skyandtelescope.com • AUGUST 2019 31The nitrogen-rich star’s powerful stellar wind, blowing at a blistering
speed of 2,000 km/s, plowed into the surrounding ISM and visiblyfractured the northwest side of the bubble.
Through my 18-inch refl ector, an exquisite 18′ × 11 ′ ellipse
tilts northeast to southwest and appears suspended in a rich
Milky Way fi eld. An irregular convex section tightly curls
between HD 192182 and the 8th-magnitude star to its south-
east. The brightest strip, though, extends 7′ further southwest,
and a fi lamentary triangular wedge forms a diffuse bridge to
the central WR star. Several 11th- to 13th-magnitude stars
pepper the southwestern fringe where hazy nebulosity leaks
inwards, condensing in a 30′′ clump. The southeast-facing side
dissolves away, but I do detect a dim yet nearly complete ring.
As the prototype wind-blown bubble, NGC 6888 is one
of the most researched Wolf-Rayet nebulae. A 2012 spectro-
scopic study found several nested shells formed by ejections
and collisions as its nitrogen-rich central star evolved. While
the progenitor star was still on the main sequence, its winds
swept up gas in the interstellar medium (ISM) and formed an
external “skin.” At the WR stage, with a wind gusting up to
1,600 km/s, an inner elliptical bubble formed and expanded
inside a larger spherical nebula. Based on the bubble’s expan-
sion rate of 75 km/s, NGC 6888’s dynamical age (the length of
time the bubble has been expanding) is 30,000 years.
Slide your scope just 2° to the south of NGC 6888 to
fi nd 8th-magnitude HD 191765 (WR 134) and HD 192103
(WR 135), two of Wolf and Rayet’s three original stars. In
1971 Canadian astronomer David Crampton identifi ed the
nearby crescent-shaped WR 134 Nebula. A follow-up investi-
gation in 2008 found that WR 135 is the source of a larger H
I bubble. Both stars lie within the Cygnus OB 3 association at
a distance of 6,000 light-years.
The Milky Way is patchy in this area, making it tough to
identify the nebula. But when I added an O III fi lter, the back-
ground sky darkened and the view improved dramatically.
At 73×, I found a broad nebulous crescent, perhaps 15′× 4 ′,
curling across the fi eld from north to south and opening to
the east. The arc appeared thicker in the middle and gradu-
ally tapered to the south. WR 134 is the brightest star in a 3′
chain and sits 10′ east of the crescent.
Sh 2-308, perhaps the quintessential celestial bubble, is a
snap to fi nd. Just point your scope at 3.9-magnitude Omicron^1
(ο^1 ) Canis Majoris — the nebula is centered only 15′ north.
The bubble is windblown material from the WR star EZ Canis
Majoris (HD 50896), which varies between magnitude 6.7 and
6.9 every 3.8 days. The nitrogen-rich star’s powerful stellar
wind, blowing at a blistering speed of 2,000 km/s, plowed into
the surrounding ISM and visibly fractured the northwest side
of the bubble. At a distance of more than 5,000 light-years,
this shell spans 50 to 60 light-years in diameter.
My 8-inch refl ector displays a ghostly arc, while my
18-inch at 73× reveals most of a delicate 35′ bubble. Thewestern fl ank is the most obvious feature — a long, wispy arc
curving gradually from north to south. A careful view teased
out subtle fi ligreed structure with weak fi lamentary strands.
The arc passes through a triangle of 8th- to 9th-magnitude
stars at the south end and disappears near Omicron^1. The
southeast section was very diffi cult to follow, but I picked it
up again along the eastern boundary. A brighter 10′ misty
patch containing several 10th- to 12th-magnitude stars is at
the north end. The rim dims again for a short stretch on the
northwest corner before completing a circuit. Nearly trans-
parent nebulosity suffuses the interior of the shell.
NGC 2359, popularly called Thor’s Helmet, is located
411 / 3 ° northeast of Gamma Canis Majoris (Muliphein). The
11.6-magnitude power source is HD 56925 (WR 7), which
carved out a complex nebula from its strong and episodic
mass loss. Its stellar wind snowplowed and ionized the
surrounding gas, resulting in its blue-green emission. At
a distance of 12,000 light-years, the central “helmet” is
expanding at a velocity of 10 to 30 km/s and spreads some 16
light-years wide. The ruddy-colored outer wings are part of an
ionized H II nebula and contain a large quantity of hydrogen
expelled during the precursor stages.pTHOR’S HELMET This view of NGC 2359, created from images
forming part of the Digitized Sky Survey 2, shows a good example of the
“nested shells” manifestation of episodic mass loss from the successive
phases of the death throes of massive stars. The exact mechanisms gov-
erning mass loss from Wolf-Rayet stars are still largely unknown. Thor’s
Helmet lies some 12,000 light-years away in Canis Major, and the main
ES bubble is around 16 light-years in diameter.
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