OF THE ROGER N. CLARK

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VISUAL ASTRONOMY OF THE DEEP SKY
A VISUAL ATLAS OF DEEP-SKY OBJECTS
M27 (NGC 6853), THE DUMBBELL
NEBULA IN VULPECULA
RA. J9h 59. Gm, Dec. 22° 43' (2000.0)
Technical. M27 was discovered in 1764 by
Charles Messier. This is one of the largest
and brightest planetary nebulae in the sky.
Astronomers have been able to measure an
annual growth in its apparent diameter of
0.068 arc-second per year. Spectroscopic
measurements show a physical expansion
rate of about 27 kilometers per second. If we
assume that both velocities are measuring the
same expansion, the nebula's distance can be
found: about 275 light-years. This is about
three times closer than distances obtained a
few years ago. This also means that M27 has
an age close to the mean for planetaries, about
20 000 years. The actual size would be a
little under one light-year.
However, this way of finding a planetary'S
size and distance should be treated with caution.
The outer edge of the nebula may not be
the actual outer edge of the gas but just the
zone where it ceases to glow. Therefore the
visible expansion of the outer edge may have
little or nothing to do with the outflow speed
of the gas.
The central star is very hot, with a calculated
surface temperature of 85 000 K. Anything
this hot radiates mostly in the ultraviolet,
and it is this radiation that excites the
gas in the nebula to glow. The gases involved
and the colors produced are similar to those
described for M57 on page 208.
al M27 is very bright and easily visible
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V1SU
. 7_power binoculars as a slightly out-of-
In
star The nebula has a total magnitude
focUS
.
.
f 8 and an apparent size of 8 by 6 arco
. tes Its mean surface brightness is 20.8
nllnu .
mag .
£I I h
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nitudes per square arc-second. The
brightest portIOn 0rms a r ctang e or ou
4
.
I h :-
lass shape 6 by arc-mmutes. t IS t IS
ourglass shape that gave the Dumbbell its
me in the 19th century. In small telescopes
:e bright portion looks mo e like a r ctang .
le.
The faint outer parts reqUire a medIUm size
telescope and at least moderate skies, or a
mall telescope and excellent skies.
5 Through the 8-inch under moderate skies,
the 13th-magnitude central star was seen
only with averted vision at magnifications
near 200 X . Because the nebula has such a
high surface brightness, better skies won't
really improve the central star's visibility. An
8-inch is probably the minimum size telescope
in which it can be detected.
The hourglass shape was easy near 200x.
The faint extensions could be seen at all magnifications
used, and a few other stars could
be seen superimposed on the nebula at high
powers. Under excellent skies, structure corresponding
to the sharp borders at the ends of
the hourglass could be glimpsed occasionally.
Larger telescopes show this detail under good
skies. Many observers have reported a distinct
greenish color to M27 even in small
telescopes, though I have observed only a
dull gray-white through the 8-inch.
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Photograph of M27. South is up. (Courtesy National
Op tical Astronomy Observatones')
Scale: 1.2 arc-min/cm
8-inch flll.5 Cassegrain
28mm Erfle (82 X)
20mm Erfle (11 7 X)
12.5mm Ortho.
(I87x, best view)
Drawing of M27.
Viewing Distance (cm)
25 X: 115 200 X: 14
50X: 57 300X: lO
lOOX: 29 400x: 7
air mass: l. 03, faintest star: 13.0 at zenith, 188x;
no tracking
8/22/82 9:00-9:40 UT at Ewa Beach, Hawaii;
R. Clark
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