OF THE ROGER N. CLARK

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VISUAL ASTRONOMY OF THE DEEP SKY
A VISUAL ATLAS OF DEEP-SKY OBJECTS
M57 (NGC 6720), THE RING NEBULA IN
LYRA
R.A. JSh 53.6m, Dec. 33° 02' (2000.0)
Technical. M57 is the classic example of a
planetary nebula. It was the first one discovered:
in 1779 by Antoine Darguier, who
described it as "resembling a fading planet".
There are probably 10 000 planetaries in our
galaxy, though less than a thousand are catalogued.
M57 consists of gas ej ected from its
central star less than 5500 years ago. It is
now expanding at a rate of 19 kilometers per
second and, at a distance of about 2000 Iightyears,
is about 1/3 light-year in diameter.
The nebula's total mass is less than that of
the Sun, but it emits about 50 times the Sun's
light. Its composition has been found by detailed
spectroscopic analysis. The percentages
of atoms are:
Hydrogen 92.55 Sulfur
Helium 7.35 Argon
Oxygen 0.054 Chlorine
Nitrogen 0.027 Fluorine
Neon 0.008
0.005
0.0007
0.00019
0.00002
Color photographs of M5 7 show a greenish
center surrounded by yellow that turns to red
on the outer edges. The different colors are
due to the various atoms that are excited to
emission. Near the center, light from oxygen
and nitrogen dominates. Near the edge, the
ultraviolet radiation from the central star is
too weak to excite these elements, so emission
from hydrogen dominates.
The central star, shining at magnitude
14.8, is either a white dwarf or evolving toward
white dwarf status. It is extremely hot,
with an estimated surface temperature of
100 000 kelvin, and is several thousand times
the density of our Sun.
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Visual. M57 is magnitude 9 and has a
apparent size of 1.3 by 1.0 arc-minutes. I
mean surface brightness is very high at 17.9
magnitudes per square arc-second. Th
nebula inside the ring is much dimmer bu
still can be seen easily in medium-size tele_
scopes. This central glow consists of several
striations parallel to the major axis. Resolv_
ing the striations requires a large amateur
telescope and excellent skies.
Through the 8-inch, the ends of the ellipse
were fainter than the other edges. The "center
of the doughnut" was markedly brighter
than the surrounding sky, but no fine detail
could be seen under good skies. The best view
was at powers near 200X, while 334x gave
about the same detail.
The central star could not be seen. It is
suspected of being variable because it is
sometimes easy in a 12-inch telescope, at
other times difficult in a 40-inch.
The ring shape can be detected in very
small amateur telescopes. I conducted an
experiment to find the minimum aperture
needed to do so. On the same night as the
drawing was made, I used the 8-inch telescope
at 188x with various masks over the
front to simulate smaller apertures. Since sky
conditions, observer and magnification were
all constant, the only variable was aperture.
The ring shape was easily detected when the
aperture was just two inches. At the next
lowest aperture, I inch, M57 could not be
seen at all. The minimum aperture required
under good skies is probably near 1.5 inches.
Photograph of M57. South is up. (Courtesy Laird
A. Thompson, Canada-France-Hawaii Telescope
Corporation.)
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-- 2' --
Drawing of M57.
Scale: 0.25 arc-min/cm
a-inch fli l.5 Cassegrain
l2.4mm Erfle (188X,
best view)
7mm Erfle (334 X)
Viewing Distance (cm)
50X:275 300X:46
lOOx: 138 400X:34
200x: 69 600X:23
air mass: 1.02, faintest star: 14.2 at zenith, 188x;
no tracking
6/ 1 7/83 11:35-1 1 :50 UT at Barbers Point, Hawaii;
R. Clark
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