OF THE ROGER N. CLARK

---
M67 (NGC 2682), OPEN CLUSTER IN
CANCER
R.A. OSh 51.0m, Dec. 1r 49' (2 000.0)
Technical. This open star cluster is unusual
in that it is one of the oldest known (estimates
range from 3 to 10 billion years) and is very
far from the plane of our Galaxy. Most open
clusters, also known as galactic clusters, lie
close to the galactic disk, whereas M67 lies
approximately 1500 light-years above it. The
cluster is about 2500 light-years distant and
12 light-years in diameter. It includes some
500 stars brighter than magnitude 16; the
brightest is magnitude 10.
VISUAL ASTRONOMY OF THE DEEP SKY
The stellar population is unlike that of
most open clusters, but rather similar to the
very old globular clusters: The only other
open cluster known to have a similar pattern
is NGC 188, in Cepheus. The most evolved
stars in M67 have only about half the luminosity
of similar stars in globulars, however.
This difference is attributed to chemical Com·
position: the typical globular is made almost
entirely of hydrogen and helium, whereas
M67 has a heavy-element ratio similar to our
Sun.
Visual. M67 has a . total vsua . 1 magnitude of
7 nd it is 15 arc-minutes In dIameter. When
esolved in very small instruments such as
inoculars, its mean surface brightness is
2\.5 magnitudes per square arc-second. The
c l any faint stars not resolved in an 8-inch
drawing shows an interesting visual effect of
losely spaced stars. The cluster contains
elescope: With averted vision, the eye can
detect their light without resolving them.
This gives an appearance of a faint background
nebulsity. The rawng is . a . representation
of thIs effect. WIth dIrect VISIOn the
"nebulosity" vanishes.
A VISUAL ATLAS OF DEEP-SKY OBJECTS
' ..
• •
•
. .
• •
• •
•
.-
•
, ..
•
•
0 ... .
,
• .
•
0 0
•
•
..
0 •
• •
• •
.
•
• •
'.
• • • •
.
.. '
o.
. . •
.-
\
•
0
, -
•
.
•
• • •
•
••
•
•
•
f
•
:
•
..
•• 0
• . •
. '
•
.
•
•
.
•
•
•
'0 •
•
•
..
•
•
•
•
•
• • f
•
•
•
•
•
•
•
• •
. "
•
•
•
•
'.
•
•
•
•
• 0
•
0
. ;
•
0
•
•
•
-- 5' ---
Photograph of M67. South is up . (Courtesy Optical
Astronomy Observatories')
Scale: 1.2 arc-min/cm
8-inch f/ 11.5 Cassegrain
20mm Erfle (117 X)
l 2 .4mm Erfle (188X)
Drawing of M67.
Viewing Distance (cm)
25X:115 200X:14
50x: 67 300x:lO
lOOx: 29 400 X: 7
air mass: 1.05, faintest star: 14.0 at zenith, 188x,
no tracking
1/16/83 10:30-10:45 UT at Barbers Point Hawaii'
R. Clark
' ,
118
119