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.
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.