OF THE ROGER N. CLARK

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MI08 (NGC 3556), GALAXY IN URSA
MAJOR
R.A. llh 1l.6m, Dec. 55° 41 ' (2 000.0)
Technical. M 1 08 is a nearly edge-on spiral
galaxy of type Sc. First discovered by Pierre
Mechain in 1781, it was not commonly included
in Messier's catalog until the early
1970s. M I 08 is about 25 million light-years
away and has many obscuring dust lanes and
no pronounced central bulge.
VISUAL ASTRONOMY OF THE DEEP SKY
Visual. MI08 appears 7.8 by 1.4 arcminutes
in angular size, with a total bright_
ness of magnitude 10.8 and a mean surface
brightness of22.0 magnitudes per square arcsecond.
It is 48 arc-minutes northwest of the
Owl Nebula, M97 (NGC 3587). Through the
8-inch under moderate skies, no detail corres_
ponding to dark lanes was seen. The nucleus
looked like a star, and an actual star is superimposed
on the galaxy's western side. The
galaxy appeared as a uniform, elongated
glow, fading at the edges into the sky background.
M97 (NGC 3587) THE OWL NEBULA:
PLANETARY NEBULA IN URSA MAJOR
R.A. llh 14.9m, Dec. 55° 02 ' (2000.0)
Technical. M97 was discovered by Charles
Messier's compatriot Pierre Mechain in
1781. When William Parsons, Lord Rosse,
observed it with his 72-inch speculum-metal
reflector in 1848, he made a drawing remarkably
like an owl's face, and M97 has been the
Owl Nebula ever since. It is thought to be
about 1600 light-years distant and about 1.5
light-years in diameter. Its gas is only 1/10 to
1/100 as dense as that of a typical planetary
nebula, suggesting that M97 is relatively old
and has been expanding and thinning for a
long time. The 14th-magnitude central star
has an effective temperature of 85 000 kelvin.
A VISUAL ATLAS OF DEEP-SKY OBJECTS
Visual. The two dark "owl's eyes" are hard
to detect in small to medium telescopes. With
a diameter of 2.5 arc-minutes and a total
magnitude of 11.0, the nebula has a mean
surface brightness of 22.6 magnitudes per
square arc-second - very low for a planetary
nebula. By comparison, the famous Ring
Nebula, M57 in Lyra, has a mean surface
brightness of 17.9 magnitudes per square arcsecond.
With the 8-inch under moderate to
good skies, the eyes were barely seen. If I had
not known of their existence I probably
would not have detected them at all. When
making the drawing I did not know the eyes'
true orientation, so the fact that they are
placed correctly indicates that they really
were seen - unless blind luck was at work.
This uncertainty shows the extreme difficulty
in observing the owl's eyes. Better skies or a
larger telescope are needed .
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---5' ---
Photograph of MI08. South is up . (Courtesy Evered
Kreimer, The Messier Album.)
Drawing of MI08.
Scale: 1.2 arc-min/cm
8-inch flll.5 Cassegrain
20mm Erfle (117X)
12.4mm Erfle (188X,
best view)
7mm Erfle (334x)
Viewing Distance
25 X:115 200x:14
50 X: 57 300 X : 10
100 X : 29 400X: 7
air mass: 1.41, faintest star: 13.8 at zenith, 188X;
no tracking
5/ 1 5/83 8:41-8:52 UT at Barbers Point, Hawaii;
R. Clark
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-- 5' ---
Photograph of M97. South is up. (Courtesy Evered
Kreimer, The Messier Album.)
Drawing of M97.
Scale: 1.2 arc-min/cm
8-inch fI 1 1.5 Cassegrain
20mm Erfle (117x)
12.4mm Erfle (l88x,
best view)
Viewing Distance (cm)
25 X:115 200 X:46
50 X: 57 300 X : 10
100x: 29 400 X: 7
air mass: 1.36, faintest star: 13.8 at zenith, 188x;
no tracking
5/ 1 5/83 8:25-8:40 UT at Barbers Point ' Hawaii' ,
R. Clark
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