Sky & Telescope - August 2019 - 33

A few years later I used an 18-inch reﬂector in rural
Australia and found NGC 3199 rich in intricate detail. With
a narrowband ﬁlter, the mottled nebula contained subtle
patches, knots, and dark veins. A thin 5′ ribbon straddled
the outer western edge, running north and south, and much
fainter nebulosity appeared to leak out and ﬂow east from the
south end. Through a 25-inch scope, I was able to trace the
full perimeter of the bubble.
The next stop is the astonishing Carina Nebula, which
surrounds the famous luminous blue variable, Eta (η) Carinae. This region, the Carina OB1 association, has produced
many of our galaxy's most massive stars, including at least
ﬁve Wolf-Rayet stars. In 1980, French astronomer MarieClaire Lortet and collaborators discovered a partial ring
centered 10′ west of the 9th-magnitude carbon-rich star
WR 23 (HD 92809).
It's easy to pass right over the WR 23 Nebula as it blends
into the dense nebulosity at the north end of the Carina
Nebula. But using a narrowband ﬁlter I traced a large 20′
semicircular arc opening to the east. The central portion is a
bit brighter and contains a wide pair of 10th- and 11th-magnitude stars. The arc hooks sharply east at the north end and
dissipates into the star ﬁeld, while the southern end leads
to a thick east-west bar of nebulosity. The ring is incomplete
along the east side due to heavy dust obscuration. Experiment with different ﬁlters - I found a hydrogen-beta ﬁlter
also enhanced its visibility.
For our ﬁnal target, we'll leave the Milky Way and head to
the neighboring Large Magellanic Cloud, which has a major
collection of more than 150 WR stars. And despite a distance of 165,000 light-years, the emission nebula NGC 2020
is faintly visible through an 8-inch scope. In images, the
nebula sports a distinctive blue ring with a double rim, suggesting a tilted short cylinder, and a circular, diffuse H II
region surrounds the ring.
I had a memorable view of NGC 2020 in April of this year
in Australia through a 25-inch reﬂector. At 264×, the roundish 3′ nebula formed a classic smoke ring, with a dark center
spanning 45″ × 30″. The 13th-magnitude WR star HD 269748
shines just north of center, and a 12th-magnitude star is
pinned against the south edge.

p YOUNG AND OLD Two rather dissimilar nebulae sit side by side in
the Large Magellanic Cloud. The pinkish nebula at right (NGC 2014) is
an emission nebula, powered by a cluster of hot, young stars. The bluish
nebula at left is a WR nebula (NGC 2020), powered by a single, massive
dying star. The color differences are due in part to the different chemical
compositions of the surrounding medium: The hot winds from the young
stars plow into a nebula composed mostly of hydrogen, whereas the
WR winds collide with heavier elements previously expelled by the red
supergiant star, primarily oxygen in this case.

What's the ultimate fate of a WR star? In a relatively short
time span, the core will become iron-rich and no longer able
to support nuclear fusion. At that point gravity will win the
ﬁnal battle with radiation, initiating a cataclysmic collapse.
Because of their extreme mass, WR stars are strong candidates to end their lives in a spectacular core-collapse supernova (Type Ib or Ic). Long-duration gamma-ray bursts may be
produced - the signature of a phenomenally high energetic
event. The blast will leave a lasting legacy by seeding our galaxy with vast amounts of heavy elements for future generations of stars and planets.

¢ A deep-sky fanatic for 40 years, Contributing Editor STEVE
GOTTLIEB keeps an eye on the sky for us from northern
California. He welcomes your questions and comments at
steve_gottlieb@comcast.net.

NGC 2020: ESO; X-R AY: ESA / X M M-NE W TON /
J. TOA L Á / D. GOLDM A N

WR Nebulae Also Shine in X-rays
Astronomers have detected X-ray emission from four Wolf-Rayet nebulae to date: NGC 6888,
Sh 2-308, NGC 2359, and NGC 3199 (all featured in this article). During the WR phase,
the star's strong and fast wind rams into the slower wind shed during the red supergiant or
luminous blue variable phase. Shock waves ripple through the surrounding medium, heating
it to tens of millions of degrees, hot enough to emit X-rays. In the image at left of NGC 3199,
obtained with the European Space Agency's XMM-Newton satellite, the hot X-ray-emitting
gas is colored blue, while the yellow-green and red (oxygen and sulfur, respectively) trace out
the visible arc - compare this with the image opposite. - DIANA HANNIKAINEN

sk yandtele scope.com * AUGU S T 2 019

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Sky & Telescope - August 2019

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