Sky & Telescope - USA (2019-09)

Planetary Nebulae

34 SEPTEMBER 2019 • SKY & TELESCOPE

very best of conditions, brighter knots high-

light the ends of the handlelike extensions.

Another popular jewel in Aquarius is the

magnifi cent Helix Nebula, NGC 7293. At

about 700 light-years distant, it’s one of

the nearest planetary nebulae to Earth. It’s

so large that it can actually be diffi cult to

view in a telescope. It shows in binoculars

and is rather lovely in rich-fi eld telescope

wide-angle views, so use low power to

enjoy it. A fi lter improves the contrast and

makes this huge object more apparent. The

giant ring is mostly round, irregularly bright, and mottled.

High-resolution images of the Helix reveal a large number

of bright knots around its circumference, the origin of which

aren’t yet fully understood. A rather remarkable Hubble image

of NGC 7293, featured in 2003 as an Astronomy Picture of

the Day and captioned “The Eye of God,” captured the public

imagination and was widely circulated. The view in a back-

yard telescope may be considerably less dra-

matic, but it defi nitely qualifi es as a celestial

showpiece nonetheless.

My planetarium software reports that

there are no fewer than 543 planetary

nebulae in Sagittarius! September fi nds most of them well

past their best availability. Still well-placed, however, is one of

the best planetary nebulae in the Archer. NGC 6818, known

as the Little Gem, is nestled between two 13.5-magnitude

stars that fl ank its east and west sides. The object itself is a

blue disk that’s darker toward the center, giving the impres-

sion of annularity. Occasionally, the center devolves into

tCAMPBELL’S HYDROGEN STAR This fascinating

star, HD 184738, snapped with the Hubble Space

Telescope, is in fact a rare Wolf-Rayet star (see S&T:

Aug. 2019, p. 28). The image shows the star at the

center of a small planetary nebula, surrounded by a

larger nebula composed of glowing gases, primarily

hydrogen and nitrogen.

CA

MP

BE

LL’

S^ H

YD

RO

GE

N^ S

TA

R:^

ES

A^ /

HU

BB

LE

/^ N

AS

A^ /

JE

AN

• C
  HR

IST

OP

HE

LA

MB

RY

;^ N

GC

67

81 :

JO

SE

F^ P

ÖP

SE

L^ /

BE

AT

E^ B

EH

LE

ETHEREAL SHELL

The author can’t resist

NGC 6781 in Aquila,

which ranks among his

favorites.

N