Sky and Telescope - July 2018 - 68
These subtle shells have nothing to do with
planets and everything to do with the death
throes of Sun-like stars. The misnomer
originated with Charles Messier's description of M57 and was later reinforced by both
William and John Herschel. Some of these
nebulae really did look like ghostly versions of
Jupiter or Uranus through their telescopes
- small, round, and evenly illuminated. So the
term "planetary nebula" made sense from
that standpoint. Even so, many of them, like
M27, don't resemble planets at all.
We now know planetary nebulae are the
glowing remains of a red giant star that
were expelled by instabilities created inside
the star as its fusion reactions sputtered
to a halt. During this process, the red giant
contracts into a tiny white dwarf star.
Once the white dwarf hits a temperature
of 25,000 K, which is hot enough to generate copious amounts of intense ultraviolet
light, the nebulous remains from the end of
the red giant phase are photoionized, causing them to glow. Until this temperature is
reached, the central star merely illuminates
this material, and the nebula is considered a
Once the star shrinks down to a white
dwarf, it can no longer sustain nuclear
fusion and instead shines by the residual, but nevertheless considerable, heat
generated by its intense contraction.
Based on this knowledge, a more accurate name for this class of nebulous
object might be along the lines of "white
dwarf nebula." On the other hand, "planetary nebula" isn't the only archaic and
misleading name still in common use,
so it seems destined to remain part of
J U LY 2 0 1 8 * S K Y & T E L E S C O P E
And get this: The densest
parts of M27's nebulosity contains up to 100,000 particles
per cubic centimeter. That
sounds like a lot, but astonishingly, is less dense than most
M27 is "A most
Ionized hydrogen (H II, colored red in the image on page 67)
dominates the brightest portions of M27. It's been ejected
outward to form the brightest segments of the northern and
southern arcs and a diagonal line of nebulosity that seems to
run through its center. Doubly ionized oxygen (O III, green
in the image) dominates in the interior and helps complete
the familiar dumbbell. The faint extensions that close M27's
oval also consist mostly of O III. Thanks to the central white
dwarf star blasting out intense ultraviolet light, all this
glowing molecular gas is about 10 times hotter than normal
diffuse interstellar gas.
The hydrogen is from the ﬁrst phase of the red giant's mass
loss, while the oxygen is from a second episode. The oxygen
was dredged from the inner layers of the quickly contracting
star by the ﬁnal bursts of nuclear fusion, along with other
elements like sulphur and nitrogen.
A delightful surprise is the fundamental nature of the
Dumbbell's three-dimensional shape. Studies have shown
that M27 and M57, the Ring Nebula, probably have much the
same intrinsic structure, and we see them as so dissimilar
because our viewing angle of the two objects is different by
roughly 90 degrees. The Dumbbell is presented essentially
broadside from our perspective, while the Ring Nebula is oriented so we're looking straight down one end. If the viewing
angles were reversed, we may well have given M57 the Dumbbell nickname and M27 would be called the Ring Nebula.
Dumbbell or Rounded Rectangle?
My earliest recorded observation of M27 is from June 1974
with my homemade 8-inch f/4 Newtonian. I'd observed it
several times before with the 8-inch and my
ﬁrst telescope, a beautiful Tasco 3-inch f/15
achromatic refractor, but I had just begun
recording my deep-sky observations that June.
My ﬁrst sketch (here at left) shows the
familiar dumbbell, but three nights later
my second sketch shows a more rectangular
shape, with an ever so slightly pinched waist
and rounded ends.
t EARLY SKETCHES My first sketch of M27 is at top,
and my second sketch, made three nights later, is on
the bottom. In June 1974 I used my 8-inch f/4 Newtonian to make these sketches that inadvertently showed
how the apparent dumbbell shape changes depending
on sky conditions.
HOWA RD BA NICH (4)
A Misleading and