The Caldwell Objects

63
63
Helical Nebula; Helix
Nebula; Sunflower Nebula
NGC 7293
Type: Planetary Nebula
Con: Aquarius
RA: 22 h 29 m 38.4 s
Dec: -20° 50' 13"
Mag: 7.3; 6.0 (O'Meara)
Dim: 12' x 10'
Dist: 522 light-years
Disc: Georg Friedrich Julius
Arthur von Auwers (pub-
lished in 1862)
H ERSCHEL: None.
GC / NGC: Remarkable, pretty faint, very large, elongated or bi-
nuclear.
THE ECLECTIC GERMAN ASTRONOMER GEORG
Friedrich Julius Arthur von Auwers (1838-1915) is
most noted for his work on Procyon's proper
motion; his observations of Venus's 1874 and
1882 transits across the face of the Sun; and his
discovery of Τ Scorpii, a nova in the globular
cluster M80. But he also published, in 1862, a
catalog of William Herschel's nebulae and
clusters — a work he began preparing for when
he was only 16 years old. Printed in Volume 34 of
the Konigsberg Observations, the work also
includes a list of 52 "very widely diffused nebulosities,"
including von Auwers's discovery of an
immense object in Aquarius. That object is
recognized today as the largest and closest
planetary nebula known in the heavens — NGC
7293, the famous Helical or Helix Nebula. For his
contributions to science, a lunar crater has been
named in von Auwers's honor.
The Helix lies about 11° northwest of 1stmagnitude
Alpha (α) Piscis Austrini (Fomal-
248
haut) and 1¼° west of 5th-magnitude Nu (ν)
Aquarii. Published visual magnitudes range from
"6th" to 7.4. Given the object's brightness and its
proximity to the globular cluster M30 in
Capricornus (the planetary is about 12° eastnortheast
of the cluster), one has to wonder how
the greatest observers, from Charles Messier to
John Herschel, failed to detect it. But the answer
is simple. First, the Helix is enormous, spanning
an impressive 12' x 10' on the sky. Second, it has a
low surface brightness. Third, its center is a
gaping "hole." And finally, the object is most
readily apparent with binoculars or a rich-field
telescope. By condensing its light, such
instruments increase the contrast between the
nebula and the sky background, making it easier
to see. By contrast, many of our astronomical
ancestors used telescopes that had limited fields
of view. The late Walter Scott Houston
concurred: "The bulk of my correspondents
indicated that the Helix
Deep-Sky Companions: The Caldwell Objects