Sky and Telescope - July 2018 - 66
DUMBBELL NEBULA by Howard Banich
The First Planetary Nebula
Explore this complex object through your telescope with the
help of an experienced observer's ever-so-elaborate sketches.
J U LY 2 0 1 8 * S K Y & T E L E S C O P E
Its size and brightness - 8 × 5 arcminutes in diameter,
magnitude 7.5 - make it an easy target even in binoculars and ﬁnder scopes. Its apparent shape can range from a
dumbbell, to a rounded rectangle, to a slightly squashed oval
depending on your observing conditions, scope, and experience. You may even see all three shapes blended together.
M27, the Planetary Nebula
Stars with 1 to 8 masses of our Sun - which make up a good
proportion of stars in the universe - will probably produce a
planetary nebula when they lose the ability to sustain fusion
during their ﬁnal red giant phase and evolve into white
dwarf stars. (See the sidebar on page 68, and below, for more
details of this process.)
But as common as this phenomenon is, it's also shortlived. It lasts only a few tens of thousands of years, a mere
ﬂicker compared to the billions of years that stars in this
mass range live on the main sequence.
As it turns out, M27 is a young planetary nebula with an
estimated age between 9,800 and 14,600 years. That's just
two to three times longer than recorded human history. The
nebulosity has a diameter of about 3.1 light-years (around
two-thirds the distance to Alpha Centauri, the closest star
system to ours) and is approximately 1,360 light-years away.
At the center of the nebula lies the largest white dwarf
thus far measured, with a radius approximately 0.055 times
that of the Sun's. A temperature of 108,600 K, about 18 times
solar, makes it hot and bluish. At magnitude 12.9, it can be
seen in a modest-size amateur telescope.
This white dwarf star has an unseen stellar companion,
which is believed to have played a major role in sculpting the
nebulous shape of M27.
ESO / I. A PPENZELLER / W. SEIFERT / O. STA HL
has everything going for it. It's a large, bright
planetary nebula with a striking and subtly
detailed shape. It's well placed in the Northern Hemisphere
sky during the peak observing season - late spring through
early fall. It has a famous nickname that captures the essence
of its shape. It's even easy to ﬁnd, especially if you triangulate
its position using Gamma (γ) and Delta (δ) Sagittae.
Almost universally referred to as the Dumbbell Nebula,
the brightest parts of M27 really do look like a handheld
workout weight. But it often looks even more like an apple
core or an hourglass. John Herschel coined the dumbbell
name and suggested the hourglass comparison in 1827. The
resemblance to an apple core, which is a more modern nickname, is just as strong.
However, I think Herschel's best description of M27 is "A
most extraordinary object," and anyone who's seen it through
a telescope will probably agree that that describes its impact
in the eyepiece better than any of its nicknames.
Charles Messier was able to discover the Dumbbell the
night before the full Moon of July 1764 because it has the
highest surface brightness of any large planetary nebula. His
description reads, in part, "nebula without a star" and "it
appears of oval shape." That general appearance ﬁt many of
his discoveries, so it became just the 27th object on his list
of enigmatic nebulae that didn't move relative to the stars.
Today we know that M27 was the ﬁrst planetary nebula ever
to be discovered.
Messier was using a 6-inch speculum mirror Gregorian
telescope at 104× when he ﬁrst found M27. His discovery
not only shows that searching for comets didn't wait for the
Moon to get out of the way but is also a dramatic illustration
of M27's conspicuousness.