouds.

In 1983-1984 the combined light of these stars faded to 15th magnitude!  As the light recovered in 1985, the star system varied regularly, showing deep eclipses from magnitude 11.5 to 15 every 15.991 days.  Another similar, but less dramatic, fading event occurred in 1996.

The differences in these two fading/eclipsing events are probably due to the variable nature of the position and density of the dust clouds.  These dust clouds probably represent matter that is being ejected perpendicular to the disk of material that surrounds the system.

The binary nature of this system seems to have lead to the complex nebula.  Apparently, when the more massive star swelled to become a red giant it enveloped its nearby companion, which lead the pair to spiral closer together.  In the process, the outer layers of the red giant were expelled into a thick disk, which can be glimpsed at the center of the HST image above.  The fast stellar wind of the evolving star blew much of this material away into the surrounding space, creating the butterfly-shaped nebula. This nebula has now nearly reached a diameter of one-third of a light year.

Millennium Star Atlas Vol I Chart 249
Sky Atlas 2000 Chart 12
Uranometria 2000 Vol I&II Chart 228
Herald-Bobroff Astroatlas B-06 C-40