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8 :^DON^ GOLDMANWOLF-RAYET NEBULAE by Steve Gottlieb
28 AUGUST 2019 • SKY & TELESCOPE
I
n 1867, French astronomers Charles Wolf and Georges
Rayet made a puzzling discovery. Using a visual spectrom-
eter, they scanned the Cygnus Milky Way with the Paris
Observatory’s 40-cm (16-inch) refl ector and spotted three 8th-
magnitude stars with remarkable spectra. Instead of the nar-
row, dark absorption lines usually seen in stellar spectra, these
stars displayed broad and strong emission lines. Adding to the
mystery, most of the prominent lines appeared to belong to an
unknown element, which was later discovered to be helium.
These and similar stars are now known as Wolf-Rayet, or
WR, stars, and they form a rare and exotic class. The bright-
est and nearest is 2nd-magnitude Gamma^2 (γ^2 ) Velorum, a
spectroscopic binary consisting of an extraordinary WR star
and O-type companion. In 1883 English-born astronomer
Ralph Copeland described its beautiful spectrum from the
high altitude of Peru’s side of Lake Titicaca:Its intensely bright line in the blue, and the gorgeous group of
three bright lines in the yellow and orange, render its spectrum
incomparably the most brilliant and striking in the whole
heavens.In the mid-1970s, astronomers determined WR stars are
highly luminous descendants of massive O-type stars. Since
then, multiwavelength studies from radio to X-ray have
investigated their copious mass loss and developed the evolu-
tionary model outlined below.
In a brief but stable main-sequence phase, a fast stellar
wind sweeps up ambient interstellar material and compresses
it into a thin shell. Behemoth O-type stars with some 60 solar
masses or more soon evolve off the main sequence, becoming
luminous blue variables. Less massive ones transition into red
supergiants. In either case, the star continues to eject a hydro-
gen-rich envelope, and a slower stellar wind forms a dusty
circumstellar nebula inside the existing interstellar bubble.
The short-lived WR stage begins as the helium-rich
core is exposed. The star then blasts a powerful wind up to
3,000 km/s (in some cases, higher speeds have been recorded),
shedding between 10-5 and 10-4 solar masses per year. This
amount may seem insignifi cant, but it’s a billion times the
Sun’s annual mass loss. The wind sweeps up the nearby cir-
cumstellar material into a glowing, ionized gas bubble.
Astronomers classify WR stars into two main types, WN
and WC, based on the optical spectra. WN stars displaystrong emission lines of helium and nitrogen, while WC stars
are carbon- and helium-dominant. These types are further
divided into subclasses (WN2 to WN9 and WC4 to WC9)
based on the relative strengths of their emission lines. Ther e’s
a third, rare class, the WO stars, which are similar to WC
stars but with more prevalent oxygen lines.
Visible nebulae surround at least 60 of the 661 known
galactic WR stars. They display a variety of features, includ-
ing complete shells, arcs (partial shells), clumps, fi laments,
and diffuse emissions. Most nebulae surround WN-type
stars, and three spectacular wind-blown bubbles headline
this class: NGC 6888 in Cygnus, NGC 2359 in Canis Major,
and NGC 3199 in Carina. Follow along and we’ll chase down
the top wind-blown bubbles across the entire sky, with the
help of O III and narrowband fi lters.
NGC 6888, the Crescent Nebula, lies 2.7° southwest
of 2nd-magnitude Gamma Cygni (Sadr), along the spine
of the Milky Way. At a magnifi cation of 25×, my 80-mm
fi nder (equipped with an O III fi lter) shows an elongated
arc of nebulosity passing through the 7.2-magnitude star
HD 192182, along with an 8th-magnitude star 7′ to its
southeast. My 8-inch refl ector captures two-thirds of the
entire oval shell. The brightest piece extends both east and
southwest from HD 192182. A broader portion of the bubble
with lower surface brightness is on the southwest end.
Although numerous Milky Way stars sparkle within the
nebula, only the WR star HD 192163 (also called WR 136)
is actually inside the nebula, slightly north of center.Mysterious in origin and ethereal in nature,
Wolf-Rayet nebulae will intrigue both
observer and reader alike.uTHE CRESCENT NEBULA This ground-based image of NGC 6888
and its central star, WR 136, highlights the cataclysms that giant stars
undergo in their death throes (the vertical lines are blooming spikes).
During the red supergiant phase, the star gently puffs some of its insides
into the surrounding space. As it transitions to the WR phase, a fast and
furious stellar wind switches on, lighting up the medium around the star.