The Caldwell Objects

2
2
Bow-Tie Nebula
NGC 40
Type: Planetary Nebula
Con: Cepheus
RA: 00 h 13 m 01 s
Dec: +72° 31' 19"
Mag: 12.3
(nebula);
11.6 (central star)
Dim: 38" x 35"
Dist: ~3,500 light-years
Disc: William
Herschel, 1787
W. H ERSCHEL: [Observed 25
November 1787] A star [of] 9th
magnitude surrounded with very
faint. . . much nebulosity. The star is
either double, or not round. Less than
1’ diameter. (H IV-58)
GC: Very faint, very small, round, very suddenly much brighter
in the middle to a star of 10th magnitude, a star of 12th
magnitude toward position angle 241.4°, diameter 25”.
THE PATCHY FAR-NORTHERN REACHES OF THE
Cepheus Milky Way all but hide the peculiar
12th-magnitude GC: Very faint, very planetary small, round, nebula very suddenly known much as NGC
brighter in the middle to a star of 10th magnitude, a star of
40. In 1918 Heber D. Curtis of Lick Observatory
12th magnitude toward position angle 241.4°, diameter 25".
first noted the object's peculiar nature on
photographic plates. Curtis saw an irregular shell
of gas that resembled a "truncated ring, from the
ends of which extend much fainter wisps."
Photographs and images taken with modern
research telescopes have revealed that Curtis's
"wisps" curve gently away from the bright
truncated ring, forming a series of looplike
structures reminiscent of a bow tie. The wisp
extending from the nebula's northern end breaks
sharply to the east, looking like a snapped twig.
Photographs taken in red light
22
NGC: Faint, very small in angular size, round, very suddenly
much brighter in the middle, a star of 12th magnitude south
preceding [to the southwest].
reveal a weak, irregular outer envelope.
How do planetary nebulae, which originate
from the largely spherical envelopes of red-giant
stars, develop such weird shapes? The answer
lies, in part, in the interaction between the central
star's fast wind of particles and material cast off
previously, when the central star was a bloated
giant. NGC 40's progenitor, which has the
spectral characteristics of a Wolf-Rayet star — a
very luminous, very hot star with an expanding
atmosphere — contains about 70 percent of our
Sun's mass. Its extremely high surface
temperature (about 90,000° Kelvin) should suffice
to strongly ionize the surrounding nebula,
causing it to glow. Yet observations show that
NGC 40 isn't quite
Deep-Sky Companions: The Caldwell Objects